ch33_Efield - ELECTRIC FIELDS AND POTENTIAL Objectives •...

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Unformatted text preview: ELECTRIC FIELDS AND POTENTIAL Objectives • Describe how to measure the strength of an electric field at different points. (33.1) • Describe how electric fields are represented by vectors and by electric field lines. (33.2) • Describe how objects can be completely shielded from electric fields. (33.3) • Explain why a charged object in an electric field is considered to have electrical potential energy. (33.4) • Distinguish between electrical potential energy and electric potential. (33.5) • Describe how electrical energy can be stored. (33.6) • Describe the operation of a Van de Graaff generator. (33.7) 33 T ELECTRIC FIELDS AND POTENTIAL ........ An electric field is a storehouse of energy. THE BIG IDEA discover! MATERIALS cell phone, aluminum foil EXPECTED OUTCOME Students will find that they have to encase most, if not all, of the cell phone in foil in order to block an incoming call. ANALYZE AND CONCLUDE he space around a strong magnet is different from how it would be if the magnet were not there. Put a paper clip in the space and you’ll see the paper clip move. The space around the sun is different from how it would be if the sun were not there. The sun’s gravitational influence affects the motions of the planets around it. Similarly, the space around a concentration of electric charge is different from how it would be if the charge were not there. If you walk by the charged dome of an electrostatic machine—a Van de Graaff generator, for example—you can sense the charge. Hair on your body stands out—just a tiny bit if you’re more than a meter away, and more if you’re closer. The space that surrounds each of these things—the magnet, the sun, and the electric charge—is altered. The space is said to contain a force field. discover! What Is Electric Shielding? 1. Wrap a cellular phone completely in aluminum foil. 2. Make a call to the wrapped phone. 3. Unwrap the cell phone, and now cover only part of it with foil. Make a call to the partly wrapped cell phone. 4. Repeat Step 3 a few more times, covering different parts of the phone. 1. See Expected Outcome. 2. Answers may include metal containers or wire mesh. 3. Electric shielding involves blocking an object from outside electrical activity. Electric shielding works because when the charge on a conductor is not moving, the electric field inside the conductor is zero. Analyze and Conclude 1. Observing What effect did completely wrapping the phone have on reception? Did wrapping only part of the phone block the incoming signal? If so, which part of the phone needed to be covered in order to block the signal? 2. Predicting What other materials do you think could be used to shield a cellular phone? 3. Making Generalizations What is electric shielding, and why does it work? 664 664 33.1 Electric Fields 33.1 Electric Fields The force field that surrounds a mass is a gravitational field. If you throw a ball into the air, it follows a curved path. Earlier chapters showed that it curves because there is an interaction between the ball and Earth—between their centers of gravity, to be exact. Their centers of gravity are quite far apart, so this is “action at a distance.” The idea that things not in contact could exert forces on one another bothered Isaac Newton and many others. The concept of a force field explains how Earth can exert a force on things without touching them, like a tossed ball. The ball is in contact with the field all the time. The ball curves because it interacts with Earth’s gravitational field. You can think of distant space probes as interacting with gravitational fields rather than with the masses of Earth and other astronomical bodies that are responsible for the fields. Just as the space around Earth and every other mass is filled with a gravitational field, the space around every electric charge is filled with an electric field. An electric field is a force field that surrounds an electric charge or group of charges. In Figure 33.2, a gravitational force holds a satellite in orbit about a planet, and an electrical force holds an electron in orbit about a proton. In both cases there is no contact between the objects, and the forces are “acting at a distance.” In terms of the field concept, the satellite and electron interact with the force fields of the planet and the proton and are everywhere in contact with these fields. Just as in the gravitational case, the force that one electric charge exerts on another can be described as the interaction between one charge and the electric field set up by the other. Key Term electric field Teaching Tip Introduce electric fields by mentioning (or showing) a Van de Graaff generator. Describe the altered space around it when it is charged. This space is an electric field. Teaching Tip Compare gravitational and electric fields. Both are regions that are altered, by mass for the gravitational field, and by charge for the electric field. Teaching Tip The easiest fields for students to visualize are magnetic fields because of the familiar iron filing patterns (Figures 36.4 and 36.5.) Explain that fields are called “force fields” because forces are exerted on bodies in their vicinity, but a better term would be “energy field” because energy is stored in a field. In the case of an electric field, any charges in the vicinity are energized. We speak about the PE that electrically charged bodies have in a field—or more often, the PE compared to the amount of charge—electric potential. Explain that the field energy, and correspondingly the electric potential, is greater nearer the charged dome and weaker with increased distance, thus following the inverse-square law. Teaching Tip Point out that measuring instruments sometimes alter that which is being measured. A cold thermometer placed in a warm liquid absorbs heat from the liquid, thereby altering the temperature of the liquid. Similarly, placing a test charge in an electric field changes the nature of the field. The test charges used to measure electric fields are small so as to minimize such changes. FIGURE 33.1 You can sense the force field that surrounds a charged Van de Graaff generator. The force on a charged particle gives electric field strength E. In equation form, E F/q. FIGURE 33.2 The satellite and the electron both experience forces; they are both in force fields. An electric field has both magnitude and direction. The magnitude (strength) of an electric field can be measured by its effect on charges located in the field. Imagine a small positive “test charge” that is placed in an electric field. Where the force is greatest on the test charge, the field is strongest. Where the force on the test charge is weak, the field is small.33.1 665 CHAPTER 33 ELECTRIC FIELDS AND POTENTIAL 665 Demonstration Hold a fluorescent lamp tube in the field of a charged Van de Graaff generator to show that it lights up when one end of the tube is closer to the dome than the other end. The direction of an electric field at any point, by convention, is the direction of the electrical force on a small positive test charge placed at that point. Thus, if the charge that sets up the field is positive, the field points away from that charge. If the charge that sets up the field is negative, the field points toward that charge. (Be sure to distinguish between the hypothetical small test charge and the charge that sets up the field.) CHECK The electric field is stronger near the dome, and weaker farther away. Charges nearer the dome experience more force, which means more work is done when they are moved in the stronger parts of the field. Thus, each charge in the stronger field has more energy. The energy per charge is what we call potential. Show that when the two ends of the fluorescent tube are equidistant from the charged dome, light emission ceases. ...... CONCEPT How are the magnitude and direction of an electric field determined? FIGURE 33.3 a. In a vector representation of an electric field, the length of the vectors indicates the magnitude of the field. b. In a lines-of-force representation, the distance between field lines indicates magnitudes. 33.2 Electric Field Lines Since an electric field has both magnitude and direction, it is a vector quantity and can be represented by vectors. The negatively charged particle in Figure 33.3a is surrounded by vectors that point toward the particle. (If the particle were positively charged, the vectors would point away from the particle. The vectors always point in the direction of the force that would act on a positive test charge.) The magnitude of the field is indicated by the length of the vectors. The electric field is greater where the vectors are long than it is where the vectors are short. To represent a complete electric field by vectors, you would have to show a vector at every point in the space around the charge. Such a diagram would be totally unreadable! A more useful way to describe an electric field is shown in Figure 33.3b. You can use electric field lines (also called lines of force) to represent an electric field. Where the lines are farther apart, the field is weaker. For an isolated charge, the lines extend to infinity, while for two or more opposite charges, the lines emanate from a positive charge and terminate on a negative charge. Some electric field configurations are shown in Figure 33.4. The photographs in Figure 33.5 show bits of thread that are suspended in an oil bath surrounding charged conductors. The ends of the bits of thread line up end-to-end with the electric field lines. In Figures 33.5a and 33.5b, we see the field lines are characteristic of a single pair of point charges. a The magnitude of an CHECK electric field can be measured by its effect on charges located in the field. The direction of the field at any point is the direction of the electrical force on a positive test charge placed at that point. CONCEPT ...... b FIGURE 33.4 a. The field lines around a single positive charge extend to infinity. b. For a pair of equal but opposite charges, the field lines emanate from the positive charge and terminate on the negative charge. c. Field lines are evenly spaced between two oppositely charged capacitor plates. Teaching Resources • Reading and Study Workbook • PresentationEXPRESS • Interactive Textbook a b c 666 666 33.2 Electric Field FIGURE 33.5 Bits of fine thread suspended in an oil bath surrounding charged conductors line up end to end along the direction of the field. The photos illustrate field patterns for a. equal and opposite charges; b. equal like charges; c. oppositely charged plates; and d. oppositely charged cylinder and plate. Lines Teaching Tip Describe the vector nature of a force field and describe the lines of force as shown in Figure 33.5. Ask If a tiny test charge were dropped in the oil bath shown in Figure 33.5, in what direction would it move? Along the same directions as the bits of thread—away from the conductor of same sign of charge and toward the conductor of opposite sign of charge. Teaching Tidbit Sharks and related species of fish are equipped with specialized receptors in their snouts that sense extremely weak electric fields generated by other creatures in seawater. a b c d The oppositely charged parallel plates in Figure 33.5c produce nearly parallel field lines between the plates. Except near the ends, the field between the plates has a constant strength. Notice that in Figure 33.5d, there is no electric field inside the charged cylinder. The conductor shields the space from the field outside. CHECK How can you represent an electric field? CONCEPT think! Teaching Resources • Reading and Study Workbook A beam of electrons is produced at one end of a glass tube and lights up a phosphor screen at the other end. When the beam is straight, it produces a spot in the middle of the screen. If the beam passes through the electric field of a pair of oppositely charged plates, it is deflected upward as shown. If the charges on the plates are reversed, in what direction will the beam deflect? Answer: 33.2 • Concept-Development Practice Book 33-1 • Problem-Solving Exercises in Physics 16-2 • Transparencies 78, 79 • PresentationEXPRESS • Interactive Textbook ...... CONCEPT ...... You can use electric CHECK field lines (also called lines of force) to represent an electric field. Where the lines are farther apart, the field is weaker. CHAPTER 33 ELECTRIC FIELDS AND POTENTIAL 667 667 33.3 Electric Shielding Teaching Tip Call attention to Figure 33.5d showing that the threads have no directional properties inside the charged cylinder. This shows that the electric field is shielded by the metal. The dramatic photo of the car being struck by lightning (Figure 33.6) also illustrates that the electric field inside a conductor is normally zero, regardless of what is happening outside. Teaching Tip After discussing Figure 33.7, go a step further and consider the test charge off center, twice as far from region A as region B, as shown. 33.3 Electric Shielding The dramatic photo in Figure 33.6 shows a car being struck by lightning. Yet, the occupant inside the car is completely safe. This is because the electrons that shower down upon the car are mutually repelled and spread over the outer metal surface, finally discharging when additional sparks jump from the car’s body to the ground. The configuration of electrons on the car’s surface at any moment is such that the electric fields inside the car practically cancel to zero. This is true of any charged conductor. If the charge on a conductor is not moving, the electric field inside the conductor is exactly zero. Charged Conductors The absence of electric field within a conductor holding static charge does not arise from the inability of an electric field to penetrate metals. It comes about because free electrons within the conductor can “settle down” and stop moving only when the electric field is zero. So the charges arrange themselves to ensure a zero field with the material. Consider the charged metal sphere shown in Figure 33.7. Because of mutual repulsion, the electrons spread as far apart from one another as possible. They distribute themselves uniformly over the surface of the sphere. A positive test charge located exactly in the middle of the sphere would feel no force. The electrons on the left side of the sphere would tend to pull the test charge to the left, but the electrons on the right side of the sphere would tend to pull the test charge to the right equally hard. The net force on the test charge would be zero. Thus, the electric field is also zero. Interestingly enough, complete cancellation will occur anywhere inside the sphere. If the conductor is not spherical, then the charge distribution will not be uniform. The remarkable thing is this: The exact charge distribution over the surface is such that the electric field everywhere inside the conductor is zero. Look at it this way: If there were an electric field inside a conductor, then free electrons inside the conductor would be set in motion. How far would they move? Until equilibrium is established, which is to say, when the positions of all the electrons produce a zero field inside the conductor. FIGURE 33.6 Electrons from the lightning bolt mutually repel and spread over the outer metal surface. The overall electric field inside the car practically cancels to zero. The dotted lines represent a sample cone of action, subtending both A and B. Region A has twice the diameter, four times the area, and four times the charge of region B. Four times the charge at twice the distance will have one fourth the effect. The greater charge is balanced by the correspondingly greater distance. This will be the case for all points inside the conductor. And the conductor need not be a sphere, as shown by the shapes in Figure 33.8. FIGURE 33.7 The forces on a test charge located inside a charged hollow sphere cancel to zero. Teaching Resources • Reading and Study Workbook • PresentationEXPRESS • Interactive Textbook FIGURE 33.8 Static charges are distributed on the surface of all conductors in such a way that the electric field inside the conductors is zero. 668 668 How to Shield an Electric Field There is no way to shield gravity, because gravity only attracts. There are no repelling parts of gravity to offset attracting parts. Shielding electric fields, however, is quite simple. Surround yourself or whatever you wish to shield with a conducting surface. Put this surface in an electric field of whatever field strength. The free charges in the conducting surface will arrange themselves on the surface of the conductor in a way such that all field contributions inside cancel one another. That’s why certain electronic components are encased in metal boxes, and why certain cables have a metal covering—to shield them from all outside electrical activity. CONCEPT CHECK How can you describe the electric field within a conductor holding static charge? ...... FIGURE 33.9 The metal-lined cover shields the internal electrical components from external electric fields. Similarly, a metal cover shields the coaxial cable. Teaching Tip Another point to consider: If the field inside a conductor were not zero, then free charges inside would move, but the movement would not continue forever. The charges would finally move to positions of equilibrium. In these positions their effects on one another would be mutually balanced. There would be complete cancellation of fields everywhere inside the conductor. This is what happens—not gradually, but suddenly. Teaching Tip Revisit the Discover! activity at the beginning of the chapter, but with a twist: Use a small portable radio instead of a cell phone, and a metal screen enclosure instead of aluminum foil. Using a metal screen or meshwork demonstrates that shielding still may occur even with small gaps in the conductor. The foil and screen are examples of Faraday cages. Named after physicist Michael Faraday, a Faraday cage is an enclosure or mesh made from conducting material. If the charge on a conductor is not moving, the electric field inside the conductor is exactly zero. CONCEPT think! It is said that a gravitational field, unlike an electric field, cannot be shielded. But the gravitational field at the center of Earth cancels to zero. Isn’t this evidence that a gravitational field can be shielded? Answer: 33.3 33.4 Electrical Potential Energy Recall the relationship between work and potential energy. Work is done when a force moves something in the direction of the force. An object has potential energy by virtue of its location, say in a force field. For example, if you lift an object, you apply a force equal to its weight. When you raise it through some distance, you are doing work on the object. You are also increasing its gravitational potential energy. The greater the distance it is raised, the greater is the increase in its gravitational potential energy. Doing work increases its gravitational potential energy, as shown in Figure 33.10a. CHECK 33.4 Electrical Potential Energy Key Term electrical potential energy Teaching Tip Briefly review the relationship between work and potential energy (Chapter 9). Explain that, just as doing work on an object increases the object’s gravitational potential energy, the work required to push a charged particle against the electric field of a charged object increases the particle’s electrical potential energy. FIGURE 33.10 a. Work is done to lift the mass against the gravitational field of Earth. In an elevated position, the mass has gravitational potential energy. When released, this energy is transferred to the piling below. b. Similar energy transfer occurs for electric charges. ...... CHAPTER 33 ELECTRIC FIELDS AND POTENTIAL 669 669 The electrical potential energy of a charged particle is increased when work is done to push it against the electric field of something else that is charged. CONCEPT CHECK ...... a 33.5 Electric Potential Key Terms electric potential, volt, voltage Common Misconceptions Electrical potential energy and electric potential are the same. FACT Electric potential is electrical potential energy per charge. b FIGURE 33.11 The small positive charge has more potential energy when it is closer to the positively charged sphere because work is required to move it to the closer location. The voltage produced by rubbing a balloon on one’s hair is low compared to the voltage of electric circuits in the household. The voltage resulting from rubbing a balloon on hair could be several thousand volts. FACT In a similar way, a charged object can have potential energy by virtue of its location in an electric field. Just as work is required to lift an object against the gravitational field of Earth, work is required to push a charged particle against the electric field of a charged body. (It may be more difficult to visualize, but the physics of both the gravitational case and the electrical case is the same.) The electrical potential energy of a charged particle is increased when work is done to push it against the electric field of something else that is charged. Figure 33.11a shows a small positive charge located at some distance from a positively charged sphere. If we push the small charge closer to the sphere (Figure 33.11b), we will expend energy to overcome electrical repulsion. Just as work is done in compressing a spring, work is done in pushing the charge against the electric field of the sphere. This work is equal to the energy gained by the charge. The energy a charge has due to its location in an electric field is called electrical potential energy. If the charge is released, it will accelerate in a direction away from the sphere, and its electrical potential energy will transform into kinetic energy. CONCEPT CHECK How can you increase the electrical potential energy of a charged particle? Teaching Tip Use a Van de Graaff generator to illustrate the difference between electrical potential energy and electric potential. Although the generator is normally charged to thousands of volts, the amount of charge is relatively small, so the electrical potential energy is relatively small. A person is not normally harmed when the charge discharges through his or her body because very little energy flows through the person. In contrast, it would be unadvisable to intentionally become the short-circuit for the household 110 V because, although the voltage is much lower, the transfer of energy is appreciable. Less energy per charge, but many many more charges! 33.5 Electric Potential Distinguishing between electrical potential energy and electric potential is high-level physics! If in the preceding discussion we push two charges instead, we do twice as much work. The two charges in the same location will have twice the electrical potential energy as one; a group of ten charges will have ten times the potential energy; and so on. Rather than deal with the total potential energy of a group of charges, it is convenient when working with electricity to consider the electrical potential energy per charge. The electrical potential energy per charge is the total electrical potential energy divided by the amount of charge. At any location the potential energy per charge — whatever the amount of charge—will be the same. For example, an object with ten units of charge at a specific location has ten times as much potential energy as an object with a single unit of charge. But it also has ten times as much charge, so the potential energy per charge is the same. The concept of electrical potential energy per charge has a special name, electric potential. electric potential electrical potential energy charge Electric potential is not the same as electrical potential energy. Electric potential is electrical potential energy per charge. 670 670 ...... FIGURE 33.12 An object of greater charge has more electrical potential energy in the field of the charged dome than an object of less charge, but the electric potential of any amount of charge at the same location is the same. The SI unit of measurement for electric potential is the volt, named after the Italian physicist Allesandro Volta (1745–1827). The symbol for volt is V. Since potential energy is measured in joules and charge is measured in coulombs, joule 1 volt 1 coulomb Thus, a potential of 1 volt equals 1 joule of energy per coulomb of charge; 1000 volts equals 1000 joules of energy per coulomb of charge. If a conductor has a potential of 1000 volts, it would take 1000 joules of energy per coulomb to bring a small charge from very far away and add it to the charge on the conductor.33.5 (Since the small charge would be much less than one coulomb, the energy required would be much less than 1000 joules. For example, to add the charge of one proton to the conductor, 1.6 × 10–19 C, it would take only 1.6 × 10–16 J of energy.) Since electric potential is measured in volts, it is commonly called voltage. In this book the names will be used interchangeably. The significance of voltage is that once the location of zero voltage has been specified, a definite value for it can be assigned to a location whether or not a charge exists at that location. We can speak about the voltages at different locations in an electric field whether or not any charges occupy those locations. Rub a balloon on your hair and the balloon becomes negatively charged, perhaps to several thousand volts! If the charge on the balloon were one coulomb, it would take several thousand joules of energy to give the balloon that voltage. However, one coulomb is a very large amount of charge; the charge on a balloon rubbed on hair is typically much less than a millionth of a coulomb. Therefore, the amount of energy associated with the charged balloon is very, very small—about a thousandth of a joule. A high voltage requires great energy only if a great amount of charge is involved. This example highlights the difference between electrical potential energy and electric potential. CONCEPT think! If there were twice as much charge on one of the charged objects near the charged sphere in Figure 33.12, would the electrical potential energy of the object in the field of the charged sphere be the same or would it be twice as great? Would the electrical potential of the object be the same or would it be twice as great? Answer: 33.5 Teaching Tip Ask students how much energy is needed to put positive charges on a spherical conductor until it has a total potential V. Tell them to think of bringing positive charges up to the conductor one by one. It takes no energy to put the first charge on the conductor because there are no electric forces acting on the charge. Now that the conductor has a positive charge, it takes a little energy to bring a second positive charge up to it. It takes more energy to bring the third charge because it is acted on by twice the force that acted on the second charge. It takes more and more energy to add each successive charge. The total amount of energy needed to put all the charges on the sphere is 0.5QV, where V is the final potential on the surface of the sphere and Q is the total charge on it. It also turns out that the charge needed to produce a certain potential on the surface of a charged sphere depends on the radius R. The charge is Q 5 RV/k, where k is the Coulomb constant. As a numerical example, suppose a conducting sphere of radius 10 cm has a potential of 45,000 V. The charge on the sphere is then Q 5 RV/k 5 5.0 3 1027 C. The total energy needed to assemble this charge is 0.5QV 5 0.011 J. As this example shows, even high potentials involve very little energy! FIGURE 33.13 Although the voltage of the charged balloon is high, the electrical potential energy is low because of the small amount of charge. Electric potential is electrical potential energy per charge. CONCEPT CHECK Teaching Resources • Concept-Development Practice Book 33-2 • Problem-Solving Exercises in Physics 16-3 CHECK What is the difference between electric potential and electrical potential energy? CHAPTER 33 ELECTRIC FIELDS AND POTENTIAL ...... ...... 671 671 33.6 Electrical Energy Storage Key Term capacitor Common Misconception A capacitor is a source of electrical energy. FACT Energy from a capacitor comes from the work done in charging the capacitor. 33.6 Electrical Energy Storage Electrical energy can be stored in a common device called a capacitor. Capacitors are found in nearly all electronic circuits. Computer memories use very tiny capacitors to store the 1’s and 0’s of the binary code. Some keyboards have them beneath each key. Capacitors in photoflash units store larger amounts of energy slowly and release it rapidly during the short duration of the flash. Similarly, but on a grander scale, enormous amounts of energy are stored in banks of capacitors that power giant lasers in national laboratories. The simplest capacitor is a pair of conducting plates separated by a small distance, but not touching each other. When the plates are connected to a charging device such as the battery shown in Figure 33.14, charge is transferred from one plate to the other. This occurs as the positive battery terminal pulls electrons from the plate connected to it. These electrons in effect are pumped through the battery and through the negative terminal to the opposite plate. The capacitor plates then have equal and opposite charges—the positive plate is connected to the positive battery terminal, and the negative plate is connected to the negative battery terminal. The charging process is complete when the potential difference between the plates equals the potential difference between the battery terminals—the battery voltage. The greater the battery voltage and the larger and closer the plates, the greater the charge that is stored. In practice, the plates may be thin metallic foils separated by a thin sheet of paper. This “paper sandwich” is then rolled up to save space and may be inserted into a cylinder. Such a practical capacitor is shown with others in Figure 33.15. (We will consider the role of capacitors in circuits in the next chapter.) Teaching Tip Show some common capacitors to your class. Teaching Tip The capacitance of a capacitor, the ratio of net change on each plate to the potential difference created by the separated charges, is measured in units of farads (F). The farad is named after Michael Faraday. FIGURE 33.14 A simple capacitor consists of two closely spaced metal parallel plates. When connected to a battery, the plates become equally and oppositely charged. FIGURE 33.15 In these capacitors, the plates consist of thin metallic foils that have been rolled up into a cylinder. 672 672 Capacitors store and hold electric charges until discharged. A charged capacitor is discharged when a conducting path is provided between the plates. Note that a capacitor might store charge even after the electricity to a device has been turned off—for seconds, minutes, or even longer. Discharging a capacitor can be a shocking experience if you happen to be the conducting path. The energy transfer can be fatal where high voltages are present. That’s the main reason for the warning labels on devices such as TV sets. The energy stored in a capacitor comes from the work done to charge it. The energy is in the form of the electric field between its plates. Between parallel plates the electric field is uniform, as indicated in Figures 33.4c and 33.5c on previous pages. So the energy stored in a capacitor is energy stored in the electric field. Electric fields are storehouses of energy. We will see in the next chapter that energy can be transported over long distances by electric fields, which can be directed through and guided by metal wires or directed through empty space. In Chapter 37 we will see how energy from the sun is radiated in the form of electric and magnetic fields. The fact that energy is contained in electric fields is truly far-reaching. CONCEPT The energy stored in a capacitor comes from the work done to charge it. CONCEPT CHECK Teaching Resources • Reading and Study Workbook • Laboratory Manual 92 • Probeware Lab Manual 15 • PresentationEXPRESS • Interactive Textbook • Next-Time Question 33-1 FIGURE 33.16 Mona El Tawil-Nassar adjusts demonstration capacitor plates. 33.7 The Van de Graaff Generator Common Misconception High voltage is dangerous under any conditions. FACT A high voltage is not dangerous if only a small amount of charge is involved. CHECK Where does the energy stored in a capacitor come from? ...... Link to TECHNOLOGY Ink-Jet Printers The printhead of an ink-jet printer typically ejects a thin, steady stream of thousands of tiny ink droplets each second as it shuttles back and forth across the paper. As the stream flows between electrodes that are controlled by the computer, selective droplets are charged. The uncharged droplets then pass undeflected in the electric field of a parallel plate capacitor and form the image on the page; the charged droplets are deflected and do not reach the page. Thus, the image produced on the paper is made from ink droplets that are not charged. The blank spaces correspond to deflected ink that never made it to the paper. Teaching Tip End your lecture on this chapter with a return to the Van de Graaff demo and discussion of the lack of current in the lamp when there was no potential difference across its ends. This is the lead-in to the next chapter. 33.7 The Van de Graaff Generator A common laboratory device for building up high voltages is the Van de Graaff generator. This is the lightning machine often used by “evil scientists” in old science fiction movies. A simple model of the Van de Graaff generator is shown in Figure 33.17. 673 673 CHAPTER 33 ELECTRIC FIELDS AND POTENTIAL ...... Demonstration Demonstration If you did not do so in Chapter 32, now is a good time to use the Van de Graaff generator to show the repulsion of like charges. Crank up the generator with a dozen 10-in. aluminum pie pans resting on top of the sphere. The weight of the pans above each pan is greater than the force of repulsion between the pans and so they remain on the sphere—all except the pan on top, which has no pans on top of it. The top pan “floats” off and the second pan becomes the top pan. It too floats off. This continues until all the pans have floated off one by one. FIGURE 33.17 In a Van de Graaff generator, a moving rubber belt carries electrons from the voltage source to a conducting sphere. An electric field is nature’s storehouse of electrical energy. If you are lucky, the pans will land one on top of another. Students usually laugh and applaud when the last one flies off. It is one of those demos that makes the class shout, “Do it again!” The voltage of a Van de Graaff generator can be increased by increasing the radius of the sphere or by placing the entire system in a container filled with highpressure gas. CONCEPT CHECK Teaching Resources • Reading and Study Workbook • Next-Time Question 33-2 FIGURE 33.18 The physics enthusiast and the dome of the Van de Graaff generator are charged to a high voltage. A large hollow metal sphere is supported by a cylindrical insulating stand. A motor-driven rubber belt inside the support stand moves past a comblike set of metal needles that are maintained at a high electric potential. A continuous supply of electrons is deposited on the belt through electric discharge by the points of the needles and is carried up into the hollow metal sphere. The electrons leak onto metal points (which act like tiny lightning rods) attached to the inner surface of the sphere. Because of mutual repulsion, the electrons move to the outer surface of the conducting sphere. (Remember, static charge on any conductor is on the outside surface.) This leaves the inside surface uncharged and able to receive more electrons as they are brought up the belt. The process is continuous, and the charge builds up to a very high electric potential—on the order of millions of volts. Touching a Van de Graaff generator can be a hairraising experience, as shown in Figure 33.18. A sphere with a radius of 1 m can be raised to a potential of 3 million volts before electric discharge occurs through the air (because breakdown occurs in air when the electric field strength is about 3 × 106 V/m).33.7 The voltage of a Van de Graaff generator can be increased by increasing the radius of the sphere or by placing the entire system in a container filled with highpressure gas. Van de Graaff generators in pressurized gas can produce voltages as high as 20 million volts. These devices accelerate charged particles used as projectiles for penetrating the nuclei of atoms. CONCEPT ...... CHECK How can the voltage of a Van de Graaff generator be increased? 674 674 ...... ...
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This note was uploaded on 11/18/2010 for the course CPHY 101 taught by Professor Jay during the Spring '10 term at University of Massachusetts Boston.

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