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Unformatted text preview: TABLE OF CONTENTS Introduction.................................................................................................. INTRO - 1 Lab 1 Estimates: ............................................................................................................................................. Lab 1 - 1 Lab 2 Velocity and Acceleration: ............................................................................................................................................. Lab 2 - 1 Lab 3 Velocity, Acceleration and Newton's Second Law: ............................................................................................................................................. Lab 3 - 1 Lab 4 Conservation of Energy: ............................................................................................................................................. Lab 4 - 1 Lab 5 Energy and Work: ............................................................................................................................................. Lab 5 - 1 Lab 6 Heat and Heat Transfer: ............................................................................................................................................. Lab 6 - 1 Lab 7 Mechanical and Thermal Energy: ............................................................................................................................................. Lab 7 - 1 Lab 8 Circuits: ............................................................................................................................................. Lab 8 - 1 Lab 9 Magnets: ............................................................................................................................................. Lab 9 - 1 Lab 10 Heat, Solar Input and Earth's available Energy: ........................................................................................................................................... Lab 10 - 1 Lab 11 Greenhouse Effect: ........................................................................................................................................... Lab 11 - 1 Lab 12 Energy and Atomic Spectra: ........................................................................................................................................... Lab 12 - 1 Lab 13 Wind & Water Energy: ........................................................................................................................................... Lab 13 - 1 Appendices: Appendix A Logger Pro Interface .....................................................................APPENDIX A - 1 Appendix B Lab Pro Software ..........................................................................APPENDIX B - 1 Appendix C Motion Detector ............................................................................APPENDIX C - 1 Appendix D Voltage Probe................................................................................APPENDIX D - 1 Appendix E Microphone ................................................................................... APPENDIX E - 1 Appendix F Energy Transfer Generator ............................................................ APPENDIX F - 1 Appendix I The Digital Multimeter (DMM) ...................................................... APPENDIX I - 1 Appendix II Graphing with Microsoft Excel ....................................................APPENDIX II - 1 Appendix III Graphed Proportions ..................................................................APPENDIX III 1 ACKNOWLEDGEMENTS The authors would like to thank all the people who have contributed to the development of this manual: Ellery Frahm David Parent Paul Knutson Jennifer Docktor And all of the teaching assistants who helped to find the 'bugs' in these instructions. Kenneth & Patricia Heller Kenneth Heller & Patricia Heller Welcome to Energy and the Environment! The lab exercises found in this manual focus on the physics that is the basis of issues concerning the generation and use of energy in our technological society. Energy use lies at the center of industrial society, the products of which, from light bulbs to cell phones, are based on our understanding of physics. The generation and the use of energy affect the environment at global, regional, and local levels. Through the exercises in this manual, you will explore the physical principles that govern the production and use of energy. The point is to get an intuitive idea of this abstract concept of energy by using it to make sense of the behavior of real objects. The lab is where you can apply the physics from the lectures and your textbook to the real world via "hands-on" experience. To do that, some degree of mathematical descriptions and calculations are necessary as tools, and you should feel free to ask your TA for help if you need it. The aim of the lab exercises, and this course as a whole, is to build your ability to think critically about the energy and environmental issues that our society faces. Before lab each week, read the pertinent sections of the textbook, as indicated for each exercise. Also familiarize yourself with the upcoming lab so that you have a reasonably clear idea of what will happen in the lab. To do well in lab be prepared before you come. If you do not come prepared, you will waste time trying to figure out what is going on and will not effectively use the lab time to challenge your ideas about physics and the real world. Use the lab periods to experience the behavior of nature and apply the concepts of this course. You must have and use a square ruled lab notebook. Keeping a good notebook and answering the prep questions in your lab notebook before you come to lab will be part of your lab grade. Your TA will explain what is expected of you. Other details are in the syllabus. A brief period at the beginning of each lab will be used to distribute answers to the week's homework problems and the two class pop quizzes each week, and to discuss--briefly--questions that they raise. INTRO - 1 INTRO - 2 LAB 1 ESTIMATES AND PREPARATION This is a short lab that will be spent on two things: a collection of estimates that we will ask you to do in groups. The problems will be passed out at the beginning of lab. The second thing we will do is to familarize you with the equipment that we will use in lab and how we expect you to prepare lab reports. 1001 LAB 1: ESTIMATES AND PREPARATION 1001 Lab 1 2 LAB 2 VELOCITY AND ACCELERATION Our first task in the course is to establish an understanding of how engineers and scientists define and calculate energy in moving objects. When we have done that, the discussion of energy utilization in society and the consequences for the environment can be discussed. The moving objects which we study at first are objects which you can easily see, like cars and baseballs. It turns out that the same concepts are used in much the same way in the study of very small objects like atoms and molecules which you cannot see without very sophisticated equipment. We will need the applications of the concepts of energy to atoms and molecules to understand energy technologies and environmental problems, but we will not discuss those applications for a few weeks, starting instead with the more easily visualized large moving objects In physics, all motion is ultimately described in terms of the positions of things at successive times. Position can be specified by giving lengths, as when one says that the physics building is about 20 feet straight north of the math building. Notice that to specify that position (even approximately) I had to give a direction as well as a length. By saying straight north I told you that you dont have to go east or west to get there from math, so I also told you the east west position of the building (again approximately.) For buildings, two lengths, say in the north direction and the west direction, are usually enough, but for an airplane position you would also have to give another length, the altitude. Generally, in our world, three lengths are enough to tell anyone where something is. That is what we mean by saying that space has three dimensions. Time requires less discussion: it is what you measure with a clock. Now a complete description of the motion of an object can be given by stating its position at each moment in time (a kind of history of the positions which the object takes). However it turns out that in order to define and measure the energy of an object and to predict its motion using physics equations, it is useful to use two other quantities which are closely related to this history of the motion and can be obtained from it. These closely related quantities are velocity and acceleration. We discuss them in terms of the motion of a car. You will do experiments on a toy car in this laboratory in order to measure them. A change in the position of a car, a baseball or any object means there is a change in its velocity. A change in velocity means there is acceleration. Notice that acceleration is not the same as velocity because there can be a velocity which is not zero, and as long as the velocity is not changing, the acceleration is zero. Here we briefly repeat the definitions and discussion of velocity and acceleration which are given in the assigned reading for this laboratory (see below). For simplicity we consider only motion along a straight line here, so that position can be specified by just one length. If an object travels along a straight line, as on the track on which the car in the experiments moves, then if it travels a certain time, then its average speed during that time is the distance gone divided by the time elapsed. If you are driving across Nevada on a very straight freeway, and you go 210 miles in 3 hours then your average speed was 70 miles per hour (210/3). However, your speedometer did not read 70mph the whole time. Sometimes it was higher and sometimes lower. The speedometer measures instantaneous speed which is the average speed averaged over an extremely, immeasurably small, time interval. When you sped up, so that the reading on the speedometer went from 65 mph to 75 mph in 30 seconds then the car had a positive acceleration. To calculate the average acceleration you divide the change in the speed by the time it took. In that case acceleration = (75mph65mph)/(1/120 hour) = 1200 miles per hour per hour. 1001 LAB 2: VELOCITY AND ACCELERATION Here I expressed 30 seconds as 1/120 of an hour. What the average acceleration means is that, if you kept on accelerating like that for one hour your speed would in crease by 1200 mph. (Not possible but fun to think about.) If you slow down, and make a similar calculation of the acceleration, you get a negative number. Negative accelerations correspond to slowing down and are sometimes called decelerations. Just as with velocity, you can also measure an instantaneous acceleration (though the measurement is a little trickier) that corresponds to the average acceleration over an extremely small time interval. PRELAB READING Hinrichs and Kleinbach, pp 6369 EQUIPMENT 1 motorized toy car 1 aluminum track 1 wooden block 1 meter stick 1 low friction cart 1 stopwatch 1 length of string 1 video camera 1 computer /LapPro interface and Motion sensor w/LoggerPro PREDICTIONS/PRELIMINARY QUESTIONS 1. What does it mean if an object moves with a constant velocity? Sketch a graph of velocity vs. time for a car (a motorized toy car in lab) moving with a constant velocity. Put velocity on the vertical axis, and time on the horizontal axis of your graph. In your own words, describe what is meant by constant acceleration. Sketch a graph of velocity vs. time for a car that is speeding up with a constant acceleration. Sketch a graph of velocity vs. time for a car that is slowing down. The following graph shows the velocity of a car in meters per second (m/s) measured at several different times. Calculate the acceleration of this car by using the slope of the graph. 2. 3. 4. 5. 1001 Lab 2 2 1001 LAB 2: VELOCITY AND ACCELERATION 6. What happens to the acceleration of a cart as it moves down an inclined track? (Does the acceleration increase, decrease, or remain the same?) Explain your reasoning. There are two parts to this exercise. The first involves a level track, and the second involves an inclined track. PART I: LEVEL TRACK, BATTERY CAR EXPLORATION Before you start to acquire computer data you need to explore a bit, to try out the other equipment. This is something you will need to do in each lab; most equipment has a range of operation that is simple and straightforward, but outside that range complicated corrections are needed. 1. Place one of the metal tracks on your lab table and place the motorized toy car on the track. Turn on the car and observe its motion. The battery powered toy car should move at constant velocity on the aluminum track. Do you think it does? Why or why not? You may also want to see if you can get a PASCO cart to travel at constant velocity for taking data. 2. It is often useful to make approximate measurements without using complicated equipment. For example, use your forearm as a unit of distance and count seconds by counting one thousandone, onethousandtwo, etc. Then you might measure, with your arm, (don't use a meter stick) three arm lengths along the aluminum track. Let the car run that three arm length distance and measure the time by counting onethousandone, etc. To determine the speed, divide three arm lengths by the time in seconds and obtain the speed in terms of armlengths per second. Each person in the group should try this approach at least once. Take some measurements with the meter stick and a stopwatch to calculate the approximate speed of the battery powered car. Explain how you determined its speed (or velocity). Be sure to record the units you are using for each value. How would a graph of horizontal distance vs. time look for the car going at a constant velocity? Sketch this graph in your lab notebook. (You don't need numbers on this graph just a sketch to show the shape of the motion.) Put distance on the vertical axis and time on the horizontal axis of your graph. Sketch a graph for velocity vs. time for the car going at constant velocity (no numbers are needed just a sketch.) Put velocity on the vertical axis and time on the horizontal axis. 3. 4. 5. You should have agreement among your lab partners about the approximate velocity of the car. You should also compare graphs. If you don't agree, continue with the lab knowing that there should be a consensus on the graphs before the end of the lab session. PROCEDURE Exploration with LoggerPro For instructions about using the LabPro interface, LoggerPro data collection software or Motion sensor, refer to the appendices at the end of this lab manual. On the computer desktop open the LoggerPro software with the motion detector connected. In the upper left corner, a small icon should show that the interface is connected and that a Motion sensor is plugged in. If you have problems, check your connections and inform your TA. 1001 Lab 2 3 1001 LAB 2: VELOCITY AND ACCELERATION To collect data you press the Go button along the top menu. Practice collecting data of the motorized toy car. Make certain everyone in your group has a chance to operate the computer. You can adjust the settings in LoggerPro to graph position vs. time, velocity vs. time and/or acceleration vs. time. Please read the appendix on using LoggerPro and take time during lab to make sure you can create all graphs necessary. If possible, every member of your group should operate the computer. ANALYSIS 1. From the graph of distance vs. time, calculate the slope of the line. How is this slope related to the velocity of the car? LoggerPro allows you to fit curves automatically, read your appendix to find out how. From the velocity vs. time graph, what can you tell about the acceleration of the car? Explain how you can tell this from the graph. 2. PART II: INCLINED TRACK, CART Use a low friction PASCO cart not the battery toy car. Put a wood block under the aluminum track to make the incline. (One block is probably all you need.) Be certain to catch the cart before it falls on the floor! Discuss with your partners and make a prediction: How would you expect a velocity vs. time graph to look for a cart rolling down an incline? Draw a sketch of the graph. Explain what coordinate system you are using EXPLORATION 1. Use a meter stick and a stopwatch to determine the average velocity of the cart rolling down the incline. 1001 Lab 2 4 1001 LAB 2: VELOCITY AND ACCELERATION 2. Use a meter stick and a stopwatch to record the distance and time for several points along the inclined track when the cart is rolling down the incline. You might need to release the cart more than once. Make a graph of distance vs. time for this measured data. 3. PROCEDURE Use the computer to obtain graphs of distance vs. time and velocity vs. time for the cart on the incline. Refer to the proper appendix if you need assistance using the software, interface or sensor. ANALYSIS 1. 2. 3. 4. How can you obtain the acceleration for the cart from the velocity vs. time graph? What is the acceleration of the cart moving down the incline? Is the acceleration of the cart increasing, decreasing, or constant as it rolls down the incline? How can you tell? How does the value of the acceleration for the cart compare to an object in free fall (9.8 meters per second every second)? CONCLUSIONS 1. 2. 3. 4. 5. 6. 7. 8. Did either the battery car or the cart have a constant velocity? If so, which one(s)? Did either object have a constant acceleration? If so, which one(s)? Sketch a graph (without numbers) of distance vs. time for an object moving with a constant velocity. Sketch a graph (without numbers) of distance vs. time for an object moving with an increasing velocity. Sketch a graph (without numbers) of velocity vs. time for no acceleration. Sketch a graph (without numbers) of velocity vs. time for constant acceleration. How do your measured graphs compare to your predictions? Can you, in the future, identify motion with a constant velocity and motion with a constant acceleration? Galileo concluded: "A body is said to be uniformly accelerated when, starting from rest, it acquires equal increments of velocity during equal time intervals." Does his definition of constant acceleration agree with your observations? HISTORICAL NOTES Galileo held ignorato motu ignorator natura, or ignorance of motion is ignorance of nature now you and your partners have a start in understanding velocity and acceleration, which will lead to ideas concerning energy and its role in the natural world. In this exercise you and your partners analyzed the motion of a motorized car on a horizontal surface (aluminum track) and the motion of a cart on that same surface when tilted. The track and wheels 1001 Lab 2 5 1001 LAB 2: VELOCITY AND ACCELERATION minimized friction. The equipment is not so different than the tilted board used by Galileo in 1608 to study acceleration: A piece of wooden moulding or scantling, about 12 cubits [about 7 m] long, half a cubit [about 30 cm] wide and three fingerbreadths [about 5 cm] thick, was taken; on its edge was cut a channel a little more than one finger in breadth; having made this groove very straight, smooth, and polished, and having lined it with parchment, also as smooth and polished as possible, we rolled along it a hard, smooth, and very round bronze ball (Discourses on Two New Sciences, 1638). Galileo hoped to investigate the motion of falling objects; however, most objects fell too quickly to measure their velocities with the equipment available to him. His solution was to use an inclined plane. He utilized a tilted board with a groove, down which he rolled a metal ball, to test Aristotelian ideas about motion. Galileo's experiments with his inclined plane were revolutionary because he focused on acceleration, a form of motion overlooked by Aristotle and his followers. 1001 Lab 2 6 LAB 3 VELOCITY, ACCELERATION, AND NEWTON'S SECOND LAW Now that you have some familiarity with velocity and acceleration, we can introduce the idea of the total force on an object which is in motion. The action of forces is the means by which energy is transferred between objects so it will be very important for our later discussions. We start with a situation in which there is very little friction impeding the motion of the object. For example consider ice skating. Suppose you are standing at the edge of a skating rink and give your little brother (weighs 50 pounds, on skates) a push while holding on to the rail. He goes flying off at some speed. Now suppose you give your sister (100 pounds) an identical push. It turns out that if you measure the final speed of both family members, the little brother will be going twice as fast. We say for a given force on the object (the push which you gave) the acceleration is less if the object has more mass. acceleration resulting from a force = (total force on the object)/(mass of the object) Here we write mass instead of weight. Mass is closely related to weight but it is not quite the same. In the example, the mass or the brother and sister is proportional to the weight. To make a determination of the mass of an object, the most conceptually straight forward way is to use a set of standard masses and a balance. The mass of the object is then the amount of standard mass required for balancing the mass of the object. Now if you look at the word equation above you see that you know how to measure acceleration (from last weeks discussion) and we just discussed how to measure mass, so the only thing left in the equation is the total force. So you can turn the equation around by multiplying by mass to solve for the total force: total force on the object= (mass of the object)x(acceleration resulting from the force) This famous equation, often written F=ma, is Newtons second law of motion. So far it basically just defines what we mean by the total force on an object. It becomes useful if we have some additional information about the force. For example consider an object dropped so that it falls straight down. You probably heard that Galileo made experiments with dropping objects and found out that massive balls falling from the top of the leaning tower of Pisa hit the ground at the same time as much less massive ones dropped at the same time. This is interpreted to mean that the acceleration of the balls is the same whether they have a lot of mass or a little mass. But now look at the equation F=ma . The only way it can be true in the case of the falling objects is if the gravitational force on a falling object increases with its mass. Suppose that is true, say that F=mg where g is some constant acceleration due to gravity. Substituting this back in Newtons second law we get F=mg =ma so a=g, that is the acceleration of falling objects is independent of mass, as Galileo(and many others) observed. So from Newtons second law plus an experiment we learned something about gravitational forces, namely that they are proportional to the mass in the moving object. But not all forces in nature have this property of being proportional to the mass in the object. For example the push that you give your sister or brother is not a gravitational force. It is generated by your muscles and which act through what are called electromagnetic forces in physics. That is why, when you push them, the little brother ends up going faster than the sister, but if you dropped them off a bridge (of course you wouldnt do that) they would both accelerate the same amount. Most of the time in this course, the forces we consider will be gravitational or electromagnetic in origin. These 1001 Lab 3 1 1001 LAB 3: VELOCITY, ACCELERATION, AND NEWTON'S SECOND LAW are two of the four main categories of force known to physicists. At the end of the course we will consider a third kind of nuclear force. In applying Newtons second law, it is often important to keep in mind that it refers to the total force on an object and that this total can be the combined result of the action of more than one kind of force. Furthermore, these different forces are not always acting in the same direction and can partially or completely cancel each other out. As an example, suppose you hold a book out in front of you over the floor. As you hold it there, it is not accelerating so the total force on it is zero by Newtons second law. But the force of gravity is pulling down on it and you are pushing up on it with your hand (with a force that is electromagnetic in origin). The two forces are exactly equal in magnitude but opposite in direction and cancel each other exactly as long as the book doesnt fall. In more complicated cases, forces, again ultimately of electromagnetic origin, of friction and air resistance play a role. These more complicated cases (which made the early study of the subject in past centuries difficult) will be discussed later. In this laboratory you will consider cases in which they can be neglected. You and your partners will measure the velocity and the acceleration of an object to understand how it obeys Newtons second law. PRELAB READING Hinrichs and Kleinbach, 6570 EQUIPMENT 1 aluminum track 1 motorized toy car 1 meter stick 1 mass hanger 1 mass set 1 wood block 1 low friction PASCO cart 1 piece of string 1 video camera 1 computer /LapPro interface and Motion sensor w/LoggerPro PREDICTIONS/PRELIMINARY QUESTIONS 1. 2. 3. 4. 5. Sketch a graph of velocity vs. time for a toy car or cart used in lab, moving with a constant velocity. Sketch a graph of velocity vs. time for a low friction cart that is moving down an incline.. On the same graph as question #2, sketch a graph of velocity vs time for a cart that is about twice as heavy and is moving on the same incline as the cart in question #2. Sketch a graph of velocity vs. time for a cart moving on a horizontal track being pulled by a weight that is connected by a string over a pulley. (See the sketch in Part III.) Sketch a graph of velocity vs. time for the same cart with extra mass attached (so the total mass of the cart and added mass is about twice the mass of the cart alone). The cart with extra mass is being pulled by the same weight as in question #4. Put the sketch on the same graph as question #4. 1001 Lab 3 2 1001 LAB 3: VELOCITY, ACCELERATION, AND NEWTON'S SECOND LAW PROCEDURE There are two situations. The first involves an inclined track (part I below), and the second involves a level track with a hanging mass pulling the cart (part II below.) In both exercises below, mass will be added to the lowfriction cart for an additional trial. PART I: INCLINED TRACK, CART Use a low friction PASCO cart not the battery toy car. Put a wood block under the aluminum track to make the incline. 1. First discuss with your partners and make a prediction: How would you expect a velocity vs time graph to look for a cart on an incline? Draw a sketch of the graph for velocity vs. time. 2. 3. Using a meter stick and stopwatch to determine the average velocity of the cart on the incline. Using the computer and motion sensor: a) Obtain a distance vs. time graph for the cart on the incline. b) Obtain a velocity vs. time graph c) How can you obtain the acceleration for the cart from the velocity vs. time graph? d) What is the acceleration of the cart? e) Is the acceleration increasing, decreasing, or constant? How can you tell? Now add some mass to the cart. Record the approximate mass of the cart and the approximate mass that you add to the cart. Repeat step #3. How does the acceleration this time compare with the acceleration without extra mass? Explain your results. 4. PART II: TENSION AND LEVEL TRACK Remove the block from under the track. Attach a string to the cart, hang it over a pulley, and tie a mass hanger with masses to the other end. Now the cart is free to move in the horizontal direction, and the only unbalanced force on it is the tension in the string from the weight hanging off the end. You and your partners will find the cart's acceleration due to this tension. Car t Tr ack A 1. 2. Using the computer and motion sensor: a) Obtain a distance vs. time graph for the cart accelerating (before the mass hits the floor.) b) Obtain a velocity vs. time graph for the cart. c) How can you obtain the acceleration for the cart from the velocity vs. time graph? d) What is the acceleration? e) Is the acceleration increasing, decreasing, or constant? How can you tell? Now add additional mass to the cart. Repeat your data collection with the more massive cart in order to calculate its acceleration. What is the acceleration of the more massive cart? How does it compare to the earlier acceleration? If you assume that the tension in the string is the 1001 Lab 3 3 1001 LAB 3: VELOCITY, ACCELERATION, AND NEWTON'S SECOND LAW same in the two experiments, what does Newtons second law tell you about what how the two measured accelerations should be related? CONCLUSIONS 1. a) Did the lowfriction cart in Part I accelerate as it moved down the incline? How do you know? b) Was the acceleration of the more massive cart greater than, less than, or equal to the acceleration of the cart with no additional mass? c) When more mass was added to the cart, did the total force on the cart change? Did the acceleration change? Explain how your observations do or do not agree with Newton's second law of motion. a) Did the lowfriction cart in Part II accelerate as it moved down the incline? How do you know? b) Was the acceleration of the more massive cart greater than, less than, or equal to the acceleration of the cart with no additional mass? c) When more mass was added to the cart, did the total force on the cart change? Did the acceleration change? Explain how your observations do or do not agree with Newton's second law of motion. a) Compare what happened to the acceleration in Part I and Part II when more mass was added to the cart (refer to your answers for questions 1b and 2b.) b) Compare what happened to the force in Part I and Part II when more mass was added to the cart. c) If there are differences between Parts I and II, what are they? If there are no differences, how are these trials the same? 2. 3. Newtons second law of motion is one of the most fundamental relationships of all physics. Together with conservation of energy, it forms the basis of all Newtonian physics. It is hard to exaggerate the importance of Newtons second law when it can be applied in a wide range of topics, from projectile motion to electromagnetic fields. It was the concept of force that was essential to the development of mechanics and all physics. Be certain you can identify the specific forces acting in both parts of this laboratory. HISTORICAL NOTES You should not be embarrassed if you find it a little tricky to sort out what is happening in the motion of these simple objects. Though the concepts of acceleration, velocity, force and Newtons second law are quite simple to state, very intelligent people studied these questions for thousands of years before finally getting them right. For example early (more than 2500 years ago) Greek natural philosophers, such as Thales, Anaximander, and Anaximenes, thought that motion was eternal and somehow contained inside all things. Aristotle, who wrote a book called Physics that was widely used in Europe during the Middle Ages, thought that rest is the natural condition of objects and that there are two types of motion natural motion is due to an objects natural tendency towards its proper place where it then rests, and unnatural motion is caused by something other than the object itself. It was not until the work of Galileo, who died about 360 years ago, that forces and acceleration started to be correctly understood. A major difference between Galileo and his predecessors is that Galileo did not just think about motion. He did experiments much like the ones you have done in this laboratory. 1001 Lab 3 4 LAB 4 CONSERVATION OF ENERGY This week we have enough of the basic concepts to begin a discussion of energy itself. Energy is sometimes introduced as if it is a concept independent of Newtons laws (though related to them). In fact, however, the idea of energy arises directly out of Newtons second law, and Newtons second law actually guarantees a central fact about energy, namely that no energy is ever lost or destroyed, but simply changes form, the socalled law of conservation of energy. This can be illustrated by the example of the falling ball (You can think of one which Galileo dropped.) The example involves just a little bit of algebra which we use to show how the energy conservation law comes out of Newtons second law. Remember that the gravitational force on the falling ball is the constant mg, where m is the mass of the ball and g is a constant acceleration due to gravity. Therefore Newtons second law takes the form mg=ma for the falling ball. Now the acceleration is the speed divided by the time elapsed. In Newtons second law, it should be the instantaneous acceleration, but because the acceleration is a constant for the gravitational force the instantaneous acceleration is the same as the average acceleration. Consider the moment just before the ball hits the ground, supposed to be a time t since it was dropped. If the speed is v then, the acceleration is v/t. Putting this in the previous equation mg=mv/t Now suppose the ball fell a distance d. To turn the preceding equation into an equation involving energy we multiply both sides of the previous equation by d mgh=(mv/t)h Now d is the average velocity times t. Here the ball was accelerating so the average velocity is not the final velocity just before it hits the ground but is of that velocity so d=(v/2)t. I substitute this for d in the righthand side of the previous equation and simplify mgh=(mv/t)(v/2)t=(1/2)mv2 This is an equation which describes how energy is conserved for a falling ball. On the left hand side, the quantity mgd is the force on the ball times its height d above the ground before it was dropped. We call it the gravitational potential energy of the ball before it was dropped. On the right hand side is a quantity (1/2)mv2 which is called the kinetic energy of the ball just before it hits the ground. We interpret the = sign to mean that the gravitational potential energy of the ball before it fell was converted into an exactly equal amount of kinetic energy which it had just before it hit the ground. To summarize, for a falling ball, two forms of energy, kinetic and gravitational potential, are involved. The sum, total energy = gravitational potential energy + kinetic energy stays the same during the motion, but at the beginning of the fall it is all gravitational potential and at the end it is all kinetic. In between, though we did not prove it, the gravitational potential energy gradually decreases while the kinetic energy increases to keep the sum constant. The fact that the sum is unchanged is what we mean by conservation of energy in this case. I showed by algebraic manipulation of Newtons second law that this follows from Newtons second law and is not an 1001 Lab 4 1 1001 LAB 4: CONSERVATION OF ENERGY independent principle. This conservation law turns out to apply for any system of objects obeying Newtons equation, though some other forms of potential energy need to be introduced. Though it follows from Newtons equation, the conservation of energy is often easier to apply and think about than the general solution to Newtons equations, so we will often apply conservation of energy considerations to study of complicated situations without worrying much about the underlying Newtonian mechanics. In this exercise, you and your partners will investigate the law of conservation of energy with an aluminum track and a lowfriction cart attached to a hanging mass. You will calculate: (i) the total change in kinetic energy of the cart and hanging mass, (ii) the change in the potential energy of the hanging mass. According to the conservation of energy and the relationship between work and energy, these quantities should be equal. Figure 1: Cart Track A Pre-Lab Reading Hinrichs and Kleinbach, pp 4850 EQUIPMENT 1 aluminum track 1 pulley 1 mass hanger 1 meter stick 1 lowfriction PASCO cart 1 piece of string 1 mass set 1 stopwatch 1 computer /LapPro interface and Motion sensor w/LoggerPro PREDICTIONS/PRELIMINARY QUESTIONS A cart is located on a level air track with a string attached between the cart and a weight. The weight is hanging by the string so it can pull the cart along the track. (See the diagram in figure 1.) 1. When the weight is hanging below the pulley but above the floor, what form of energy could be used to describe the energy of the weight relative to the floor? What quantities would you need to know or measure to calculate this energy? 1001 Lab 4 2 1001 LAB 4: CONSERVATION OF ENERGY 2. 3. 4. 5. 6. Does the potential energy of the hanging weight increase, decrease, or stay the same as it falls to the floor? Does the potential energy of the cart on the track increase, decrease, or stay the same as it is pulled by the hanging weight? Suppose the weight falls from a certain height and pulls the cart along the horizontal track. Just before the weight hits the floor, what form of energy does it possess? Using the law of conservation of energy, describe the energy situation just before the hanging weight starts falling compared with the energy situation just before the weight hits the floor. Write an expression for the work done by gravity in pulling the weight from its initial height to the floor. What quantities do you need to measure to calculate the work done on the hanging mass? PROCEDURE In this exercise, you and your partners will calculate :i) the total change in kinetic energy of the cart and hanging mass and ii) the change in potential energy of the hanging mass. 1. The change in kinetic energy of the cart and hanging mass can be calculated. The two objects will both start at rest, so their initial kinetic energies are zero. The cart and the hanging mass will accelerate at the same rate (a string connects them.) The velocity of one object will always be equal to the velocity of the other, as long as the string remains taut. With the computer data analysis software, you can find the velocity of the cart or hanging mass at any point in time. You and your partners are interested only in the final velocity. (After the weight hits the floor, what happens to the speed of the cart?) The mass of the cart and that of the hanging mass can be determined with a triplebeam balance. Refer to the appendix on using LoggerPro if necessary. With this information, calculate the kinetic energy of each object separately. Recall that kinetic energy is: K = E 1 2 m v 2 where m is mass of an object and v is the velocity. Be sure to use the proper SI units: kilograms for mass and meters per second for velocity. To calculate the total change in kinetic energy, you must add the final kinetic energy for both the hanging mass and the cart. 2. There is no change in the gravitational potential energy of the cart if the track is level, its height off the floor does not change. Only the height of the hanging mass changes. As a result, only the potential energy of the hanging mass changes. Define the floor as the point of zero potential energy. Recall the equation for gravitational potential energy: P =m E gh where m is the mass of the object, g is the acceleration of gravity on Earths surface, and h is the height from the point of zero potential energy. Use this equation and the measured mass of the hanging mass and the distance if fell to calculate the potential energy, 1001 Lab 4 3 1001 LAB 4: CONSERVATION OF ENERGY CONCLUSIONS 1. 2. a) Do your values support the conservation of energy? Explain. b) How can you account for any discrepancies between these values? Comment on your values for the total change in kinetic energy of the cart and the hanging mass and the change in potential energy of the hanging mass. Consider the following situation: A physics instructor demonstrates the conservation of energy with a bowling ball attached to the ceiling with a string. It is simply a large pendulum. When the pendulum is pulled back, it has all potential energy, and at the bottom of its swing, it has all kinetic energy. The physics instructor releases the bowling ball from the tip of his nose, and it swings across the room and back toward him. Based on the conservation of energy and your results for this exercise, will this physics instructor get smacked by the bowling ball? Will it touch his nose? Explain. What if he gave the bowling ball a slight push when he let go? Explain your answers. If there is a difference, distinguish between what happens in theory and what happens in the real world. 1001 Lab 4 4 LAB 5 ENERGY TRANSFER THROUGH WORK In previous laboratories you have studied the energy of a body or collection of bodies which could be regarded for the purposes of the experiment as isolated. We did not have to consider the transfer of energy from one body to another explicitly. However in energy technology such as the delivery of electricity to your home where it is converted to other forms for your use or the delivery of the chemical energy in gasoline to your car where it is converted through two steps to the kinetic energy of your car, energy transfer from one body to another is of utmost importance. Energy transfer from one body to another takes place through the action of work. To illustrate this, we will turn back to the falling ball discussed in laboratory 2, but this time, suppose that, before the ball was dropped, you picked it up off the floor and lifted it up to height d for dropping. For simplicity we will suppose that it was lifted slowly and steadily so that we can ignore any accelerations and decelerations during the lift. From an energetic point of view, what is happening to the ball is that gravitational potential energy is being added to it. Where did that energy come from? It came from your body, where the energy was stored in the form of chemical energy in your muscles. We want to focus on the mechanism by which the transfer took place: As you lift the ball, a gravitational force mg (massxgravity) acts down on it and you exert a slightly larger force upward on it with your hand. Since we are ignoring accelerations and decelerations as small we can suppose that this upward force is also of magnitude mg (though it has a completely different origin than the downward force). Now the net energy added to the ball is the gravitational potential energy mgd. Therefore Energy transferred to the ball during lift =mgd = (magnitude of the upward force due to your hand X distance through which the force acted) We say that your hand performed work on the ball equal to the force you exerted on the ball times the distance through which it acted. The conclusion from this example can be generalized: When a body exerts a force on another body and the body moves through a distance, energy is transferred to the other body. The magnitude of the energy transferred is the product of the magnitude of the force times the distance, along the direction of the force, through which the force acts. When energy transfers of this sort occur it is often relevant to the engineering application to know how fast they occur. Therefore we introduce the concept of power, which is the rate at which energy is transferred due to the performance of work and is given by the amount of work done divided by the time elapsed. In this laboratory, you will perform two experiments in which work is done and energy is transferred to a body. The first case is a little tricky because it appears that the body, actually your body, is doing work on itself: You will be running up stairs and measuring the rate at which you add gravitational potential energy to your body. However, one can regard this as a transfer of chemical energy stored in your muscles to the gravitational energy of your body as a whole through the performance of work which is done as you push your self upstairs with your legs. (If you think about it you will find it a little tricky to analyze how you push yourself upstairs, but it is clear that a net upward force must result.) In the second experiment you will turn a crank which causes a wheel to rub against an aluminum cylinder. In this case chemical energy is converted ultimately to the thermal energy in the aluminum, manifested as a rise in its temperature. As we will discuss, thermal energy is actually a form of kinetic and potential energy associated with the random motion of the molecules of a substance (aluminum here). 1001 Lab 5 1 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK PART 1. TRANSFER OF CHEMICAL ENERGY TO GRAVITATIONAL POTENTIAL ENERGY THROUGH WORK As you run up stairs, you use over two hundred muscles. When you walk or run, your muscles use energy both to propel your body forward and to move it against gravity. The energy to do this comes from your food. Your muscles convert the chemical energy of adenosine triphosphate (ATP) into mechanical work. In this exercise, you and your lab partners will find your own work and power output when running up a single flight of stairs and then four flights of stairs. PRELAB READING Hinrichs and Kleinback, pp 4757 EQUIPMENT 1 stopwatch 1 meter stick 1 bathroom scale PREDICTIONS/PRELIMINARY QUESTIONS (Look at the lab procedure and prelab reading for help in answering these questions. For converting energies into different units, you can use Appendix B of your book (page B.I) or Table 3.4 (p.91)) 1. Estimate the work you would do in climbing a set of stairs with a vertical height of 4 meters. (Use the weight of a person in kg) If you know the weight of a person how do you find the force of gravity on the person? 2. 3. 4. 5. 6. What is the relationship between the work you do in climbing a set of stairs and the potential energy you gain in going up the stairs? Estimate the time it would take to walk up the set of stairs and use that to determine the power you would develop in climbing those stairs. Estimate the amount of horsepower you develop in climbing those stairs. Note: 746 watts of power are equivalent to one horsepower. If you run up the stairs (instead of walking) will you be developing more horsepower or less? Explain your reasoning. If you walk up several flights of stairs do you think the power you expend would be greater, the same or less than when you walk up only one flight? Explain your reasoning. PROCEDURE In this exercise, you and your partners will calculate your work and maximum power output when climbing stairs. Remember that work is a change in energy. When you climb a flight of stairs, you have increased your potential energy (you now have the potential to fall down the stairs under the influence of gravity). How fast this change in energy occurs will allow you and your partners to determine the power output involved. This exercise requires physical activity. If you are unable to participate due to an injury or asthma, tell your instructor. Be careful on the stairs so you dont trip or collide with anyone else. Someone acting as a lookout is a good idea. 1001 Lab 5 2 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK 1. 2. To calculate your potential energy, you must know your mass. Measure your weight in pounds on the scale and multiply by 0.45 to find your mass in kilograms. You must also know your change in height to calculate your potential energy. Use the meter stick to determine the height of one flight of stairs and four flights of stairs. Be careful the physics building is a little strange. The flights of stairs have different numbers of steps, and some of the flights even have steps with a different height! In each case, the student has the same gravitational potential energy, and she has done the same amount of work to reach the top because (neglecting friction) she does no work when she moves horizontally. Work is required, though, for her to move against earth's gravity. 3. Each member of your lab group should run up one flight of stairs (from one landing to the next) while someone records the time taken. Next, each group member should run up four flights of stairs (two stories) while someone records the time taken. 4. Calculate the work you did in lifting your body against the gravitational force. The work done is equal to your change in potential energy. To find your power, divide the change in potential energy by the time it took to climb the stairs. Report the results. CONCLUSIONS 1. Report your leg power in watts (Joules per second). Divide your result by 746 in order to find your power in horsepower. How does your work and power compare to that of your partners? Did anyone do more work than you but used less power? Vice versa? Who did the most work? Who had the greatest power output? The least? As you climb the stairs, you convert food energy into kinetic energy. A major part of this conversion, however, is heat that is lost to the air. In fact, about 75 percent of the energy from food becomes heat. Only 25 percent of the food energy is converted to work. Based on your leg power and the efficiency of converting food energy into work, how long would it 2. 1001 Lab 5 3 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK take you, climbing stairs, to burn off a Big Mac (570 kilocalories)? How about a Double Whopper with cheese (1010 kilocalories)? One calorie (not Calorie) equals 4.2 Joules. 3. How does your power for a single flight compare to that for four flights of stairs? Read the following article about the Annual Empire State Building RunUp. Consider the record holders and the 88yearold orchestra conductor. Assume that the men Paul and Chico weigh 170 pounds and that Belinda weighs 130 pounds. Calculate their work and power. Compare to your work and power, and explain the differences. Also consider long distance runners and sprinters. A typical long distance runner has a maximum power of 9 watts/kg body weight, and a typical sprinter has a maximum power of 15 watts/kg body weight. 22nd Annual Empire State Building RunUp February 23, 2000 This aint no party, this aint no fooling around. This is the 23rd annual Empire State Building RunUp, the worlds most elevating workout. Organized by the New York Road Runners Club (NYRRC), the race offers 30 minutes of uphill torture. Starting at 10:30am, beginning in the lobby of the Empire State Building on 34th Street about 150 runners attack flights of stairs in one of the world's most famous landmarks. According to our abacus, that comes to 1576 steps, for an ascent of 1050 feet approximately 1/5 of a mile. The stairs themselves are only 40 inches wide plenty of room for cockroaches, but not much for humans. And the view? There is none, until you reach the top. A ceremony immediately follows on the 86th floor, where the male and female winners each get a trophy. Why run up all those stairs in the first place? Running from New Yorks finest muggers and assailants? Hardly. The real reason lies with Fred Lebow, founder of the NYRRC. Twenty years ago Fred lived in a walkup a type of apartment building without an elevator and he figured that if he walked up stairs every day, why not run a race up them? While the idea might seem simple, it has caught on in a big way. Not only is this the NYRRCs 23rd run, but the concept itself has been appropriated by copycats in Chicago, in Toronto, in Moscow, and in Sydney. If you find yourself inspired to rush over to scale those steps with the other runners, dont bother. You see, this is a special race in a special town. You must apply in writing to the NYRRC for approval and explain in an emotional essay how youve been deprived since birth and why you deserve this type of punishment. If approved, youll be notified. Dont call them, they will call you. Oh yeah, and you will also sign every medical release known to man. So who made the cut? Last year, 150 runners from 9 countries and 12 US states crammed their way up those stairs. Ages ranged from 20 to 87, including an optometrist, a nanny, and an air traffic controller. Watch out for the Australians; they hold the record time for the men (Paul Crake made a new record last year, at 10:15 thats 155.3 steps per minute!) and the women (Belinda Soszyn, 12:19, set in 1996). And look for Chico Scimone, the 88yearold orchestra conductor from Italy, who has run the race several times, logging last year at 34:21. 1001 Lab 5 4 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK Hell be running with some family members this year; and, reportedly, hes in even better health than last year! Not bad for 88. Certainly a lot faster than most of you. WhatsGoingOn.com; Copyright 19962001 Nother Company LLC Note on Energy Use by the Human Body ATP is the primary energy source of cells. As its name would suggest, this compound has three phosphate groups, and energy is stored in these chemical bonds in the same way that electrical energy is stored in a battery. With this energy, your muscles produce a force by contracting. When attached to bones by a series of tendons, a contracting muscle can pull two bones together, using the bones as a lever. If you exercise more than a minute or so, then the energy in the ATP in your muscles gets converted to its lower energy form and has to be restored. The slow step in doing the restoration is getting enough oxygen into your blood which is why you start to breath hard if you run for a while. If you are running hard, use up the ATP and your lungs arent pumping oxygen in fast enough, then your body will switch over to another source, a kind of energy in sugar called glycogen in your blood. The use of glycogen results in an excess of lactic acid in the blood, which can cause cramps and discomfort. The glycogen supply only lasts a few minutes if its being used at the maximum rate. Its used by athletes in sprints and dashes and the lactic acid results in incapacitation for as much as an hour after such events while the glycogen is restored and the lactic acid is cleared from the blood. If you run at a pace which can be sustained for a long time, because the oxygen is supplied fast enough to keep up with its use, then the glycogen is only involved in a minor way and you can run for hours (as in marathons). For most humans the power production under those conditions is around 1/4 horse power. PART 2: TRANSFER OF MECHANICAL TO THERMAL ENERGY THROUGH WORK Thermal energy is central to the operation of many of the energy technologies which we discuss in this course. In most electrical power generation, coal is burned to convert chemical energy to thermal energy which is then converted to mechanical energy in turbines and finally to electrical energy through processes we will discuss later. Most transportation involves conversion of chemical energy to thermal energy which is in turn converted to kinetic energy of the vehicle. So what is thermal energy? Basically, though it took more than a century to establish this, thermal energy is understood to be a form of kinetic and potential energy arising from the random motion of the atoms and molecules in the material, gas or liquid which is heated. Temperature, as measured by a thermometer, is directly related to the amount of kinetic energy in the molecules and atoms. However, because there is also potential energy associated with the forces between the atoms, the temperature alone does not give enough information to calculate the amount of energy added to a material when its temperature rises by a fixed number of degrees. To take account of the added potential energy one needs to know the specific heat c of the material whose temperature is changing. One calculates the added thermal energy through the formula Added thermal energy = (specific heat c) x (change in temperature)x(mass of material) In this experiment, you will turn a crank which causes a cable to rub against an aluminum cylinder. The rubbing causes an increase in the random motion of the aluminum atoms in the metal drum, a rise in its temperature and an increase in its thermal energy. You can calculate the work done by multiplying the magnitude of the force by the distance through which it acts and you can calculate the added thermal energy by use of the above equation, knowing the specific heat of aluminum 1001 Lab 5 5 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK (which is given below) if you measure the temperature rise and the mass of the aluminum. By comparing these two numbers, you can confirm the conservation of energy in this process. Historically this kind of experiment, which was done very early by some of the people who discovered that thermal energy was a form of mechanical energy, was regarded in a somewhat different way, as establishing the relationship between the units of heat (calories) and the units of mechanical energy (joules). It was eventually found that one calorie equals 4.186 joules. PRELAB READING Hinrichs and Kleinback pp97105 EQUIPMENT 1 Mechanical Equivalent of Heat apparatus 1 weight on a rayon string 1 digital multimeter 2 wires with banana plugs PREDICTIONS/PRELIMINARY QUESTIONS You and your partners will do work on the aluminum cylinder by turning a crank. A rayon string will be wrapped around the cylinder several times. When you turn the crank, friction between the cylinder and the string will hold up the weight and will heat the drum. Since you are applying force over a distance, you can calculate the work done on the cylinder. You can also measure the cylinders temperature and will know the specific heat of aluminum, so you can calculate the heat energy produced. The work done depends on the number of times you turn the crank. You will calculate the number of turns needed, in theory, for a specific increase in temperature. You and your partners will compare this theoretical number to your experimental result and consider any differences. 1. Before you do your first trial, you must find the theoretical number of turns of the cylinder needed to raise its temperature by a particular amount. With your lab partners, choose a temperature change of at least 8 degrees Celsius. This will be the temperature increase of the drum that you will use in the socalled heat equation: Added thermal energy = (specific heat c) x (change in temperature)x(mass of material) . The value for the specific heat of aluminum in SI units is 900 joules/kg degrees C. 2. According to the conservation of energy, the amount of heat transferred Q must equal mechanical energy input, or the work you will do on the drum. After you calculate the heat energy, you and your lab partners must consider the work you will do on the drum. Remember that work is a force applied over a distance. The force will equal the pull of gravity on the hanging weight. The distance will equal the drums circumference times the number of times you turn the drum with the crank. In the form of an equation, the work done on the aluminum drum will be: Work gravitational = (mweight g )(Dcylinder )(N ) Where m is the mass of the hanging weight, D is the cylinder diameter, and N is the number of times you turn the crank. With this equation, find the number of turns necessary to increase the heat energy of the drum by the amount you calculated in the first step. 1001 Lab 5 6 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK PROCEDURE With your theoretical number of turns, you are now ready to conduct your trial. Secure the drum apparatus to the table with the clamp. Place your weight on the floor beneath the drum. Wrap the string around the drum two or three times, enough that you hardly need to pull on the string when turning the drum to keep the weight at a constant height off the floor. Practice turning the drum while pulling gently on the string to raise the weight a few centimeters off the floor. As the drum turns and the string slides over it, the friction will heat the drum. It is a very good idea not to place your feet under the weight. It has a mass of over 8 kilograms and is sufficiently heavy to break your toes if it accidentally falls from a height of several centimeters. Also, never let go of the crank when the weight is off the floor or it will spin rapidly and bruise fingers. 1. You will measure the aluminum cylinders temperature with a temperaturedependent resistor cleverly called a thermistor. As the temperature of the aluminum increases, the resistance of the thermistor will decrease. The multimeter will be used to measure the resistance of the thermistor. So, set the multimeter to measure resistance (the symbol for which is ), and attach it to the apparatus with the wires provided. Record the resistance, and use the conversion table on the apparatus to find the initial temperature. If this resistance is not close to one of the temperatures on the table, grasp the drum with your hand to increase its temperature to a more convenient one. To the initial temperature, add the temperature change on which your group decided in the first step. This will be your final temperature. Use the conversion table to ascertain the resistance of the thermistor when it reaches this final temperature. 2. Choose someone to turn the crank (if the counter is not working, this person must also count the number of turns), someone to hold the end of the string, and someone to monitor the resistance. The person who is monitoring the resistance should watch for the thermistors resistance to decrease to that which corresponds to the desired final temperature (use the conversion table printed on the apparatus to decide these values beforehand.) Start turning the handle at a rapid pace. Be certain that the apparatus is properly attached to the table so nothing will fall. The hanging weight should stay a constant height off the floor as you 3. 1001 Lab 5 7 1001 LAB 5: ENERGY TRANSFER THROUGH PERFORMANCE OF WORK turn the handle this way the gravitational potential energy of the hanging weight does not change. It is easy for the cord to start wrapping around the cylinder, and it might take some practice to keep this from happening. 4. When the resistance reaches that of your final temperature, stop turning the crank, and record the number of turns. Then compare the number of actual number of drum turns to the number predicted in the second step. Your instructor may ask you to do a second trial with a different change in temperature. CONCLUSIONS 1. 2. What was the amount of thermal energy transferred to the aluminum cylinder for each trial? Compare the number of actual turns needed to add this amount of thermal energy to the cylinder to the number predicted by your calculations in the second step. a) Did you have to turn the crank more times or fewer than predicted? b) What are possible explanations for differences between your predicted and measured number of turns? c) Is there any source of error or uncertainties in your measurements? Was there any energy lost? How do you know? Are there assumptions in your prediction that account for differences? What could you and your lab partners change to increase the accuracy of the theoretical number of turns? What would you expect if you performed your trials outside instead of indoors? Explain your answers. Using what you have learned about the flow of heat, discuss how the specific heat capacities of aluminum and copper affect the use of these metals in flatplate solar collectors. (Refer to the Historical Notes below. The specific heat of copper is 387 J/kgC) 3. 4. 5. HISTORICAL NOTES Flatplate solar collectors were invented late in the seventeenth century, but it was not until the 1960s that the collectors were developed and used around the world. The most common types in the world are allcopper solar collectors and copperaluminum hybrid collectors. In fact, some indigenous communities in the Amazon use collectors described as an aluminum module with the copper soul to provide power to their villages. A typical flatplate collector is comprised of a rectangular surface, called an absorber, with a series of fluid tubes running along it. It is the absorber that is commonly aluminum, copper, or a composite of both. 1001 Lab 5 8 LAB 6 HEAT AND INSULATION As a child, you were probably told to turn off the lights when you left a room and to turn off the television when you were not watching it. It definitely makes sense to turn lights and appliances off when they are not in use; however, lights, televisions, computers, and other appliances only account between 10% and 30% of the energy used in an average American home. On the other hand, heating and cooling (collectively known as space conditioning) devour between 50% and 70% of the energy used in most homes. Unfortunately, most houses in the United States are not well insulated, and inadequate insulation is a major cause of energy waste. When thermal energy is transferred from your house to the outdoors we say that heat is transferred or that there was a flow of heat. Thus we will reserve the word heat for the cases where thermal energy is being transferred from one place to another. We will not speak of the heat in a body but only of the thermal energy in a body though we say that energy is transferred as heat from one body to another. This is similar to the way we use the word work. We never speak of the work in a body but only of the work done in transferring energy from one body to another. Insulation decreases heat flow between the outdoors and a house interior by providing a resistance to such flow. There are many different types of insulation: fiberglass blankets, loose mineral fibers and fiber pellets, polyurethane foam, fibrous or plastic boards, plastic fiber made mainly from recycled milk bottles, cementbased foam, adobe (clay), natural fibers such as cotton and straw, and more. Different insulation materials have different degrees of thermal resistance, or resistance to heat transfer. You and your partners will examine the thermal resistance of four different insulators in an effort to determine the characteristics of an effective insulator. You will measure the temperature change of hot water in a pop (soda) can that is wrapped in an insulator, and this change will be divided by the thickness of the material in order to compare the effectiveness of the different materials. PRELAB READING Hinrichs and Kleinbach, pp. 106109,133146 EQUIPMENT 5 empty pop cans 5 alcohol thermometers Set of various insulators (bubble wrap, felt, can cozies) 1 digital calipers hot tap water PREDICTIONS/PRELIMINARY QUESTIONS 1. 2. Would a substance with high thermal resistance be considered a good insulator or a poor insulator? a) Rank the following substances according to their effectiveness as an insulator: aluminum, first aid gauze, foam "cozy" for a pop (soda) can, rubber sheet, and bubble wrap. List the best insulator first. (Note: These comparisons are to be made in terms of effectiveness per the same unit of thickness.) b) On what reasoning did you base your decisions about ranking? PROCEDURE As previously mentioned, you and your partners will investigate the thermal resistance of four insulators to establish the characteristics of an effective insulator. You will measure the temperature change of hot water in a can that is wrapped in an insulator, and this will be divided by its thickness to compare the effectiveness of the different insulators. 1001 Lab 6 1 1001 LAB 6: HEAT AND INSULATION 1. 2. 3. 4. 5. Make a table in which to record your groups temperature measurements. You will be measuring the temperatures of the water in five cans each minute for 15 minutes. Determine the mass of each can. Take the five cans to the restroom and fill them with hot water. Be sure each can has the same amount of water almost filled to the top and bring them back to the lab. Determine the mass of each can when filled with water and record it in your notebook. Leave one can unwrapped. Put one can in the "cozy" and wrap the other cans with a layer of the various insulators: bubble wrap, first aid gauze, and rubber sheet. Secure the insulators around the cans with some tape. Tape a thermometer in each can, so that it does not touch either the sides or bottom of the cans and so there is no heat loss through the hole in the can. Record the temperatures of the water in five cans each minute for 15 minutes. While you are taking your temperature measurements, use the calipers to measure the thickness of the insulator around each can. Determine the temperature in the room with another thermometer and record it in your notebook. 6. 7. 8. 9. ANALYSIS For each can for which you have recorded data, graph the temperature minus the room temperture as a function of time. You can put the data for all five cans on the same graph for comparison. Based on your data, rank your materials from "best" insulator to "worst". Is the water cooling at the same rate during the entire experiment for one of the cans? If not, when is it cooling fastest? slowest? Can you explain why it cools faster sometimes than others? (Hint: use the expression for rate of heat transfer by conduction on p. 107 of your book.) Calculate the rate of cooling during the first minute, during the time between the sixth and seventh minute and during the time between the 14th and 15th minute for one of the cans. How much thermal energy left the water in the first minute? How much thermal energy left the water between the 6th and 7th minute? (Use the expression: change in thermal energy=(specific heat)x(mass of water)x(change in temperature)) The specific heat of water is 4186 J/kg/oC .) Now use the expression (p. 133 of your book) rate of cooling =(1/R) (area of can surface)x(temperature room temperature) to calculate the R value of the insulation around this can. (You need to measure the height and diameter of the can to calculate its surface area A. You need to then solve the above equation for R and use A and the rates of cooling that you determined to get R.) Do this both for the first minute and for the the period between the 6th and 7th minute. Are the answers the same? Should they be? Compare the R value with those in table 5.2 of the book. Would this insulation be adequate for a house? 1001 Lab 6 2 1001 LAB 6: HEAT AND INSULATION CONCLUSIONS 1. a) What material was the best insulator? The second best? The worst? The second worst? How does your measured ranking compare to your predicted one? b) Based on your results, what are the properties of a material that you think will make it a good insulator? 2. 3. 4. Why do most houses in Minnesota have double or triplepane windows? Is it the glass that is an efficient insulator, or is something else involved? How can the Inuit construct igloos out of snow and still expect them to remain warm inside? Why do forest animals burrow into snow to find shelter from the winter cold? The following materials (at the stated thicknesses) have the same thermal resistances: 13 cm of polyurethane foam, 58 cm of white pine, 5.5 meters of window glass, and 2.25 km of silver. How does this fit with your results in this exercise? Consider the insulation with which we who live in Minnesota are terribly familiar: winter clothes. a) What kind of clothing do people usually wear in winter? b) What about these winter clothes makes them better insulators than our summer clothes? Relate your answers to the observations in this exercise. 6. What heat transfer mechanism or mechanisms were involved in this exercise? (Radiation? Conduction? Convection?) Draw an illustration to help explain your answer. 5. ADDITIONAL NOTES Research at Oak Ridge National Laboratory, which started in 1943 as a nuclear weapons laboratory, now includes the development of new insulation materials as a part of the Building Envelopes Program. Insulation materials differ and must be chosen to fit several factors: the local climate; the size and construction of the home; the type and efficiency of the heating and cooling systems; and the fuel used. This is because different insulation materials have different degrees of thermal resistance. In construction, the measurement of thermal resistance is called the Rvalue. The greater the Rvalue, the greater the insulation effectiveness. The Rvalue depends on the type of material and its thickness. The Uvalue, which is simply the reciprocal of the Rvalue, is another common measure of thermal resistance. Physicists typically use a quantity called the thermal conductivity to report a material's resistance to heat transfer. 1001 Lab 6 3 1001 LAB 6: HEAT AND INSULATION 1001 Lab 6 4 LAB 7 MECHANICAL FROM THERMAL ENERGY: A SIMPLE THERMAL ENGINE, THE MASS LIFTER Most of the energy which is delivered for use to consumers in our society takes the form of thermal energy at some point before it's used. In automobiles, trucks, buses, airplanes and most railroads, hydrocarbons (storing chemical energy) are burned to produce thermal energy which is then converted back to gravitational potential and kinetic energy of the vehicles. In most electrical power plants (but not nuclear or hydropower ones) hydrocarbons are burned and the resulting thermal energy is converted to mechanical energy in turbines and then to electrical energy which is carried by the power lines to consumers. This week we are considering how the thermal energy is converted to kinetic or gravitational potential energy of large (that is not molecular size) objects in engines. That is the central physical process behind the extraction of energy from fossil fuels discussed in your book and the lectures. The simple engine you will use uses thermal energy to lift objects. A piston that lifts the objects moves up and down inside of a cylinder filled with air. The air inside the cylinder is connected with a tube to a sealed metal can that can be heated and cooled by placing it in hot or cold water. A cycle starts with an object on top of the piston and the can immersed in cold water. The can is then immersed in hot water and the cylinder rises. At its highest point you remove the object and the piston rises some more. Next the can is placed into cold water. The piston descends. When it stops descending, you put an object on it causing it to go lower. The system is now in its original state In this experiment we would like you to verify experimentally that the useful mechanical work done in lifting a mass, m, through a vertical distance, y, is equal to the net thermodynamic work done during a cycle as determined by finding the enclosed area on a P-V diagram. Essentially you are comparing useful mechanical "mgy" work (which we hope you believe in and understand from earlier studies) with the accounting of work in an engine cycle as a function of pressure and volume changes given by the area on a pressure versus volume plot of the cycle. The following figure shows an apple lifted through a height y and the area on a P-V plot corresponding to the work done in a cycle that lifts the apple through the height y. The heat engine we use consists of a hollow cylinder with a graphite piston that can move along the axis of the cylinder with very little friction. The piston has a platform attached to it for lifting masses. A short length of flexible tubing attaches the cylinder to an air chamber (consisting of a small can sealed with a rubber stopper that can be placed alternately in the cold reservoir and the hot reservoir. A diagram of this mass lifter is shown below: 1001 Lab 7 1 1001 LAB 7: MECHANICAL FROM THERMAL ENERGY If the temperature of the air trapped inside the cylinder, hose, and can is increased, then its volume will increase, causing the platform to rise. Thus, you can increase the volume of the trapped air by moving the can from the cold to the hot reservoir. Then, when the apple has been raised through a distance y, it can be removed from the platform. The platform should then rise a bit more as the pressure on the cylinder of gas decreases a bit. Finally, the volume of the gas will decrease when the air chamber is returned to the cold reservoir. This causes the piston to descend to its original position once again. The various stages of the mass lifter cycle are shown in the next Figure. Before taking data on the pressure, air volume, and height of lift with the heat engine, you should set it up and run it through a few cycles to get used to its operation. A good way to start is to fill one container with room temperature water and another with hot tap water or preheated water at about 6070C. The engine cycle is much easier to describe if you begin with the piston resting above the bottom of the cylinder. Thus, we suggest you raise the piston a few centimeters before inserting the rubber stopper firmly in the can. Also, air does leak out of the cylinder slowly. If a large mass is being lifted, the leakage rate increases, so we suggest that you limit the added mass to something between 100 g and 200 g. After observing a few engine cycles, you should be able to describe each of the points a, b, c, and d of a cycle carefully, indicating which of the transitions between points are approximately adiabatic and which are isobaric. You can observe changes in the volume of the gas directly and you can predict how the pressure exerted on the gas by its surroundings ought to change from point to point by using the definition of pressure as force per unit area. Description of the Engine Cycle 1001 Lab 7 2 1001 LAB 7: MECHANICAL FROM THERMAL ENERGY a. Predicted transition ab: Close the system to outside air but leave the can in the cold reservoir. Make sure the rubber stopper is firmly in place in the can. What should happen to the height of the platform when you add a mass? Explain the basis of your prediction. b. Observed transition ab: What happens when you add the mass to the platform? Is this what you predicted? c. Predicted transition bc: What do you expect to happen when you place the can in the hot reservoir ? d. Observed transition bc: Place the can in the hot reservoir and describe what happens to the platform with the added mass on it. Is this what you predicted? (This is the engine power stroke!) e. Predicted transition cd: Continue to hold the can in the hot reservoir and predict what will happen if the added mass that is now lifted is removed from the platform and moved onto an upper conveyor belt. Explain the reasons for your prediction. f. Observed transition cd: Remove the added mass and describe what actually happens. Is this what you predicted? g. Predicted transition da: What do you predict will happen if you now place the can back in the cold reservoir? Explain the reasons for your prediction. h. Observed transition da: Now it's time to complete the cycle by cooling the system down to its original temperature for a minute or two before placing a new mass to be lifted on it. Place the can in the cold reservoir and describe what actually happens to the volume of the trapped air. In particular, how does the volume of the gas actually compare to the original volume of the trapped air at point a at the beginning of the cycle? Is it the same or has some of the air leaked out? i. Theoretically, the pressure of the gas should be the same once you cool the system back to its original temperature. Why? Determining Pressures and Volumes for a Cycle In order to calculate the thermodynamic work done during a cycle of this engine, you will need to be able to plot a P-V diagram for the engine based on determinations of the volumes and pressures of the trapped air in the cylinder, tubing, and can at the points a, b, c, and d in the cycle. Volume and Pressure Equations a. What is the equation for the volume of a cylinder that has an inner diameter of d and a length L? b. Use the definition of pressure to derive the equation for the pressure on a gas being contained by a vertical piston of diameter d if the total mass on the piston including its own mass and any added mass is denoted as M. Hints: (1) What is the definition of pressure? (2) What is the equation needed to calculate the gravitational force on a mass, M, close to the surface of the Earth? (3) Don't forget to add in the atmospheric pressure, Patm, acting on the piston and hence the 1001 Lab 7 3 1001 LAB 7: MECHANICAL FROM THERMAL ENERGY gas at sea level. Now that you have derived the basic equations you need, you should be able to take your engine through another cycle and make the measurements necessary for calculating both the volume and the pressure of the air and determining a P-V diagram for your heat engine. Instead of calculating the pressures, if you have the optional equipment available, you might want to measure the pressures with a barometer or a barometer sensor attached to a computer-based laboratory system. Determining Volume and Pressure a. Take any measurements needed to determine the volume and pressure of air in the system at all four points in the engine cycle. You should do this rapidly to avoid air leakages around the piston and summarize the measurements with units in the space below. b. Next you can use your measurements to calculate the pressure and volume of the system at point a. Show your equations and calculations in the space below and summarize your results with units. Don't forget to take the volume of the air in the tubing and can into account! P a= V a= c. Use the measurements at point b to calculate the total volume and pressure of the air in the system at that point in the cycle. Show your equations and calculations in the space below and summarize your results with units. P b= V b= d. What is the height, y, through which the added mass is lifted in the transition from b to c? e. Use the measurements at point c to calculate the total volume and pressure of the air in the system at that point in the cycle. Show your equations and calculations in the following space and summarize your results with units. P c= V c= f. Remove the added mass and make any measurements needed to calculate the volume and pressure of air in the system at point d in the cycle. Show your equations and calculations in the space below and summarize your results with units. P d= V d= g. We suspect that transitions from ab and from cd are approximately "adiabatic". Explain why. 1001 Lab 7 4 1001 LAB 7: MECHANICAL FROM THERMAL ENERGY h. You should have found that the transitions from bc and from da are "isobaric". Explain why this is the case. Finding Thermodynamic Work from the Diagram In the next activity you should draw a P- V diagram for your cycle and determine the thermodynamic work for your engine. Plotting and Interpreting a P-V Diagram a. Fill in the appropriate numbers on the scale on the graph frame that follows and plot the P-V diagram for your engine cycle. Alternatively, generate your own graph using a computer graphing routine and affix the result in the space below. b. On the graph in part a, label each of the points on the cycle (a, b, c, and d). Indicate on the graph which of the transitions (ab, bc, etc.) are adiabatic and which are isobaric. Next you need to find a way to determine the area enclosed by the P- V diagram. The enclosed area doesn't change very much if you assume that P is approximately a linear function of V for the adiabatic transitions. By making this approximation, the figure is almost a parallelogram so you can obtain the enclosed area using one of several methods. Three of the many possibilities are listed below. Creative students have come up with even better methods than these, so you should think about your method of analysis carefully. Method I Since the pressure doesn't change from point b to point c, you can take the pressure of those two points as a constant pressure between points. The same holds for the transition from d to a. This gives you a figure that is approximately a parallelogram with two sets of parallel sides. You can look up and properly apply the appropriate equation to determine the net thermodynamic work performed. Method II Display your graph with a grid and count the boxes in the area enclosed by the lines connecting points a, b, c, and d. Then multiply by the number of joules each box represents. You will need 1001 Lab 7 5 1001 LAB 7: MECHANICAL FROM THERMAL ENERGY to make careful estimates of fractions of a box when a "leg" of a cycle cuts through a box. Comparing the Thermodynamic and Useful Mechanical Work a. Choose a method for computing the thermodynamic work in joules, describe it in the space below, and show the necessary calculations. Report the result in joules. b. What is the equation you need to use to calculate the useful mechanical work done in lifting the mass from one level to another? c. Use the result for the height that the mass is lifted in the power stroke of the engine to calculate the useful mechanical work performed by the heat engine. d. How does the thermodynamic work compare to the useful mechanical work? Please use the correct number of significant figures in your comparison (as you have been doing all along, right?) Caveat: This Mass Lifter Engine Is Not So Simple Understanding the stages of the engine cycle on a P-V diagram is reasonably straightforward. However, it is difficult to use equations for adiabatic expansion and compression and the ideal gas law to determine the temperature (and hence the internal energy of the air throughout the cycle. There are several reasons for this. First, air is not an ideal gas. Second, the mass lifter engine is not well insulated and so the air that is warmed in the hot reservoir transfers heat energy through the cylinder walls. Thus, the air in the can and in the cylinder are probably not at the same temperature. Third, air does leak out around the piston, especially when larger masses are added to the platform. This means that the number of moles of air decreases over time. You can observe this by noting that in the transition from point d to point a, the piston can actually end up in a lower position than it had at the beginning of the previous cycle. However, this mass lifter engine does help us understand typical stages of operation of a real heat engine, including the fact that difficult to eliminate problems reduce the efficiency of a heat engine below the thermodynamic ideal. 1001 Lab 7 6 LAB 8 OHM'S LAW AND CIRCUIT EXPLORATION Toasters, televisions, computers, and flashlights all involve electric circuits. In a flashlight, for example, charge flows from a battery through a wire to a bulb. When the charge moves through the filament of the bulb, some energy is transferred to the filament. As a result, the filament heats up and radiates energy as light. The rate of charge flow is called current. One can only have charge flow (current) for an extended period of time if the charge has a closed circuit, or path to flow around. This is because electrical charge can never be created or destroyed. If the charge starts to flow and there is not a closed circuit, then it starts to build up somewhere, where the path it is following ends. The pileup of charge is accompanied by a buildup of stored energy and eventually the charge escapes by arcing, often destructively. The fact that charge never disappears in nature is another of the great conservation laws of physics. A closed circuit involves charges leaving the positive terminal of a battery, passing through the wires or light bulbs, and returning to the negative terminal of the battery. The bulb acts as a resistor because it resists the flow of charges. The bulbs resistance causes the filament inside to increase in temperature and start to glow. Your battery separates positive charges from negative charges within its structure. The work that is done to separate these charges increases their electrostatic potential energy, and a voltage difference is created. The simplest circuit involves a resistor and battery connected in a single current path. When the batterys voltage is increased and the resistance is held constant, the current increases. If the voltage is held constant and the resistance is decreased, the current should again increase because the flow of charges is not resisted as much as before. These relationships can be written as V = IR, in which V is voltage, I is current, and R is resistance. This is Ohms Law. It holds for the resistors you will study in this laboratory, but it not for all possible circuit elements. In this exercise, you and your lab partners will investigate Ohms Law by measuring voltage, current, and resistance in a simple closed circuit. PRELAB READING Hinrichs and Kleinbach pp 330339 EQUIPMENT 1 sixvolt battery 2 digital multimeters 1 light bulb 3 resistors 1 set of wires PREDICTIONS/PRELIMINARY QUESTIONS V 1 1 2 2 44 resistor 3 3 Figure 1 1001 Lab 8 1 1001 LAB 8: OHMS LAW AND CIRCUIT EXPLORATION 1. Suppose a voltmeter were connected across a battery to measure the voltage from point 1 to point 2. (See figure 1.) Suppose you moved the voltmeter and connected it from point 3 to point 4. Compare the voltage reading at the new location with the reading at the first location. Explain your reasoning. Compare the current reading, in amps, on an ammeter connected between point 2 and point 3 with the reading on the ammeter if it were connected between points 1 and 4. (The ammeter is connected between the points so all the current runs through it. See figure 2.) Explain your reasoning. If you were to double the voltage (for example, using two batteries) what would happen to the amount of current in the circuit if the resistance in the circuit stayed the same? (See figure 2) If the resistor in the circuit were replaced with a resistor with twice the resistance, what would happen to the current in the circuit assuming you used the same battery with the same voltage? 2. 3. 4. battery 1 2 A DMM set to amps 4 Figure 2 3 PROCEDURE The first part of this exercise is to become familiarized with two quantities: voltage (known also as potential difference; measured in volts) and current (measured in amps). Voltage is the work done transferring one coulomb of charge from one place to another; you do not need a complete circuit to measure a potential difference. The digital multimeter (DMM) has various settings to measure voltage. It also allows you to measure the current (Amps) by turning the switch to the amps or milliamps settings. Furthermore, you can measure resistance by turning the switch to the ohms settings. See Appendix I for a more thorough description of how to use the DMM. When you set up a circuit in the lab with the necessary wires, meters, and resistors, etc. make the circuit look as simple as possible. Do not let the wires cross unless necessary. Keep the wires as straight as possible and as short as possible. Make the circuit so you can easily tell which objects are connected to each other. 1. First, set the digital multimeter to measure volts and connect it across the terminals of the battery. Observe what happens as you change the range settings on the meter. Observe what happens when you reverse the connection. The meter adds a minus sign if the potential difference is reversed and indicates (by a blank screen) if the potential difference is too large to be 1001 Lab 8 2 1001 LAB 8: OHMS LAW AND CIRCUIT EXPLORATION measured. Choose a range where the most digits are displayed so that you will have the greatest precision in your measurements. 2. You can leave the multimeter connected across the battery terminals, but to measure the current through the circuit another meter must be connected in series with the resistor and the battery. Connect a complete circuit from the positive terminal of the battery, through a second multimeter (set to measure the current through the resistor), and back to the battery. The second multimeter is now able to measure current. Record the current and the voltage across the battery. 3. Disconnect the circuit. Set a multimeter to measure resistance, and connect it across the terminals of the resistor. Record the value of your resistor in ohms. Why must you disconnect the circuit before trying to measure the resistance of the resistor? 4. Repeat these steps several times, using a different resistor in the circuit each time. Also take measurements using a light bulb as the resistor. Be sure to take measurements of the resistance, voltage, and current for each circuit. ANALYSIS Record the values that you measured using the multimeter in a data table, similar to the one shown below. Also calculate theoretical resistances using your measurements for voltage and current (R=V/I) and then record your calculations. Tabulate your experimental and theoretical resistances for all the resistors and the bulb. Explain the values that you have obtained. Measured Voltage Measured Current Measured Calculated (in Volts) (in Amps) Resistance Resistance (in Ohms) (R = V/I) Resistor 1 Resistor 2 Resistor 3 ... Light Bulb CONCLUSIONS 1. Compare the measured and calculated resistance values in your data table. How close are they? If there are differences, what are some possible reasons for these differences? The German physicist Georg Ohm showed that, when the voltage across a resistor changes, the current through it changes. He expressed this relationship as: I = V R Current is directly proportional to voltage and inversely proportional to resistance. In other words, as the voltage increases, so does the current. Ohm's Law also means that current is inversely proportional to resistance and voltage is directly proportional to resistance. 1001 Lab 8 3 1001 LAB 8: OHMS LAW AND CIRCUIT EXPLORATION 2. Superconductors are materials which, when cooled sufficiently, offer almost no resistance to electrical current. The copper wires in your circuits do, however, add to the total resistance. a) What if the wires used in the circuit with the light bulb were replaced with superconducting wires and the resistance became almost nothing? Would the current in the circuit change, or the does voltage change, or both? b) What would happen to the light bulb? Explain your answer using Ohm's law. 1001 Lab 8 4 LAB 9 MAGNETS AND MOVING CHARGES The interaction of magnetism and electric charges underlies how motors and generators function. Motors and generators look very similar inside and are mechanically very similar. They work by closely related, but different principles. In this laboratory you will study the principle by which motors work in Part 1 and the principle by which generators work in part 2. Magnetic fields arise from magnetic materials and also from moving charges in the currents running in circuits or beams of electrically charged particles. The picture in most television sets is made by such beams (of electrons) which move from the back of the picture tube to the screen where they hit the screen and excite phosphors which make the light which you see. (Actually the magnetic fields arising from magnets can also be regarded as arising from moving microscopic currents inside the material.) If one has a magnetic field produced by one of these means (a magnet, a current carrying wire (often wound in a coil) or a charged beam) then if other moving electrical charges move through this magnetic field, the moving charges will experience a force. This force is what makes the rotor of an electric motor go around. To get an idea how that works you should study Figure 11.7 of your book. The rotor (the thing that goes round and round ) in the motor has a wire in it that carries a current. The rotor moves in a magnetic field provided by two pole magnets labeled N and S in the figure. Because the charges in the wire in the rotor are moving, they feel a force which pulls on the rotor and makes it turn. What is strange about this force is its direction: It is not in the direction of the magnetic field and it is not in the direction of the velocity of the moving charges but is perpendicular to both those directions as illustrated in Figure 11.4 of the book. In part 1 of this laboratory, you will do some experiments using a magnet and the currents in a cathode ray tube (a simplified TV tube) to help you to understand this peculiar directional aspect of magnetic forces better. Whereas in a motor, electrical energy is being converted into mechanical energy through the action of the forces arising from the motion of charges through magnetic fields, in a generator, mechanical energy is turned into electrical energy. That is possible because of a different principle illustrated in Figure 11.10 of the book, Notice that Figures 11.10 and 11.7 look very similar. You should make sure you understand how they are different. In 11.10, illustrating a generator, the wire, which does not have a current being forced through it by an external battery, is being mechanically forced to turn by forces exerted on some kind of a handle on the shaft. As a result, the magnetic field through the coil is changing. The new principle is that a changing magnetic field through a coil will cause a voltage in the coil to develop and then a current will flow. (This principle is a form of what physicists call Faradays law.) In part 2 of this laboratory you will explore this principle by moving a magnet around a coil of wire and observing the resulting currents. Then you will construct and measure aspects of the operation of a simple generator, operating on the same principles as the one sketched in Figure 11.10, but different in some details. PRELAB READING Hinrichs and Kleinbach pp 359372 PART 1. THE PRINCIPLE BY WHICH MOTORS WORK: THE EFFECT OF A MAGNETIC FIELD ON A MOVING CHARGE EQUIPMENT 1 cathode ray tube (CRT) 1 highvoltage (CENCO) power supply 3 bar magnets 1 compass PREDICTIONS/PRELIMINARY QUESTIONS 1001 Lab 9 1 1001 LAB 9: MAGNETS AND MOVING CHARGES 1. If a beam of positively charged particles moves from right to left as shown in the figure below what would be the effect of a bar magnet on the beam if it were located as shown in the diagram? (Would the beam move up? down? Out of the paper? Into the paper? Or, would there be no effect?) 2. S N Beam of + charges If the beam of particles, in the above diagram, were electrons (negative charges) what would be the effect of the magnet on the beam? (Would the beam move up? Down? Out of the paper? Into the paper? Or, would there be no effect?) If a beam of positive charges were coming out of the paper what would be the effect of the magnet on the beam? (The black dot represents the beam coming out of the paper.) Would the beam move up? down? Out of the paper? Into the paper? Left? Right? Or, would there be no effect?) 3. N S PROCEDURE A cathode ray tube (CRT) produces an electron beam that can be seen as a glowing spot when it hits a phosphorescent surface inside the tube. Holding a magnet close to the CRT produces a magnetic field in the tube and causes the electron beam to deflect, which will make the spot on the phosphorescent screen move. The lab instructor will show you how to connect the CRT to the high voltage power supply properly. You and your partners will work with equipment that produces large voltages, and improper use can cause injury. To avoid danger, the power should be off and you should wait at least one minute before any wires are connected to or disconnected from the power supply. Do not grasp a wire by its metal ends. 1. How do you think the electron beam will be deflected in the presence of a magnetic field? Record your predictions in your notebook. 2. Connect the CRT and the power supply. Consult with your instructor about the ideal accelerating voltage with which to start the exercise. Record this value. Also record the location of the nondeflected beam spot on the screen. 3. Use your compass to determine which pole on your bar magnet is the north magnetic pole. Below is the magnetic field that is produced by a bar magnet: 1001 Lab 9 2 1001 LAB 9: MAGNETS AND MOVING CHARGES Based on this, describe what the magnetic field will be like within the CRT. 4. Place the magnet at the side of the CRT. In what direction does the beam deflect? Does the deflection of the beam match your prediction? Why or why not? Repeat the procedure for the south pole. Should there be a difference? Why or why not? 5. If you position a bar magnet at the front of the screen of the CRT, do you see any deflection? What does the right hand rule predict? Try this with both poles of the bar magnet. What are the results? Are they what you expected? Can you orient the bar magnet so that it attracts or repels the electron beam? 6. Place the north pole of your magnet a fixed distance from the side of the CRT screen. Record the initial deflection of the beam. Increase the electrons speed by increasing the accelerating voltage as much as possible. CAUTION: Always turn off the power supply and wait a 10 seconds before removing and reconnecting wires, or you could get a dangerous shock! Never grasp a wire by its metal end. How much does the deflection change? Try with both poles of the magnet. Were the results what you anticipated? 7. Place the north pole of the bar magnet a fixed distance from the side of the CRT near the screen. Record the initial deflection. Increase the magnetic field by adding more magnets. Also try the south poles. Was the result what you would expect? 8. What effect does Earths magnetic field have on the CRT electron beam? What is the direction of Earths magnetic field in your lab room? Based on your earlier findings, orient the CRT to see the maximum effect. Also orient it for the minimum effect. Is there also an effect on the beam from Earths gravitational field? CONCLUSIONS 1. When you bring the north end of a bar magnet near the side of the CRT: ? ? ? ? N ? ? ? ? 1001 Lab 9 3 1001 LAB 9: MAGNETS AND MOVING CHARGES a) in what direction is the electron beam deflected? b) Does the beam deflection increase or decrease as your magnet is moved closer to the CRT? c) Does the beam deflection increase or decrease as the magnetic field strength increases? How can you tell? d) Is the direction different for the south end of the magnet? 2. The electron beam in a television is steered with magnetic fields. a) What is the direction of the field that pushes the electrons up and down? (Draw a sketch and use words to explain it.) b) What is the direction of the field that pushes the electrons left and right? (Draw a sketch and use words to explain it.) In the future, the use of superconductors will allow construction of smaller, lighter, and more efficient generators and motors. In a superconductor, there is no resistance to the flow of electrons. This has two effects: (1) no energy is lost in the form of heat, and (2) the velocity of the electrons would be greater. a) Does the deflection of an electron beam in a magnetic field depend on the speed of the electrons? b) Discuss what effects a higher electron velocity in the wires of a generator and a motor would have. 3. PART 2. THE PRINCIPLE BY WHICH GENERATORS WORK: THE EFFECT OF A CHANGING MAGNETIC FIELD ON THE VOLTAGE IN A COIL In the previous part of this laboratory you studied the way in which forces are exerted on moving charges, either in a beam or in a wire, when they move through a magnetic field. That is the principle by which motors work. (The magnetic force is sometimes called the Lorentz force.) In this part of the laboratory you will study how a changing magnetic field can cause the charges in a wire which has no current flowing in it to start moving, thus producing a current, as a result of the changing magnetic field. This principle is a version of Faradays law. In generators it is used to produce a current by mechanically forcing a wire coil to move through a stationary magnetic field (as in Figure 11.10 of your book) or by magnetically causing a magnet to move in and out of a coil (as in the simple generator you will build in the laboratory.) In the first part of the exercise, you and your lab partners will investigate the currents and voltages which are induced in wire coils when the magnetic fields around them change in time. Then you will construct a simple generator using which uses Faradays law to convert mechanical to electrical energy. EQUIPMENT 1 long wire solenoid 1 short wire solenoid 1 digital multimeter 3 neodymium magnets 1 roll of masking tape 1 pencil or wood dowel 1 compass PREDICTIONS/PRELIMINARY QUESTIONS 1001 Lab 9 4 1001 LAB 9: MAGNETS AND MOVING CHARGES 1. 2. What, if anything, happens to the charges inside a coil of wire when a magnet moves through the coil? What happens to the charges if the magnet is close to the coil but not moving? Does the same thing happen to the charges if you move the North end of a magnet toward a coil of wire as when you move the South end toward the coil? Explain your reasoning. Does the speed at which the magnet moves make a difference? What happens to charges in the wire if you hold the permanent magnet stationary and move the coil? 3. PROCEDURE In this exercise, you and your partners will use a permanent magnet to produce changing magnetic fields. This causes an electric field to be produced, which you will detect using a solenoid (a spool of wire) connected to a multimeter. The electric field drives electrons around the solenoid, producing a current that can be detected by the multimeter. Consult Appendix I at the back of this manual for information about the use of your multimeter. If you still have difficulties, ask your instructor for assistance. 1. Use a compass to determine which ends of the magnets are north and which are south. Keep in mind a compass needle is a lightweight magnet balanced on a pivot point, but the end labeled as north points to Earth's magnetic north pole. 2. Start with the longer wire solenoid it has more loops of wire than the other one. Do you detect any current with your digital multimeter when a magnet is simply placed at rest inside the solenoid? Try the lowest setting to measure current. Do you detect any current when you move the magnet or wiggle it inside the solenoid or near it? What happens if you change the speed at which you move the magnet? If you reverse the direction of the magnets poles, what happens? What happens if you hold the magnet still and move the solenoid? What happens if you change the speed at which you move the solenoid? What happens if you reverse the direction of the magnets poles? What happens when you repeat the above steps with second magnets? With three? Now you and your partners will construct a generator. Tape a magnet securely to the end of a pencil so that its looks like the head of a hammer. Mark the north and south poles of the magnet (if not marked already) for later reference. Rotate the pencil near the solenoid and observe the induced current with the digital multimeter. Change the orientation of the pencil and note its effect on the current. Which orientation induces the largest current? The smallest current? If you change the speed at which you rotate the pencil near the solenoid, does it have any effect on the amplitude of the induced current? Use the second wire solenoid with the same diameter but fewer loops. Compare the strength of the induced current in the two solenoids. How does the current produced by your generator change with the distance of the bar magnet? 3. 4. 5. 6. 7. 8. 9. 1001 Lab 9 5 1001 LAB 9: MAGNETS AND MOVING CHARGES CONCLUSIONS 1. 2. What factors affect the induced current in the wires? In this exercise, you and your partners used a magnetic field to induce a current. In the previous exercise, you and your partners used a current to produce a magnetic field. How do the factors that affected the magnetic field in the previous exercise compare to the factors in this exercise? Based on your experience in this exercise and the previous one: a. describe how an electric generator works. (What basic components are needed? What needs to happen with these components?) b. Describe how this happens for each of the following sources of electricity: solar, geothermal, hydroelectric, tidal, wind, and nuclear power. Based on what you know about generators and motors, how could you convert your generator in this exercise into a motor? 3. 4. 1001 Lab 9 6 LAB 10 HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY Note: This exercise must be done on a clear day and/or during a 20minuteperiod when there are no clouds. Almost all of the Earth's available energy comes from the sun (geothermal and nuclear energy do not). Fossil fuels such as oil and coal are a result of ancient photosynthesis. Renewable biomass fuel, as well as our food supply, is also a product of photosynthesis. Wind and waves are driven by heat from the sun, and solar panels directly collect the sun's radiant energy. Consequently, the planet's available energy and our food supply are both restricted by the amount of energy received from the sun. Humans and other animals depend on the plant kingdom for food, which is completely dependent on solar radiation. In this exercise you and your partners will determine how much radiant energy the earth receives from the sun at the earths surface. From this and some additional information you will be able to draw a lot of interesting conclusions such as how much of this energy is used in photosynthesis, how much of total solar energy is used by plants humans can eat; how many people the earth could support if all we did was feed them, whether there is enough solar energy to run our economy in the United States, and how much of the solar energy coming from the sun gets all the way to the earths surface. To measure the energy coming from the sun you and your partners will determine the amount of the suns energy that is absorbed by a quantity of water or copper, which will increase in temperature as the sun heats it. PRELAB READING Hinrichs and Kleinbach 161170 EQUIPMENT 1 parabolic mirror setup 1 digital thermometer 1 triplebeam balance 1 styrofoam cup 1 stopwatch Alternate equipment 1 rectangular box with Aluminum metal plate and leads to connect to a DMM 1 DMM (digital multimeter) PREDICTIONS/PRELIMINARY QUESTIONS 1. To determine how much solar energy reaches the earth's surface you can let sunlight heat a substance such as water or metal, then calculate how much heat has been gained by the substance and from that obtain an estimate of the amount of solar energy reaching the earth. Suppose the substance you are heating is water. a) What would you need to measure to be able to determine the radiant heat absorbed by the water? b) How would you calculate the amount of heat gained? a) To determine the power supplied by the sun what must you measure in addition to the heat gained? b) After measuring the quantity needed in part a) how do you calculate the power supplied by the sun? a) To determine the power supplied by the sun per square meter what must you measure? b) After measuring the quantity in part a) how do you calculate the power per square meter? Once you know the power supplied by the sun per square meter how would you calculate the power supplied to the whole earth? 2. 3. 4. 1001 Lab 10 1 1001 LAB 10: HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY PROCEDURE In this exercise, you will put a parabolic mirror in the sun. It will focus the sun's rays on a container of water or an piece of copper that lies at the focal point of the mirror. Refer to the Historical Notes at the end of this lab to learn more about the use of parabolic mirrors in the past. You will use the focused rays to heat water or copper, much as a solar power plant uses mirrors to heat water in order to drive a turbine. Based on the increase in the heat energy of the water or metal , you will find how much energy Earth receives from the sun. From this calculation, you and your partners can estimate how much of this energy is used in photosynthesis; how much of that is used by plants we can eat; and how large a human population Earth can support. 1. Determine the crosssectional area of the parabolic mirror. In other words, if you look at the mirror headon, what is the apparent area of the circle you see? If you have difficulties with this, ask your instructor for assistance. 2. If you are using a water sample, fill the receiver at the focal point with water. The temperature of the water should be close to the temperature outside. You and your partners should determine the mass of this water using a styrofoam cup and the triplebeam balance. Find a location outside that has direct sunlight and little or no wind. Try to stay away from buildings or walls because they can reflect additional sunlight toward the mirror. If the wind picks up, shield the setup as best as you can, but don't block any sunlight! Be sure to aim your parabolic mirror directly at the sun. Be very careful about this. All members of your group should check the container the one with water or a metal plate to be certain it is pointing directly at the sun. You will probably have to adjust the rectangular container (if you are using that) during the time it is receiving solar energy. (The parabolic mirror or its container might also need adjustment during collection time.) Also be sure that you shield the mirror from the sunlight until you start the stopwatch. Record the initial temperature of the water or metal. Record the temperature once every three minutes for 15 minutes (or, if you are using water, until the water starts to boil). Be careful the water, metal and receiver might become very hot. After your last measurement, make sure you have recorded the mass of the water or copper. If you were using water, pour it out. Gather your equipment and return to the lab. 3. 4. 5. Alternate equipment: A rectangular box containing an aluminum plate to absorb solar energy can be used to collect solar energy. The principles are the same as when using water. It works best to cool the plate 10oC to 20oC below the outside temperature before starting. (Be certain the plate is dry before you start using it to collect solar energy.) Heat the box until it is 10oC to 20oC above the outside temperature. You need to align the container so the sun's rays are perpendicular to it. (You can tell the rays are perpendicular when the white plastic peg in the middle casts no shadow.) The temperature can be obtained by connecting leads from the terminals on the side of the box to an ohmmeter. Ask your TA for a conversion chart to change resistance into temperature. The rest of the process uses the same principles as used with the water. ANALYSIS The solar input is the amount of power that an area receives from the sun when directly overhead. Using your measurements, you and your partners can determine the heat energy absorbed by the water or copper based on the temperature increase. We will assume the receiver on the parabolic mirror is a perfect insulator, meaning that it gains no heat from nor loses any to the environment. Computation of the solar input involves solving an equation that equates the thermal energy gained by the copper or water equal to the radiant energy received from the sun. 1001 Lab 10 2 1001 LAB 10: HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY 1. You and your partners should have already calculated the crosssectional area of your parabolic mirror. As stated previously, the solar input is the amount of power that an area receives when the sun is directly overhead. When the sun is at an angle (which is always true in Minnesota and all places outside the Tropics of Capricorn and Cancer), an area receives a fraction of the full energy. Since you aimed the mirror directly at the sun, we do not need to make a correction for this. Next, you and your lab partners should determine the thermal energy gained by the water or copper due to the solar input. Use the equation: 2. thermal energy transferred=(mass of water or copper)x(specific heat of water or copper)x change in temperature The change in the temperature is the final temperature of the water or copper minus the initial temperature of the water or copper. The specific heat of water in SI units is 4186 Joules/kg degrees C. The specific heat of copper is 387 Joules/kgC Be sure to use SI units in your calculations. 3. You and your partners can now determine the value of the solar input. As mentioned already, you can find the solar input by setting the thermal energy gained by the water or copper equal to the radiant energy which came in from the sun: Energy received from sun = thermal energy gained by the water or copper Remember, however, that the solar input is a measure of the sun's power, not energy. Since power is energy over time, we should change our equation to: Power from the sun = thermal energy added to water or copper Time in the sun 4. Power from the sun for the mirror: Power from the Sun Area receiving sunlight (in square meters) thermal energy added to water or copper mc T = = (Mirror Cross - sectional Area ) (Time ) (Mirror Cross - sectional Area ) (Time in Sun Solar input = The solar input is the amount of power that an area receives from the sun when it is directly over that area. If you used only SI units, your result for the solar input is the amount of power (in watts) received by one square meter. 5. To determine the total solar power received by Earth, you and your partners should multiply your result for the solar input by Earth's crosssectional area. (Use the radius of Earth, 6.38 x 106 m, to calculate its crosssectional area.) ) solar power received by Earth = (S olar Input ) (Earth' s cross - sectional Area ) 6. Determine the solar power used in photosynthesis. It has been estimated that about 0.0757% of the solar power that reaches the Earth is captured by plants and utilized in the photosynthesis process. Knowing this, how much solar power is used by plants to make their food? 1001 Lab 10 3 1001 LAB 10: HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY 7. Now you can estimate the amount of food could be produced with this much sunlight, if it were all used to produce food. In The Hungry Planet, George Borgstrom, a professor of Food Science and Geography at Michigan State University, states that 10% of the solar power used by plants in photosynthesis can be used by humans as food. Determine how many watts of power this equals. 8. Knowing that one watt equals 0.24 cal/sec and one food Calorie equals 1000 calories, calculate how many food Calories could be produced on the Earth in one day. 9. Lastly, estimate the maximum population of Earth. The current opinion is that the normal food requirement of a fullgrown person is about 3,000 food Calories per day. Since most humans eat animal products as well as plants, their intake includes the energy needed to feed the animal products they consume. So, although the normal food requirement is about 3,000 food Calories per day, this figure increases to about 10,000 food Calories a day when the conversion of plants to animal products is taken into account. If you use a 10,000Calorie intake as a standard, what is the theoretical maximum number of humans that Earth can support? 10. The amount of solar radiant energy reaching the top of the earths atmosphere from space was first measured by the first space satellites in the middle of the last century and is about 1360 W/m^2. How does this number compare with the radiant power per meter squared which you measured? Do you expect it to be bigger or smaller? Explain the factors which would lead it to be different. 11. In the US economy, you can deduce from Figure 1.15 of your book that energy usage is about 10 kilowatts per person on average. If the solar energy were coming in to the surface of the earth at the rate which you measured, assuming that the US population is 300 million, and assuming that 1/5 of the solar energy could be converted to an easily used form such as electrical energy, how much area of the surface of the US would be needed to collect enough solar energy to power the economy? CONCLUSIONS 1. In the experiment, you assumed that all the energy in the sunlight which was incident on the mirror was transformed into thermal energy in the water or copper. What effects might make this assumption invalid? Which of them is the most important? Can you make any quantitative estimate of how big they might be and how much they could affect the result? Do you expect these effects will make your result for the energy coming in from the sun too big or too small? How could you alter the experiment to make the effects less important? 1. How does your result compare with the current population of Earth, approximately six billion? Based on current rates of growth, the global population has a doubling time of about 40 years. What are the implications of that figure based on your results? In how many years will the number of humans reach your maximum population estimate? 2. What if all humans became vegetarian? Go back to the final step of your analysis, and assume that all our food came from photosynthesis. This would mean that each person would need about 3,000 food Calories per day, not 10,000 Calories. If everyone was a vegetarian, how many people could inhabit the planet? How does this compare to your previous result? In about how many years will the population reach your new estimate? 3. Check the website http://www.oksolar.com/images/daily_solar_radiation1.gif about energy from the sun. It discusses the sun input and shows a map of the minimum solar energy received in the United States per day. How does your result compare with the website 1001 Lab 10 4 1001 LAB 10: HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY values? Redo steps 5 through 9 using the numbers form the website. What are your new answers? HISTORICAL NOTES The use of curved mirrors to heat water or some object dates back millennia. In fact, the ancient Greeks, Romans, and Chinese fashioned mirrors that could focus the sun's energy onto an object and cause it to catch fire in mere seconds. Parabolic mirrors, like the one you and your lab partners used, focus the sun's rays to a very small point. This was discovered by the mathematician Dositheius, who built the first parabolic mirror in the third century B.C. Plutarch wrote that, when the temple at Delphi was sacked and the sacred flame extinguished, it was relit with the "pure and unpolluted flame from the sun" using "concave vessels of brass." The ancient Chinese used concave mirrors for similar purposes a book of ceremonies states that "Directors of the Sun Fire have the duty of receiving, with a concave mirror, brilliant fire from the sun...to prepare brilliant torches for sacrifice." Later, it was imagined that large parabolic mirrors could be used as weapons. One apparently apocryphal legend has it that Archimedes used parabolic mirrors to set fire to the vessels of Romans at Syracuse in 212 B.C. In the thirteenth century, Roger Bacon, a monk, suggested that a parabolic mirror could be used as a weapon of mass destruction in the crusades. In the seventeenth century, Giambattista Porta imagined a mirror that so destructive that it would "cast forth terrible fire" and set ablaze "the enemy's ships, gates, bridges, and the like. A parabolic mirror uses the sun's rays to boil a kettle of water in Tibet. 1001 Lab 10 5 1001 LAB 10: HEAT, THE SOLAR INPUT, AND EARTH'S AVAILABLE ENERGY 1001 Lab 10 6 Lab 11 GREENHOUSE EFFECT Everyday the Earth receives energy from the Sun. On a daily basis the Earth reflects about 1/4th of the incoming solar radiation. The remainder is absorbed and contributes to warming the land, atmosphere and oceans. For the Earths temperature to be steady and not rapidly heating or cooling, this absorbed energy must be very nearly balanced by energy radiated back to space in the infrared wavelengths. Luckily the amount radiated out in as infrared energy increases with the temperature of the earth, allowing us to reach steady states without over heating. The average temperature of the earths surface, and our climate, is therefore determined by how effectively the infrared radiation coming from the warm surface of the earth can be reradiated into space. The reason that scientists are concerned about global warming at this time is because some human activities are making this re radiation less effective. These human activities release molecules into the atmosphere which cause the radiated infrared radiation to reflect back down to the earths surface, rather than escaping into space. Thus the atmosphere acts like the window on the ceiling of a greenhouse, which also causes infrared radiation coming from the plants inside to be trapped inside the greenhouse, keeping it warm in (mild) winters. So the effect of certain gases in the atmosphere which trap infrared radiation and cause the earths surface to be warmer than it would otherwise be is called the greenhouse effect. Greenhouse gases are molecules such as carbon dioxide and water, which absorb and reradiate infrared radiation as the atoms in them vibrate against each other. The human activity which is having the biggest effect on this balance is the burning of fossil fuels which is releasing much more carbon dioxide into the atmosphere than was released in preindustrial times. Measurements confirm that the amount of carbon dioxide in the atmosphere is steadily rising and that the average temperature of the earths surface is also rising. Because the climate is complicated and because other things can also cause the temperature to change there has been some discussion about whether the temperature rise is caused by the carbon dioxide increase. However in the last few years as a result of many measurements as well as extensive computational modeling, a very wide scientific consensus has emerged which attributes much of the observed temperature increase to the human activities leading to increased carbon dioxide in the atmosphere. In the laboratory this week you will carry out an experiment in a model earth consisting of glass case (an empty aquarium) in which you measure the temperature at the bottom surface of the tank when it is heated with visible light (from a lamp, modeling the sun) first when there is no carbon dioxide in the tank, and again when the tank is filled with carbon dioxide. Trial 1: Aquarium Trial 2: Aquarium with CO2 Trial 3: Aquarium with black paper on bottom Trial 4: Aquarium with CO2 and black paper on bottom Trial 5: complex system (8oz. of coke in bottom) EQUIPMENT 5 gallon aquariums Dry ice for CO2 Black Paper cut to fit bottom of aquarium Heat Lamp SaranWrap Can of Coke Vernier LoggerPro interface, temperature sensor and computer 1001 Lab 11 1 1001 LAB 11: GREENHOUSE EFFECT PREDICTION Given the 5 models, predict possible outcomes for each of the systems, rank in order of highest to lowest equilibrium temperatures. Do you think the tank with only C02 will have a higher equilibrium temperature than the tank with only black paper on the bottom? How does the model that uses coke compare to the 4 previous trials, how does it differ? WARMUP QUESTIONS 1. What effect is the black paper going to have on the model system? 2. From your knowledge of CO2, what effect might it have on the system? 3. Do you think that the effects of the black paper and CO2 gas will be additive? If so, why? If not, why? 4. What will Trial 5 be like? Is there another greenhouse gas other than CO2 in this system? How about the color of the coke, how does that compare to the black paper? 5. Which of the 5 trials do you believe will have the hottest equilibrium temperature? EXPLORATION Familiarize yourself with the apparatus. When using dry ice to create CO2 you will experience a certain amount of cooling, how can you minimize the effect on your data? It is useful to know that CO2 is heavier than air, so it will sit in the aquarium like water in a bathtub if not disturbed too much How much effect do you think the distance of the lamp from the aquarium surface will have on your results? How can you minimize this between the different lab groups? How important is the temperature probe positioning? If the lamp directly shines on the probe, does this effect your measurements? Make a plan that determines a good amount of time to collect data. How can you tell if your system has reached equilibrium? MEASUREMENT Run the trials and collect temperature data for each setup. If you are going to compare data as a class, the setup needs to be standardized so each of the lab groups has the exact same distance from the tank surface to the heat lamp bulb. The temperature probes should be shielded with a Styrofoam cup to keep the light from directly striking it. ANALYSIS List the equilibrium temperatures for each group and compare. Were the results you got for the coke trial what you expected? 1001 Lab 11 2 1001 LAB 11: GREENHOUSE EFFECT CONCLUSION Changes in the vibrational and rotational motions of molecules are the main reason for the absorption of IR energy The direct absorption of infrared radiation occurs only if there is a change in the dipole moment of the molecule. Consider the dipole moment as the molecule undergoes each of these motions. The vibrations will result in an absorption in the infrared region. Infrared absorption frequencies for several atmospheric gases are given in Table 1. 1001 Lab 11 3 1001 LAB 11: GREENHOUSE EFFECT Table 1: Principal IR Absorptions for Various Atmospheric Gases Type of Gas Absorption Ranges (in cm-1) [Look for one peak within the following ranges.] Type of bond causing absorption Water (H20) 3800-3600 1600-1400 2400-2200 Asymmetric Stretch Bend Asymmetric Stretch Bend Symmetric Stretch Asymmetric Stretch Asymmetric Stretch Symmetric Stretch Asymmetric Stretch Asymmetric Stretch Carbon Dioxide (CO2) 800-600 2400-2200 1400-1200 Nitrous Oxide (N20) Carbon Tetrachloride (CCl4) Ozone (O3) 1000-600 2400-2200 1200-1000 3200-3000 Methane (CH4) 1400-1200 Bend Nitric Oxide (NO) 2000-1800 Stretch 1001 Lab 11 4 LAB 12 ENERGY AND ATOMIC SPECTRA It turns out that the principles which govern the operation of generators and motors also allow for the existence of waves of electrical and magnetic fields propagating in space and carrying energy. When this was first discovered in the nineteenth century (by Maxwell on the theoretical side and by Hertz experimentally) then people realized for the first time that the light by which we see is an electromagnetic wave. From the point of view of energy technologies, light is important as a source of energy for solar energy devices and from the point of view of climate change, it is important in understanding the temperature of the earths surface which is determined by a balance of energy flows of which the incoming electromagnetic wave flow from the sun and the outgoing infrared radiation are dominant. The use of solar radiation to generate useful energy proceeds by three major pathways. One is direct conversion to thermal energy, which has been discussed earlier. Another is the use of solar radiation to produce electrical energy in photovoltaic cells (as in solar powered calculators) and the third is in the conversion of the energy of solar radiation to chemical energy in plants. The conversion of solar electromagnetic radiation to chemical energy in plants is not only the source of the energy in food and biofuel but is also the source of energy in fossil fuels, which store the energy converted in this way by plants which lived millions of years ago. Of course electromagnetic waves are also central to many information technologies, such as radio, TV, and cell phones. To understand more fully how these various conversions of solar radiation to other forms works, one has to introduce the idea of the spectrum of light and to discuss the nature of waves in a little more detail. A wave is a disturbance in a medium or the vacuum of space which is characterized by a propagation speed, a frequency and a wavelength (see Figure 4.14. of your book.) In a traveling wave (the kind we will mainly consider) the speed is the speed at which the bumps in the wave move in the direction of travel. The wavelength is the distance between the bumps on the top of the wave and the frequency is one over the time that passes between the moments when successive bumps pass some fixed point in space. The three quantities are related by (see p. 111 of your book) speed =wavelength x frequency Wave speed is characteristic of the type of wave and the medium in which it is propagating. It does not depend on the wavelength or the frequency. Therefore, from the previous equation, for a given medium and type of wave, once you know the wavelength you know the frequency and vice versa. The other important feature of a wave is the amplitude of the wave, which is the height of the bumps, and which determines how much energy the wave is carrying. Solar radiation consists of electromagnetic waves, which travel in vacuum at a universal (and very high) speed of 3x108 m/s. It is a mixture of waves at different frequencies and wavelengths however. The color which we see in lighted objects arises because our eyes distinguish between electromagnetic radiation of different wavelengths. We see longer wavelengths of light as red and shorter ones as blue with the other colors appears as yellow, green, purple etc depending on wavelength. We cant see all the electromagnetic radiation which exists with our eyes however. The waves which carry cell phone, TV and radio signals are of longer wavelength and our eyes do not detect them and we also cant see ultraviolet and Xray radiation which are electromagnetic waves of shorter wavelengths than we can see (see Figure 41.5 of your book.) You can be convinced that solar radiation is a mixture of different wavelengths by using a prism to split the light into a rainbow of different colors, an experiment first done by Newton. In a more quantitative experiment, one can use an instrument to measure how bright the light at each color (that is, how much energy is coming in at each color). Making a graph of this measurement as a function of wavelength gives the quantitative spectrum of the light (see 1001 Lab 12 1 1001 LAB 12: ENERGY AND ATOMIC SPECTRA Figures 4.16 and 6.2.) Looking more closely at such spectra, one finds that they consist of two different types of features. There is a smoothly varying background, as sketched in figure 4.16, as well as some sharp peaks. You can see hints of the sharp peaks in Figure 6.2 . The shape of the smooth background tells you the temperature of the source (a very high 6300 kelvin degrees for the sun) and the peaks are characteristic of the atoms present in the sun. These socalled line spectra were first discovered about 100years ago. They serve as a fingerprint of the chemical composition of the source and have been used to learn the composition of the stars by astronomers. In this laboratory, you will observe the spectra of sources made of various elements in the periodic table, in order to get an idea how the spectra vary depending on the chemical composition of the source. The reason why atoms give out line spectral is because the electrons which move around the heavy nuclei of the atoms can only move in certain ways. When they jump from one mode of motion to another, they emit radiation characteristic of the difference between the rates at which they orbit the nucleus in the two modes. Just as materials which are emitting electromagnetic energy do so only at certain well defined wavelengths, so materials can only absorb electromagnetic radiation at certain wavelengths. Thus one can also observe absorption spectra, which tell you which radiation failed to get through the material. The absorption spectrum of chlorophyll, which is the material in plants which harvests sunlight for conversion to chemical energy, is shown below. Chlorophylls absorb only certain portions of the visible spectrum: the blueviolet range (420460 nm) and the red range (630660 nm). The other colors, particularly green, are not absorbed by the chlorophylls and are reflected. Because plants therefore reflect green light to our eyes, we see them as green in color. . Absorption vs. Wavelength for Chlorophylls 1001 Lab 12 2 1001 LAB 12: ENERGY AND ATOMIC SPECTRA We show some characteristic emission spectra of gases of atoms of the periodic table below. Examples of Atomic Spectra Lithium Red Beryllium Red Sulfur Red Potassium Red Violet Violet Violet Violet You and your partners will view the atomic spectra of several different elements. You will record the characteristic spectrum for each element. Using the periodic table and your observations, you and your lab partners will get some understanding of the role of electron energy levels in creating atomic spectra. EQUIPMENT 1 handheld quantitative spectrometer (per student) 1 small strip of diffraction grating (per student) 1 highvoltage power supply (per class) 1 set of spark tubes filled with different gases (per class) PREDICTIONS/PRELIMINARY QUESTIONS Use the website www.colorado.edu/physics/2000 to help gain some understanding of the topics in this lab. Also check the website www.webelements.com for a periodic table. 1. What are electromagnetic waves? What is electromagnetic radiation? 2. 3. What do we suggest an electron, within an atom, does when the atom produces a certain color in the spectrum? In terms of electrons what is a difference between atoms of one element and atoms of another element that produce spectra that are very different? Compare for example an atom with a complex emission spectrum and an atom with a spectrum having only a few lines in its spectrum. 1001 Lab 12 3 1001 LAB 12: ENERGY AND ATOMIC SPECTRA PROCEDURE You and the rest of the class will observe and record the spectra emitted from a series of different elements. The spectra are your data for this exercise. From your observations, you will get some insight into the role of electron energy levels in creating spectra. 1. The elements you will observe first are gaseous and are contained in spark tubes. The spark tubes will allow us to excite the electrons of these gases. The electrons in a gas can be excited by running an electric current through the gas. The electricity will be supplied by a highvoltage power supply; it is similar to the equipment that is used in neon signs. A spark tube is placed within an electric circuit, and an electric current is run through the circuit. The gas within the tube is excited, and as a result, light of specific wavelengths is emitted. The lab instructor will handle the spark tubes as they are fragile and involve high voltage. 2. Instead of using a prism to spread out the emitted light, you will use a spectrometer with a thin piece of plastic called a diffraction grating. It is essentially a plastic slide with a large number of evenly spaced parallel grooves. The grooves are very fine, with 4000 to 8000 lines per centimeter. When you hold the spectrometer in front of your eye, you will see the light from the windows and the overhead lights spread out into a rainbow. The scale indicates the wavelengths of the light you see. To cut out this extraneous light, the shades will be pulled down and the lights turned off. Your instructor will put spark tubes with different gases in the power supply. Observe the spectrum of each and record it as accurately as possible. Note the color, brightness, spacing, and number of the lines. Before using the spectrometer look at the light from the fluorescent lights and light from the spark tubes using a diffraction grating that looks like just a piece of plastic. You should see a spectrum off to one side in fact, you can probably see a spectrum off to the left and to the right. To use the blue spectrometer, hold it with the writing up and point the rectangular opening (located on the right side) toward the light source. Look through the eyepiece in the smaller end. There is a note about calibration, on the spectrometer, close to the eyepiece: Point the rectangular opening toward a fluorescent light and rotate the small plastic eyepiece until the green line is aligned with the mark at 546 nm. Then it should be ready to use if you don't adjust the eyepiece any more. Observe the spectra and try to identify the elements. On your computer desk top open "The Elements Spectra" you can use this program to obtain the spectra of the elements. Click on one of the elements in the periodic table to see the spectrum. The program shows both the absorption spectrum and the emission spectrum that you see looking through the spectrometer. The most noticeable or significant lines in the absorption spectrum are probably the easiest ones for you to see in an emission spectrum. Your instructor may show you the spectra from the gas tubes using a special spectrometer that can be used to project the spectra on the screen. Examine a periodic table of the elements. An excellent periodic table can be found online at WebElements (www.webelements.com). You should also visit the Physics 2000 website www.colorado.edu/physics/2000 . Go to the Science Trek section and read the sections on Electromagnetic Waves, Quantum Atom, and the Periodic Table. Use this information to help answer the questions in the conclusion. 3. 4. 5. 6. 7. 8. 1001 Lab 12 4 1001 LAB 12: ENERGY AND ATOMIC SPECTRA CONCLUSIONS 1. a) Describe why light emitted by a particular electron would have only a specific set of energies. b) Which of the elements had the fewest spectral lines? Which element had the most? c) What causes the complexity of the spectra? Explain in detail. (Remember that if there are more protons in the nucleus of an atom, there are more electrons and therefore more available levels for the electrons to populate.) The equipment used in this exercise is very similar to that used in neon signs. Use your knowledge of atomic theory and explain how you think all of the different colors in neon signs are made feel free to look up more information on the web to answer this one. Identification of elements by spectra lines is often employed in environmental science. In particular, it is used in the fields of environmental contaminants, atmospheric chemistry, hydrology, waste management, soil chemistry, and various others. In addition, it used by other scientists. For instance, astronomers use spectra to investigate stars, and geologists use them to investigate microscopic variations in minerals. Can you think of some other applications of atomic spectra? Explain your answers. 2. 3. 1001 Lab 12 5 1001 LAB 12: ENERGY AND ATOMIC SPECTRA 1001 Lab 12 6 LAB 13 CONVERSION OF GRAVITATIONAL POTENTIAL AND WIND ENERGY TO ELECTRICAL POWER: HYDRO AND WIND POWER Now that you have some familiarity with the way an electrical generator works, you will use a somewhat more sophisticated generator to study how effectively it converts the gravitational potential energy of water lifted above the earths surface (in applications, above sea level) to electrical energy. As discussed in your book, this source supplies about 3 % of the US electricity supply (using the water stored behind dams to drive turbines which turn generators.) It is ultimately a form of solar energy because the water gets into the rivers and flows into the dams through the collection of rain and snow falling on mountains and other highlands. The rain, in turn arises because the heating of the ocean surface by the sun evaporates water which rises as vapor to the clouds and is dropped as rain on the mountains. Unfortunately most of the promising sites for hydropower plants are already developed in the United States, and constructing new dams has some negative environmental consequences, so there are not prospects for strong growth in this energy sector in the United States. You will use the same generator to study the effectiveness of the generation of electrical energy from the wind. Wind energy is also partly due to solar heating of the atmosphere, but the wind is partly extracting energy from the kinetic energy of the earths rotation. Wind energy generation is a rapidly growing industry in the United States and Europe and can supply a significant portion of electrical energy needs in the future (10% or more). PRELAB READING Hinrichs and Kleinbach pp 400417 EQUIPMENT 1. Generator Assembly 2. Pulley, threestep 3. Resistor Plug (1),100 ohm (1) 4. LED plug (1) 5. Spool of thread (1) 6. Turbine 7. Windmill PREDICTIONS/PRELIMINARY QUESTIONS If you have 500 cubic centimeters of water an average height of 0.5 meters above the turbine, what is the maximum energy which you can generate with the available energy? If there is a 100 ohm resistor across the output terminals of the generator, and the water takes a minute to run through the turbine, what is the average voltage which you expect to see across the output terminals of the generator? PROCEDURE Part 1. Electrical Energy from Gravitational: Potential Energy of a Weight. First use the generator to convert the gravitational potential energy of a weight to electrical energy. You will do one qualitative experiment in which the electrical energy is converted to light and a second experiment in which you measure the output energy quantitatively. : 1.Use the rod clamp on the side of the generator to mount the generator to a rod stand. 2. Insert the LED plug into the jacks on the generator (Figure 1a). 1001 Lab 13 1 1001 LAB 13: HYDRO AND WIND POWER 3.To attach the pulley, align the indent marks on the pulley with the indented portion of the black shaft on the generator; then slide the pulley onto the shaft. Fasten the nut tight over the screw to hold the pulley in place. 4. Cut a small piece of nobounce foam, and place the foam underneath the hanging mass . 5. Cut a piece of string to tie to the hanging mass and pulley (Figure 1b). 6. Tie a double knot in the string and hook the knot in the slot on the pulley. 7. Attach the other end of the string to the hanging mass. 8. Wind the string up on the smallest pulley, such that the string falls out when the mass reaches the bottom of the foam pad. 9. Adjust the position of the hanging mass or the height of the generator so that the knot slips out just as the mass reaches the foam.) 10. Now wind up the string and let the mass fall, observing the light emitted by the light emitting diode. You will see it changing from green to red and back, corresponding to current flowing in one direction and then in the opposite direction. What does this tell you about this generator? In particular, does it have a commutator, as the one illustrated in Figure 11.11 does? Does the rate at which the light changes from green to red and back change as the weight falls? Note the answer in your notebook, together with a brief discussion of the possible reason for any changes. 11. Now for the quantitative measurement, remove the LED plug and insert the resistor plug into the jacks on the generator. Your instructor will show you how to attach a voltage probe to the resistor jacks and to the computer and to use the software LoggerPro to make a graph of the power coming out of the generator at each moment in time. The instructor will also explain how to get the total electrical energy produced during the fall from the power data plotted on the screen. Note the total energy in your notebook and make a sketch of the graph of power versus time. 12. Measure the distance which the weight fell and note it in your notebook and weigh the weight to determine its mass (if it is not marked on the weight) and make a note of it. Part 2. Electrical Energy from Gravitational Potential Energy of Water 1. Attach the ETHydro accessory housing to the molded case of the generator using the two captured screws and the supplied screwdriver 2. Insert the pointed end of a plastic nozzle into the spring clip underneath the housing 3. Connect the nozzle to a piece of tubing connected to an external water supply.(figure on next page). Have a beaker or container below the housing to collect water exiting the turbine. 1001 Lab 13 2 1001 LAB 13: HYDRO AND WIND POWER 4. Run the water supply through the nozzle of the turbine and watch the turbine spin. 5. Shut off the water with the valve and refill the water container. Measure the vertical distance from the center of the cylinder to the turbine and the diameter of the cylinder. Plug the resistance into the generator and connect the voltage detector to the resistance and the computer as before in order to measure energy output as you did in Part 1. 6. To start taking power data, open the valve and let the water run out through the turbine. Determine and record the energy output in your notebook. Part 3. Electrical Energy from Wind Energy 1. Remove the turbine from the generator and attach the windmill assembly. 2. Turn on the fan and point it toward the windmill. Observe the windmill turning 3. Use both the Hall wind device and/or the anemometer provided to measure the velocity of the air as it hits the windmill. Which is more accurate? Does it matter? 4. Turn off the fan and measure the diameter of the windmill. 5. Now turn the fan back on and measure the power output. ANALYSIS 1. Calculate the efficiency of conversion for each of the three experiments. a. For the weight, divide the total electrical energy output by th change in gravitational potential energy during the drop of the weight. b. For the hydro experiment, similarly divide the total electrical energy output by the gravitational potential energy change of the water. To calculate the change in gravitational potential energy of the 1001 Lab 13 3 1001 LAB 13: HYDRO AND WIND POWER water, you need its mass, which is given by the volume of water times the mass density of water which is close to 103 kg/m3. c. For the wind experiment, calculate the input power from the wind as (1/2) x(mass density of air)x (air speed)3x (crosssectional area of the windmill) Calculate the efficiency of the wind generator by dividing the measured electrical power by the input power of the wind. The density of air is approximately .094 kg/m3 CONCLUSIONS Compare the efficiencies of the three conversions you have determined. Discuss the reasons for the differences. 1001 Lab 13 4 Appendix A: LoggerPro data acquisition software: Before launching, you should: Power the LabPro using the AC power supply or AA batteries. Connect a sensor to LabPro. Connect the USB or serial cable to LabPro. Attach the other end of the interface cable to any unused serial port or USB port on your computer. Start Logger Pro Locate the Logger Pro icon and doubleclick on it. Logger Pro can collect data from multiple interfaces and devices. In the figure above, a Go!Link, Go!Temp, LabPro and Ohaus balance are connected to the computer. Logger Pro is a program that allows you to collect and analyze data from Vernier LabPro. Among its many features, data may be manually entered from the keyboard, pasted from the clipboard, or retrieved from a file saved on disk. Logger Pro is also a document creator, with the ability to include several pages in one document. With Logger Pro you can also: Perform data analysis, such as statistical analysis, curve fitting, and calculated columns. Add pictures to lab reports. Add movies and synchronize movies with data collection. Manually enter data. Export data to Excel or other spreadsheets. Have students draw a prediction on a graph and see how well the experimental data matches it. AcquiringData Configure Logger Pro for Your Experiment There are several ways to set up Logger Pro for an experiment. Most often you will just start Logger Pro with autoID Sensors connected. AutoID Sensors With autoID sensors, all you need to do is connect the sensor to one of the LabPro channels and start Logger Pro software. The sensor will be identified, and default datacollection parameters loaded for that sensor. You are ready to click the button and begin data collection! Probes & sensors experiment files If you are using an older Vernier sensor that is not autoID, or if you want to use some other specially configured file, you will need the Probes & Sensors folder. Choose Open from the File menu, and open the Probes & Sensor folder. There you will find a folder for each of the Vernier sensors. Open up a file with a name that best fits the data collection you want to perform. 1001 Appendix A : Page 1 LoggerPro Sensor dialog This dialog, called up by clicking the LabPro icon in the upper left side. It shows your sensor information, the units and current readings. The details of the dialog vary, depending on the specific interface used. Use the scroll bars in the Sensor Catalogs to view all sensors. By clicking in a Channel box with a displayed sensor, you may be able to: Calibrate a sensor: Perform a new calibration. Get sensor information: Includes collection details and calibration information Zero a sensor: Sets the current reading of a sensor to zero. Choose sensor: Select another sensor for the channel. Change the current settings: Select the available units or calibration settings. Analog out (CH 4 only): This channel can be used as a function generator. Remove sensor: Clears the channel of the sensor. Collect Data Click the green GO button. Logger Pro will begin plotting data in the graph window. Tip: Normally, the data collection stops at the time indicated on the time axis. You can stop the data collection early by clicking the stop button. If you discover you want the data collection to go even longer, you can add additional time by choosing Extend Collection from the Experiment menu. Now that you have collected data, there are many things you can do with it. Keep reading for descriptions of some of the most commonlyused Logger Pro features. Store Data The Store Latest Run option in the Experiment menu saves the most recently collected data in memory, allowing you to do another collection while keeping the first one. The run labeled "Latest" always contains the most recently collected data, and is overwritten when you collect more data. When you store the latest run, a new data set named Run 1, Run 2, etc, is created and the values of the latest data set are copied to it. Columns in the latest data set are drawn with thick lines on the graph, while other columns are drawn with thin lines. 1001 Appendix A : Page 2 LoggerPro Note: Using Store Latest Run does not save data on disk, nor will it preserve your data between sessions or when you turn off your computer. You must select Save As from the File menu to save your session (data and analysis) to disk. Data Collection and Modes You are not limited to timebased data collection. Choose Data Collection from the Experiment menu to modify data collection parameters for your experiment. See Logger Pro's Help for information about your choices. Time Based Set the data collection rate as a function of time. Events With Entry A data point is recorded whenever the Keep button is clicked. You are then prompted to enter values for a new data column. Selected Events A data point is recorded whenever the Keep button is clicked. Digital Events You can set data collection to run continuously (until the Stop button is clicked) or at a certain number of events. Manually Enter Data You can type data directly into the data table from the keyboard. To manually enter data, disconnect all interfaces, and start Logger Pro. Click on a cell and type in a number. You can use the mouse or the Enter/Return key to move and edit within the table. Your values will also be plotted on the graph. Logger Pro Screen and Toolbar Toolbar The toolbar contains buttons for the most commonly used features of Logger Pro. All of the features in the toolbar are also available in the menus. As you move the pointer across a button, you are given a short description of each feature. Menu bar Logger Pro has a menu bar across the top of the main window. Use the menu bar to access all of the software's features. 1001 Appendix A : Page 3 LoggerPro Manage Your Page and Data There are numerous ways that you can customize the presentation of your data. Move Objects Select the object by clicking on it once. Move the cursor over the edge of the selected object until the cursor turns into a hand, then hold down the mouse button while grabbing and moving the object. Resize Objects Singleclick on an object to select it. When an object is selected, its border becomes visible, along with eight resizing handles. Clicking and dragging a resize handle will resize the object in the appropriate direction. Graph Axes Customize your graph by changing the columns that are displayed. 1. Position the mouse over the Xaxis or Yaxis label and click. A list of available columns will appear. 2. Select the column you want displayed. If you want more than one column displayed, select More and check the boxes next to the desired columns. 1001 Appendix A : Page 4 LoggerPro Graph Options Doubleclick on the graph or select Graph Options from the Options menu to call up the Graph Options dialog. This allows you to change the graph object appearance, including point protectors, legend, line style, and other graph features. Click the Axis Options Tab to adjust scaling and select what columns are plotted or add a Y axis to the righthand side of the graph. Create a Column You can create a new column that is manually entered or that consists of calculations based on other data in Logger Pro. Create a new column by selecting New Manual Column or New Calculated Column from the Data menu. Select and Edit Cells in a Data Table Manuallyentered data and data collected using the Events with Entry mode can be edited. To edit a single cell in a table, singleclick in the cell. To select a single cell in a table, click and drag the selection within that cell. You can select entire columns by clicking on the title of the column. You can select an entire row of data by clicking on the row numbers. The Data Browser The Data Browser is a container for all data in a Logger Pro file. In contrast, a given Data Table will not necessarily show all data columns. Data Browser Features Drag and drop from the Data Browser to objects such as tables and graphs. Drag a column to a y or xaxis to plot it there. When the Data Browser is selected, edit menu items like Cut, Copy, Paste, etc. will apply to the selected items within the Data Browser. 1001 Appendix A : Page 5 LoggerPro Analyze Data The Analyze menu contains functions for examining and analyzing your data. A few of the most commonly used analysis functions include: Examine: Shows the x and yvalues of the data point closest to the mouse pointer. Statistics: Calculates the minimum, maximum, mean, and standard deviation. Curve Fit: Fits a selected or userdefined function over the data. Draw Prediction: Sketch a prediction on a graph using the mouse. Several of the analysis features are available from the Toolbar. Additional Analysis Features Movie: You can synchronize data collection and a movie of the experiment so that as you Replay the movie, the data are replayed on the graph. Video Analysis: You can create a graphical representation of the motion you see in a movie. Model: Perform a manual curve fit of your data. Histogram: Displays the distribution of values for a data set in bar graph format. Strip Chart: Each new collected data point gets added to the right most edge of the graph. FFT: Gives the amplitudes of the frequency components of a data series, and displays a bar graph showing the amplitudes of the frequencies shown along the xaxis. Set Preferences The Preferences dialog box contains several options that pertain to the general operation of the software. These preferences will apply every time Logger Pro is launched. In Windows, choose Preferences from the File menu. In Mac OS X, choose Preferences from the Logger Pro menu. 1001 Appendix A : Page 6 LoggerPro Printing Print Graphs and Data 1. Arrange the objects on the screen as you want them to appear on the paper. If you choose, you can print several graphs and data tables at once. 2. Select Print from the File menu or click on the toolbar. 3. When the printer is ready, click in the Print dialog box. You can print the graphs and data tables individually by selecting Print Graph or Print Data Table from the File menu. Where to Next? The extensive Help system in Logger Pro provides detailed instructions for these and other features. Also available are the Logger Pro Tutorials. Choose Open from the File menu to access them. These short tutorials are suitable for reference or for use in your classes. Firsttime users should work through one introductory tutorial and the advanced tutorials as needed. If you have questions as you are using our software, call or email us. We will be happy to help! 1001 Appendix A : Page 7 LoggerPro 1001 Appendix A : Page 8 Appendix B: LabPro Computer Interface The LabPro interface has three buttons, three LEDs, four analog channels (CH 1, CH 2, CH 3, and CH 4), two digital channels (DIG/SONIC 1 and DIG/SONIC 2), a serial computer connection, a USB computer connection, a piezo speaker, and a calculator I/O port. Buttons: The three buttons on the top of LabPro are used for the following purposes: TRANSFER begins transfer of calculator programs or applications (apps) between LabPro and an attached TI graphing calculator. QUICK SET-UP clears any data stored in LabPro's memory, then polls all channels for auto-ID sensors and sets them up to collect data. QUICK SET-UP is used when neither a computer nor a calculator is attached to LabPro and works only with auto-ID sensors. START/STOP begins sampling for Quick Set-Up. Sampling continues until the default number of samples is collected or you press START/STOP again. This button also acts as a manual trigger for certain data-collection modes. Lights LabPro has three lights that indicate the LabPro status. Red indicates an error condition. Yellow indicates that LabPro is ready to collect data samples. Green indicates LabPro is collecting data. Beeps LabPro has a piezo speaker that makes four kinds of sounds or "beeps." A "good" beep is a medium tone followed by a high tone. A "caution" beep is a medium tone followed by another medium tone. A "bad" beep is a medium tone followed by a low tone. A "tick" sound is made when a key is pressed. Connecting Sensors Sensors can be divided into two basic types--analog and digital. Examples of analog sensors are Temperature Probes, pH Sensors, Force Sensors, Oxygen Gas Sensors, etc. Up to four analog sensors can be connected to LabPro. Four jacks for the analog sensors (CH1-CH-4) are located on the same side as the AC Adapter Port. The analog ports accept British Telecom-style plugs with a right-hand connector. Examples of digital sensors are Motion Detectors, Radiation Monitors, Photogates, and Rotary Motion Sensors. Up to two digital sensors can be connected to LabPro. The digital ports (DIG/SONIC), which accept British Telecom-style plugs with a left1001 Appendix B : Page 1 Appendix B: LabPro hand connector, are located on the same side as the serial and USB computer connections. Connect the sensor to the appropriate port. An important feature of LabPro is its ability to detect auto-ID sensors, and automatically set up an experiment. If you are not using sensors with the auto-ID ability, you can easily set them up with the experiments provided with the LoggerPro software. Attaching LabPro to a Computer The LabPro computer connection is located on the right side of the interface. The sliding door found there can be moved left to reveal the serial port or to the right to reveal the USB port. Serial Port and Digital Channels (USB Port is behind the sliding door) Starting Logger Pro Software and Preparing to Collect Data Locate the Logger Pro icon on your computer and double-click on it. An important feature of LabPro is its ability to detect auto-ID sensors, and automatically set up an experiment.3 If Logger Pro does not detect your LabPro and/or a sensor is not connected, the Collect button will be faded. Choose Connect Interface from the Experiment menu. The computer will attempt to communicate with LabPro. If you see the following message4, then Logger Pro is having trouble communicating with LabPro. Select the correct port or check your current port and click OK. If you have connected a Stainless-Steel Temperature Probe and the computer has detected the LabPro interface, for example, you will see the following screen, which shows a graph of Temperature vs. Time along with the current reading at the top. 1001 Appendix B : Page 2 Appendix B: LabPro Notice how the program automatically identified the temperature probe (an auto-ID sensor). The current temperature reading is displayed in the status bar at the top of the screen. The default data collection mode is time graph. In this example, you have a Temperature Probe, reading in Celsius, and collecting data as a function of time for 120 seconds. If you now disconnect the Temperature Probe, connect a different auto-ID sensor, and choose New from the File menu, Logger Pro will set up a new experiment for the new sensor. Tutorial: Temperature Measurement with Logger Pro A good way to get acquainted with LabPro is by performing a simple experiment. Using this tutorial, you will take measurements using a temperature sensor. Follow the simple steps and you are well on your way to collecting and analyzing data with a computer. MATERIALS Computer & LabPro interface Vernier Stainless-Steel Temperature Probe One 250-mL beaker or cup 100 mL hot water PROCEDURE 1. Place about 100 mL of hot water into a 250-mL beaker or a cup. 2. Plug the Stainless-Steel Temperature Probe into channel CH 1 on LabPro, and lay the temperature probe on the tabletop. 3. Start the Logger Pro software. Logger Pro will detect the auto-ID sensor, set the data collection parameters, and computer display. In this case, collection parameters are 1.0 sample per second and 120 samples. The program displays a graph and data table on the computer. The vertical axis of the graph will have temperature scaled from 0 to 100C. The horizontal axis will have time scaled from 0 to 120 seconds. You are ready to collect data. 4. Click GO to begin data collection. 5. Wait about 10 seconds and place the Temperature Probe into the cup of hot water, as shown above. Allow Logger Pro to complete data collection. 6. Notice that the sensor does not read the new temperature instantly; it takes a moment to respond. 7. Now that the run is complete, pull down the Analyze menu and choose Examine. The cursor will become a vertical line. As you move the cursor across the screen, temperature and time values corresponding to the cursor position will be displayed. Move the cursor to the point when the probe was first placed in the hot water. Record that time. Move the cursor to find the highest temperature, and record that time. How long did it take for the temperature sensor to stabilize at the water's temperature? This tutorial is an example of how easy it is to collect and analyze data with LabPro and a computer. The auto-ID Stainless-Steel Temperature Probe simplified the experiment setup, but the use of non auto-ID sensors is almost as easy. All you have to do is connect the sensor or sensors, and open up an appropriate experiment file. Logger Pro software has over 500 experiment files. This set of files is also includes experiments in our lab books. 1001 Appendix B : Page 3 Appendix B: LabPro 1001 Appendix B : Page 4 Appendix C: Motion Detector The Motion Detector is used to collect position, velocity and acceleration data of moving objects. Students can study a variety of motions with the Motion Detector. This is a sample of motion data collected with Logger Pro and a computer. How the Motion Detector Works This Motion Detector emits short bursts of ultrasonic sound waves from the gold foil of the transducer. These waves fill a cone-shaped area about 15 to 20 off the axis of the centerline of the beam. The Motion Detector then "listens" for the echo of these ultrasonic waves returning to it. The equipment measures how long it takes for the ultrasonic waves to make the trip from the Motion Detector to an object and back. Using this time and the speed of sound in air, the distance to the nearest object is determined. Note that the Motion Detector will report the distance to the closest object that produces a sufficiently strong echo. The Motion Detector can pick up objects such as chairs and tables in the cone of ultrasound. The sensitivity of the echo detection circuitry automatically increases, in steps, every few milliseconds as the ultrasound travels out and back. This is to allow for echoes being weaker from distant objects. Features of the Motion Detector The Motion Detector is capable of measuring objects as close as 0.15 m and as far away as 6 m. The short minimum target distance (new to this version of the Motion Detector) allows objects to get close to the detector, which reduces stray reflections. The Motion Detector has a pivoting head, which helps you aim the sensor accurately. For example, if you wanted to measure the motion of a small toy car on an inclined plane, you can lay the Motion Detector on its back and pivot the Motion Detector head so that it is perpendicular to the plane. The Motion Detector has a Sensitivity Switch, which is located under the pivoting Motion Detector head. To access it, simply rotate the detector head away from the detector body. Connecting the Motion Detector to a LoggerPro Interface To use the Motion Detector with LabPro, the Motion Detector is connected to the interface with the included cable. Plug the cable into the DIG/SONIC port on the right side of the Motion Detector. Plug the other end of the cable into the interface. Using the Sensitivity Switch Slide the Sensitivity Switch to the right to set the switch to the "normal" setting. This setting is best used for experiments such as studying the motion of a person walking back and forth in front of the Motion Detector, a ball being tossed in the air, pendulum motion, and any other motion involving relatively large distances or with objects that are poor reflectors, e.g., coffee filters. The other sensitivity setting, which we call "Track", works well when studying motion of carts on tracks like the Vernier Dynamics System, or motions in which you want to eliminate stray reflections from object near to the sensor beam. 1001 Appendix C : Page 1 Appendix C: Motion Detector 1001 Appendix C : Page 2 Appendix D: Voltage Probe The Differential Voltage Probe is designed for exploring the basic principles of electricity. Use this probe to measure currents in low voltage AC and DC circuits. With a range 6.0 V, this system is ideal for use in "battery and bulb" circuits. Use it with the Current Probe to explore Ohm's law, phase relationships in reactive components and much more. This differs from the Voltage Probe that comes with your interface in that neither clip is connected to ground. Use multiple sensors to explore series and parallel circuits. This sensor has the same characteristics as the Voltage Probe from the Vernier Current and Voltage Probe System. Using the Voltage Probe with a Computer 1. Connect the Differential Voltage Probe to any of the analog ports on LabPro (in most cases, Channel 1 is used) 2. Start the Logger Pro software on the computer. 3. You are now ready to collect data. Logger Pro will identify the Differential Voltage Probe and load a calibration. Click on Collect and begin collecting data. Specifications Differential Voltage Probe input voltage range: 6.0V Max. voltage on any input: 10 V Input Impedance (to ground): 10 M Linearity: 0.01% Resolution (using LabPro): 3.1 mV Supply voltage: 5 VDC Supply current (typical): 9 mA Output voltage range: 0 - 5 V Transfer function : Vo = 0.4 (V+ V-) + 2.5 How the Differential Voltage Probe Works The Differential Voltage Probe measures the potential difference between the V+clip (red) and the V- clip (black). The voltage probes have differential inputs. The voltage measured is with respect to the black clip and not circuit ground. This allows you to measure directly across circuit elements without the constraints of common grounding. The voltage probes can be used to measure negative potentials, as well as positive potentials. This is a nice improvement for people using one of our 0 to 5 volt interfaces. The voltage probes are designed to be used like voltmeter leads. They should be placed across a circuit element. The differential input range is 6 volts to +6 volts. Over-voltage protection is provided so that slightly higher voltages will not damage the sensor. You should NEVER use high voltages or household AC with these probes. Do I Need to Calibrate the Differential Voltage Probe? "No" You should not have to perform a new calibration when using the Differential Voltage Probe in the classroom. We have set the sensor to match our stored calibration before shipping it. You can simply use the appropriate calibration file that is stored in your data-collection program from Vernier in any of these ways: 1. If you are using the Differential Voltage Probe with a LabPro, then a calibration is automatically loaded when the Differential Voltage Probe is detected. 2. If you manually load an experiment or calibration file, choose the Current & Voltage Probe system. The calibration for the Differential Voltage Probe is the same as for the Voltage Probe in this older system. The output of this system is linear with respect to the measurement it is making. As mentioned before, the amplifier allows you to measure positive and negative voltages on any of our interfaces. Since many lab interfaces can read voltages only in the range of 0 to 5 volts, the amplifier offsets and amplifies the incoming signal so that the output is always in the range of 0 to 5 volts. If an input is zero volts, for example, the amplifier will produce an output of 2.5 volts. The output varies from this 2.5 volt level, depending on the input. To collect data as differential current, use either the calibration supplied with your program, or calibrate the unit using known voltages. A standard, two-point calibration is done, as with any Vernier sensor. Another option to consider instead of calibrating is "zeroing" the sensor. This is done by 1001 Appendix D : Page 1 Appendix D: Voltage Probe shorting out the leads of the sensor, then choosing the Zero option in the data-collection software. This option adjusts the calibration offset but does not adjust the calibration gain. The default calibration slopes and intercepts for these sensors are: Potential in volts: Slope: -2.5 V/V Intercept: 6.25 V 1001 Appendix D : Page 2 Appendix E: Microphone The Microphone can be used for a variety of activities with sound waves: Demonstrate how the wave pattern changes when frequency and amplitude are changed Compare the waveforms from various musical instruments Have students capture the waveform of the sound of a tuning fork and model the sine wave using a function Measure the speed of sound by using reflected sound waves in a tube Demonstrate beat patterns Determine the period and then the frequency of a sound by measuring the time between peaks on the waveform Display the fast Fourier transform (FFT) of a sound Collecting Data with the Microphone This sensor is used with the Vernier LabPro with a computer. Here is the general procedure to follow when using the Microphone: 1. Connect the Microphone Probe to the interface. 2. Start the data-collection software. 3. The software will identify the Microphone and load a default data-collection setup. You are now ready to collect data. Specifications Frequency range approximately 20 Hz to 16,000 Hz Maximum frequency LabQuest 10,000 Hz LabPro, SensorDAQ, or CBL 2 5,000 Hz Power 7.5 mA @ 5 VDC Stored calibration Slope 1 Intercept 0 (arbitrary units) Note: The maximum data collection rate of the interface affects the maximum frequency you can effectively sample. How the Microphone Works The Microphone uses an electret microphone that has a frequency response covering essentially the range of the human ear. An op-amp circuit amplifies the signal and sends it to the British Telecom connector. Actually the signal is sent to the interface on two different lines. A signal centered at 2.5 volts is on the Vin-low line and a signal centered at 0 volts is on Vin. More information about the input lines on LabPro a is available in appendix B. The best sound sources to use with the microphone are tuning forks, but you may want to investigate a human voice or a whistle, electronic keyboards, and other musical instruments. Try comparing the wave pattern for different sound sources. Try playing two sounds of nearly the same frequency to produce beat patterns. Make sure the sound level is in the correct range to produce good wave patterns. If the sound is too loud, the wave pattern will be "clipped off" at the top or bottom. Move the microphone further from the sound source, or turn down the volume of the sound. 1001 Appendix E : Page 1 Appendix E: Microphone 1001 Appendix E : Page 2 Appendix F: The Energy Transfer Generator Used for demonstrating the conversion of stored gravitational potential energy into electrical energy. The generator includes a 3/4 inch neodymium magnet, which spins between two 400 turn coils, and is visible inside a semiclear, plastic housing. A plastic rod clamp is molded to the generator for attaching the housing to a rod stand. Two plug attachments, an LED (lightemitting diode) plug and 100ohm resistor plug are included. Both the LED and resistor plug insert into the banana jack outputs on the generator's housing. Each plug contains two jacks for connecting a voltage sensor to the generator for collecting data with a computer. The LED plug allows the user to visually see the electrical effects of turning the shaft on the generator. As the student rotates the generator, the LED emits light. The resistor plug is recommended for use with sensors, so the student can measure the effects of generator rotation on power, voltage, and energy. A threestep pulley keys into the shaft on the side of the housing and is removable. Basic Setup Note: The basic setup uses a pulley and hanging mass. If you have purchased accessories for the ETGenerator, see "Attaching Accessories to the Generator" on pages 69 of this manual. Attaching a Pulley with Hanging Mass to the ETGenerator 1. Use the rod clamp on the side of the ETGenerator to mount the generator to a rod stand. 2. Insert the LED plug into the jacks on the generator (Figure 1a). 3. To attach the pulley, align the indent marks on the pulley with the indented portion of the black shaft on the generator; then slide the pulley onto the shaft. Fasten the nut tight over the screw to hold the pulley in place. 4. Cut a small piece of nobounce foam, and place the foam underneath the hanging mass (Figure 1b). 5. Cut a piece of string to tie to the hanging mass and pulley (Figure 1b). 6. Tie a double knot in the string and hook the knot in the slot on the pulley. 1001 Appendix F : Page 1 Appendix F: Energy Transfer Generator Collecting Data with the ETGenerator Note: For realtime data collection, a computer interface is required. 1. Insert the resistor plug into the jacks on the generator. Note: For electrical studies, use the resistor plug and a Voltage Probe. 2. Connect a Voltage Probe to the LabPro interface 3. With the Voltage Probe, connect the black banana plug to the black jack and red banana plug to the red jack on the ETGenerator. 4. Setup your experiment in LoggerPro. 5. In LoggerPro, click the Start button. Let the mass fall, and record the power or other measurement. Attaching the ET-Hydro Accessory (ET-8772) to the ET-Generator 1. Attach the ET-Hydro Accessory housing to the molded case of the ET-Generator using the two captured screws and the supplied screwdriver (Figures 6-7) 2. Attach the turbine blade and tighten the standoff nut over the shaft screw. Keep your fingers in the recess of the housing and hold the shaft to keep the turbine from spinning (Figure 7a). 3. Insert the pointed end of a plastic nozzle into the spring clip underneath the housing (Figure 7a). Note: The clip is spring-loaded and can turn to adjust where the water stream hits the turbine. To increase the water flow, cut (trim) the nozzle end. 4. Connect the nozzle to a piece of tubing connected to an external water supply. Note: Have a beaker or container below the housing to collect water exiting the turbine. 5. Run the water supply through the nozzle of the turbine and watch the turbine spin. Note: To collect data, insert the 100-ohm resistor plug into the banana jacks, connect banana plugs from a Voltage Probe into the resistor plug, and click the Start button in LoggerPro. when water is running through. 1001 Appendix F : Page 2 Appendix F: Energy Transfer Generator Sample Results a) This graph is of average power, and for each turn of the magnet, it shows the average power generated for that small time. The area under the curve is energy (notice almost the same value as the energy for instantaneous power) but should be easier to comprehend for the beginner. b) The graph below is the actual instantaneous power, and the area under the curve is energy. For beginning students, this graph would be hard to understand. They might ask, why is it going up and down? 1001 Appendix F : Page 3 Appendix F: Energy Transfer Generator 1001 Appendix F : Page 4 Appendix I The Digital Multimeter (DMM) The digital multimeter is a piece of equipment that can be used to measure a number of electrical quantities, most often current, resistance, and potential. The DMMs that you will be using are capable of measuring both direct current (DC) and alternating current (AC) circuits. Be careful about knowing which type of measurement you need to make, then set your DMM accordingly. The layout of some DMMs might be slightly different from the one pictured to the right. 13.6 V AC Voltage Range Selection Dial Display DC Voltage Range Resistance Range Current Range V ~ OFF V A Probe Sockets V COM 200 mA 10 A The DMM can measure currents from 10 amps to one microamp (106 amps). This versatility makes the DMM fragile, since measuring a large current while the DMM is prepared to measure a small one will certainly harm the DMM. For instance, measuring a 1-ampere current while the DMM is on the 2-milliamp scale will blow a fuse. If this happens, the TA may be able to change the fuse. However, if you damage the DMM beyond repair, you will have to finish the lab without it. Measuring Current: 1. Set the selection dial of the DMM to the highest current measurement setting (10 amps). Insert one wire into the socket labeled "10A" and a second wire into the socket labeled "COM." 2. Attach the DMM into the circuit as shown below: To measure current, the DMM must be placed in the circuit so that all the current you want to measure goes through the DMM. 3. If no number appears while the DMM is at the 10A setting, move the wire from the 10A socket to the 200mA socket and then turn the selection dial to the 200 milliamp (200m) setting. If there is still no reading, change the dial to the 20 milliamp setting, etc. 4. When you have taken your measurement, return the DMM selection dial to the highest current setting (10 amps) and move the wire back to the 10 A socket. APPENDIX I - 1 APPENDIX I: THE DIGITAL MULTIMETER (DMM) Measuring Voltage: 1. Set the DMM selection dial to read DC volts. Insert one wire into the socket labeled 'V?' and a second wire into the socket labeled 'COM'. 2. Set the selection dial of the DMM to the highest voltage measurement setting. Connect the two wires from the DMM to the two points between which you want to measure the voltage, as shown below. To measure voltage, the DMM must be placed in a circuit so that the potential difference across the circuit element you want to measure is across the DMM. 3. If no number appears, try a different measurement scale. Start at the highest voltage scale and work your way down the scales until you get a reading. Measuring Resistance: The component whose resistance you are measuring must be disconnected from all other currents (due to other batteries, power supplies, etc.) for the DMM to work. That means you must remove it from a circuit first. 1. Set the DMM selection dial to measure ohms (). Insert one wire into the socket labeled 'V' and a second wire into the socket labeled 'COM'. 2. Make sure that the circuit element whose resistance you wish to measure is free of any currents. 3. Attach the wires across the circuit element, as shown in the example below: 4. If no number appears, try a different scale. Use a method that covers all the scales, such as beginning at the largest scale (20 M) and working your way down. APPENDIX I - 2 Appendix II Graphing with Microsoft Excel You will find that numerous exercises in this manual will require graphs. Microsoft Excel is a spreadsheet program that can create fourteen types of graphs, each of which have from two to ten different formats. This results in a maze of possibilities. There are help screens in Excel; however, this overview is covers the type of graph you should include in your lab reports. This is meant to be a brief introduction to the use of Microsoft Excel for graphing scientific data. If you are acquainted with Excel already, you should still skim through this appendix to learn about the type of graph to include in reports. Microsoft Excel is available to all students on the computers in the University labs: East Bank: Eddy Hall Annex 54 Elliott Hall 121 Folwell Hall 14 Walter Library 103 HHH Center 50 Classroom Office Building 135 Classroom Office Building 17 Magrath Library B50 McNeal Hall 305 West Bank: St. Paul: Step 1. Input your measurements and highlight the data using your cursor. APPENDIX II - 1 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL Step 2. Click on the "Chart Wizard" on the toolbar. Step 3. Choose XY Scatter, not Line, from the list and click the "Next" button. Step 4. Select the APPENDIX II - 2 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL "Series in: Columns" option and click the "Next" button. Step 5. Fill in the chart title and axis labels, and click the "Next" button. Step 6. Click "Finish" button. the APPENDIX II - 3 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL Step 7. Your graph will appear on the worksheet. Step 8. Click on data points highlight the to APPENDIX II - 4 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL them. Step 9. Select "Add a Trendline" from the "Chart" menu. Step 10. Choose the best APPENDIX II - 5 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL type of trend line for your data. Step 11. The trend line will appear is it a good fit to your data? Step 12. If equation of line is the the APPENDIX II - 6 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL needed, choose "Display equation on chart." Step 13. The equation of the trend line should appear on your graph. APPENDIX II - 7 APPENDIX II: GRAPHING WITH MICROSOFT EXCEL APPENDIX II - 8 Appendix III Graphed Proportions You will find that numerous exercises in this lab manual will require you to determine the relationship between two variables. This section includes graphs of common proportions that you may encounter in these exercises. y x (x and y are directly proportional) y x2 (y is proportional to x squared) APPENDIX III - 1 APPENDIX III: GRAPHED PROPORTIONS y x3 (y is proportional to x cubed) y 1 (x and y are inversely proportional) x y 1 x2 (y is APPENDIX III - 2 APPENDIX III: GRAPHED PROPORTIONS inversely proportional to x squared) y inversely proportional to x cubed) 1 (y is x3 y x (y is APPENDIX III - 3 APPENDIX III: GRAPHED PROPORTIONS proportional to the square root of x) y 1 (y is inversely proportional to the square root of x) x APPENDIX III - 4 ...
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This note was uploaded on 10/05/2011 for the course PHYS 1001W taught by Professor Unkown during the Spring '10 term at Minnesota.

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