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Unformatted text preview: UCSD Physics 12 Conservation and Exchange of Energy
Nothing Comes for Free UCSD Physics 12 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 amount of energy in the Universe is constant!! Don't we create energy at a power plant? Oh that this were true--no, we simply transform energy at our power plants Doesn't the sun create energy? Nope--it exchanges mass for energy Spring 2010 2 UCSD Physics 12 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 simple 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 Spring 2010 P.E. = 0; K.E. = mgh Q 3 UCSD Physics 12 Perpetual Motion Why won't the pendulum swing forever? It's hard 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 Spring 2010 4 UCSD Physics 12 Some Energy Chains: A coffee mug with some gravitational potential energy is dropped potential energy turns into kinetic energy kinetic energy of the mug goes into: ripping the mug apart (chemical: breaking bonds) sending the pieces flying (kinetic) into sound into heating the floor and pieces through friction as the pieces slide to a stop In the end, the room is slightly warmer
Spring 2010 5 UCSD Physics 12 Gasoline Example Put gas in your car, containing 9 Cal/g Combust gas, turning 9 Cal/g into kinetic energy of explosion 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 Spring 2010 6 UCSD Physics 12 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
Spring 2010 7 UCSD Physics 12 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 F = T 4 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 Spring 2010 8 UCSD Physics 12 Rough numbers How much power does the earth radiate? F = 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 comparison, U.S. 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...)
Q 9 UCSD Physics 12 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 Spring 2010 2 Q 10 UCSD Physics 12 Shift Gears: The Global Energy Scene Global energy production is about 400 QBtu/yr a QBtu is a quadrillion Btu, or 1015 Btu so about 4 1020 J per year U.S. share is about one fourth of this (1020 J) 1996 value in book (1st edition) is 93 QBtu/year 2003 value in second edition is 98.3 QBtu/year 1020 J/yr = 3 1012 W divided by 300 million people (3 108) = 104 W per person (10 kW) Spring 2010 11 UCSD Physics 12 Reminder: how do we stack up? world average = 10 bbl/yr Spring 2010 12 UCSD More Countries Logarithmic scale Fills in the gaps 1971 data Physics 12 Spring 2010 13 UCSD Evolution of Energy Sources Physics 12 Spring 2010 14 UCSD Physics 12 U.S. Consumption in 2003
Source Coal Natural Gas Petroleum Nuclear Renewables Total Spring 2010 Amount QBtu Percent 1018 Joules 23% 22.9% 39.8% 8.1% 6.3% 100% 15 23.8 23.7 41.3 8.4 6.5 103.7 1.08 109 tons 22.6 21.8 1012 ft3 6.72 109 bbl 22.5 39.1 757 109 kWh 7.97 578 109 kWh 6.15 98.3 UCSD Physics 12 The Fall of the Work Animal Used to rely completely on animals for transportation Trains entered the picture in the mid-1800s Cars entered the scene in a big way around 1920 World has never been the same Work animal fell off the map around 1940 Today automotive is over 95% of the story Spring 2010 16 UCSD Physics 12 Energy Sources and Destinations Spring 2010 17 UCSD U.S. Consumption vs. Production Physics 12 policy change Spring 2010 18 UCSD Physics 12 Where is our energy produced, and of what flavor? Spring 2010 19 UCSD Physics 12 Lessons Our energy production is completely dominated by fossil fuels, with only about 15% coming from nuclear and hydroelectric hydroelectric is the only truly renewable resource of the two Part of our enormous appetite is due to the expanse of our country: transportation is important Space heating is also an issue in a country where detached houses are the rule Any industrial society (at our current scale) is going to have a large demand for energy
Spring 2010 20 UCSD Physics 12 References & Assignments Very good book on energy: ENERGY: A Guidebook, by Janet Ramage (more global perspective) A recent amazing book: Sustainable Energy--without the hot air, by David MacKay www.withouthotair.com (get book for free!) see 10-page synopsis for quick-read/intro Assignments Read Chapter 2 Homework #1 due Friday, April 9 Homework #2 will be found on the web by week's end: go to Assignments page for link start early on this one (toughest of quarter?) Spring 2010 21 ...
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