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Unformatted text preview: The Flow of Energy
Where it comes from; where it goes Energy as a tool in physics UCSD: Physics 8; 2 Energy is a very abstract notion, but it is a very useful and quantifiable notion We use the conservation of energy to predict behavior by setting E = mgh + mv2 = constant we can elucidate the value of the velocity at any height: v2 = 2g (height fallen from rest) We rely on the fact that energy is not created out of nowhere Where did the energy we see around us come from? most of what we use derives from the sun some derives from other, exploded stars (nuclear fission) ultimately, all of it was donated in the Big Bang but surprisingly, the net energy of the universe can be (and looks to be) zero! UCSD: Physics 8; 2 Energy is Conserved Conservation of Energy is different from Energy Conservation, the latter being about using energy wisely Conservation of Energy means energy is neither created nor destroyed. The total amount of energy in the Universe is constant!! Don't we create energy at a power plant? No, we simply transform energy at our power plants Doesn't the sun create energy? Nope--it exchanges mass for energy UCSD: Physics 8; 2 Energy Exchange Though the total energy of a system is constant, the form of the energy can change A simple example is that of a pendulum, in which a continual exchange goes on between kinetic and potential energy
pivot K.E. = 0; P. E. = mgh h K.E. = 0; P. E. = mgh
height reference P.E. = 0; K.E. = mgh Perpetual Motion UCSD: Physics 8; 2 Why won't the pendulum swing forever? It's impossible to design a system free of energy paths The pendulum slows down by several mechanisms Friction at the contact point: requires force to oppose; force acts through distance work is done Air resistance: must push through air with a force (through a distance) work is done Gets some air swirling: puts kinetic energy into air (not really fair to separate these last two) Perpetual motion means no loss of energy solar system orbits come very close UCSD: Physics 8; 2 Some Energy Chains: A toilet bowl with some gravitational potential energy is dropped potential energy turns into kinetic energy kinetic energy of the toilet bowl goes into: ripping the toilet bowl apart (chemical: breaking bonds) sending the pieces flying (kinetic) into sound into heating the ground and pieces through friction as the pieces slide to a stop In the end, the local environment is slightly warmer UCSD: Physics 8; 2 How Much Warmer? A 20 kg toilet bowl held 1 meter off the ground has 200 J of gravitetional potential energy mgh = (20 kg)(10 m/s2)(1 m) = 200 kgm2/s2 = 200 J A typical heat capacity is 1000 J/kg/C (a property of the material) So 200 J can heat 0.2 kg of material by 1C or 1 kg by 0.2C or 20 kg by 0.01C heat capacity follows intuitive logic: to get same T, need more energy or less mass given fixed energy input, get smaller T for larger mass for a given mass, get larger T for more energy input So how much mass is effectively involved? initially not much (just contact surfaces): so hot at first but heat diffuses into surrounding bulk: cools down so answer is ill-defined: depends on when But on the whole, the temperature rise is hardly noticeable UCSD: Physics 8; 2 Gasoline Example Put gas in your car Combust gas, turning chemical energy into kinetic energy of the explosion (motion of gas particles) Transfer kinetic energy of gas to piston to crankshaft to drive shaft to wheel to car as a whole That which doesn't go into kinetic energy of the car goes into heating the engine block (and radiator water and surrounding air), and friction of transmission system (heat) Much of energy goes into stirring the air (ends up as heat) Apply the brakes and convert kinetic energy into heat It all ends up as waste heat, ultimately UCSD: Physics 8; 2 Bouncing Ball Superball has gravitational potential energy Drop the ball and this becomes kinetic energy Ball hits ground and compresses (force times distance), storing energy in the spring Ball releases this mechanically stored energy and it goes back into kinetic form (bounces up) Inefficiencies in "spring" end up heating the ball and the floor, and stirring the air a bit In the end, all is heat UCSD: Physics 8; 2 Why don't we get hotter and hotter If all these processes end up as heat, why aren't we continually getting hotter? If earth retained all its heat, we would get hotter All of earth's heat is radiated away as infrared light hotter things radiate more heat If we dump more power, the temperature goes up, the radiated power increases dramatically comes to equilibrium: power dumped = power radiated stable against perturbation: T tracks power budget UCSD: Physics 8; 2 Another Piece of the Energy Zoo: Light The power given off of a surface in the form of light is proportional to the fourth power of that surface's temperature! P = AT4 in Watts the constant, , is numerically 5.67 10-8 W/K4/m2 easy to remember constant: 5678 A is surface area of hot thing, in square meters temperature must be in Kelvin: K = C + 273 C = (5/9) (F 32) Example: radiation from your body:
(1 m2)(5.67 10-8) (310)4 = 523 Watts (if naked in the cold of space: don't let this happen to you!) UCSD: Physics 8; 2 Radiant Energy, continued Example: The sun is 5800K on its surface, so:
P/A = T4 = (5.67 10-8) (5800)4 = 6.4 107 W/m2 Summing over entire surface area of sun gives 3.9 1026 W Compare to total capacity of human energy "production" on earth: 3.3 1012 W Single power plant is typically 0.51.0 GW (109 W) In earthly situations, radiated power out is partially balanced by radiated power in from other sources Not 523 W/m2 in 70F room, more like 100 W/m2 goes like Th4 Tc4 UCSD: Physics 8; 2 Rough numbers How much power does the earth radiate? P/A = T4 for T = 288K = 15C is 390 W/m2 Summed over entire surface area (4R2, where R = 6,378,000 meters) is 2.0 1017 W For reference, global "production" is 3 1012 W Solar radiation incident on earth is 1.8 1017 W just solar luminosity of 3.9 1026 W divided by geometrical fraction that points at earth Amazing coincidence of numbers! (or is it...) UCSD: Physics 8; 2 No Energy for Free No matter what, you can't create energy out of nothing: it has to come from somewhere We can transform energy from one form to another; we can store energy, we can utilize energy being conveyed from natural sources The net energy of the entire Universe is constant The best we can do is scrape up some useful crumbs UCSD: Physics 8; 2 Energy and Calories A calorie is a unit of energy (1 cal is the amount of energy required to raise the temperature of 1 cc of water 1C.) 1 cal = 4.184 J Food Calories are measured in kcal (1 Cal = 1000 cal) 1 Cal = 4184 J 250 Calories is enough energy to raise 250 liters (about 66 gallons) of water 1C. UCSD: Physics 8; 2 Human Energy Requirements 1,500 Calories per day just to be a couch-potato 6,280,000 J Average human power consumption is then: 6.28 MJ / 86,400 seconds 75 W We're like light bulbs, constantly putting out heat Need more like 2,000 Cal for active lifestyle 100 W of power UCSD: Physics 8; 2 Energy from Food Energy from fat, carbohydrates, protein 9 Calories per gram for fat 7 Calories per gram for alcohol 4 Calories per gram for carbohydrate Fiber part doesn't count 4 Calories per gram for protein Calculate 63 fat, 84 CH, 40 protein Cals total is 187 Calories (180 is in the ballpark) 1 Calorie (kilo-calorie) is 4,187 J 180 Cal = 753 kJ set equal to mgh climb 1100 m vertically, assuming perfect efficiency UCSD: Physics 8; 2 Not So Fast... Human body isn't 100% efficient: more like 25% To put out 100 J of mechanical work, must eat 400 J 180 Calorie candy bar only gets us 275 m, not 1100 m Maximum sustained power output (rowing, cycling) is about 150-200 W (for 70 kg person) Consuming 600-800 W total, mostly as wasted heat For 30 minutes 800 J/s 1800 s = 1.44 MJ = 343 Cal Can burst 700 W to 1000 W for < 30 sec put out a full horsepower momentarily! UCSD: Physics 8; 2 Most impressive display of human power The Gossamer Albatross crossed the English Channel in 1979, powered by Bryan Allen Flight took 49 minutes, wiped Bryan out! Sustained power out ~250 W UCSD: Physics 8; 2 Human Energy Requirements Summarized We need chemical energy from food to run Ultimate source is sun, long chain of events to twinkies Constantly burn energy at rate of 75-100W We spend energy at about 25% efficiency Maximum sustained power is 150-200 W actually burn 4 times this due to inefficiencies UCSD: Physics 8; 2 Exercise Ways to transform chemical energy of food work and heat When we exercise, we do a little bit of both, but mostly we transform energy from food into heat 3:1 ratio, given 25% efficiency UCSD: Physics 8; 2 Air Resistance We're always "neglecting air resistance" in physics Can be difficult to deal with Affects projectile motion Friction force opposes velocity through medium Imposes horizontal force, additional vertical forces Terminal velocity for falling objects Dominant energy drain on cars, bicyclists, planes UCSD: Physics 8; 2 Drag Force Quantified With a cross sectional area, A (in m2), coefficient of drag of 1.0 (most objects), sea-level density of air, and velocity, v (m/s), the drag force is: Fdrag = cDAv2 Newtons cD is drag coefficient: ~1.0 for most things, 0.35 for car is density of medium: 1.3 kg/m3 for air, 1000 kg/m3 water typical object in air is then Fdrag 0.65Av2 Example: Bicycling at 10 m/s (22 m.p.h.), with projected area of 0.5 m2 exerts 32.5 Newtons requires Fv of power 325 Watts to maintain speed "Free" Fall For 75 kg person subtending 0.5 m2, vterm 50 m/s, or 110 m.p.h. UCSD: Physics 8; 2 Terminal velocity reached when Fdrag = Fgrav (= mg) which is reached in about 5 seconds, over 125 m of fall actually takes slightly longer, because acceleration is reduced from the nominal 10 m/s2 as you begin to encounter drag Free fall only lasts a few seconds, even for skydivers UCSD: Physics 8; 2 Announcements/Assignments Next up: a simple model for molecules/lattices electrons, charge, current, electric fields Assignments: read chapter 7, pp. 212214, 225228 read chapter 3, 8387; chapter 9 265269, 278279 HW1: 1.E.4, 1.E.7, 1.E.8, 1.E.20, 1.E.25, 1.E.34, 1.P.1, 1.P.8, 1.P.10 (in Newtons), 1.P.14, 1.P.16, 1.P.18, 1.P.22, 2.E.28, 2.P.10, 2.P.11: due 4/13 First Q/O due Friday, 4/14 by 6PM via WebCT ...
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