PffpPossibleEssay - Physics for Future Presidents Possible...

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Physics for Future Presidents Possible Essays HYDROGEN ECONOMY Difficulties o Hydrogen is not very dense; takes up too much space Compared to gasoline, liquid hydrogen has 3x more energy per gram 3x less energy per gallon o Boils at -423 F Transported in special thermos bottles Transported in a form in which combined with other materials at room temperature, although more weight Transported as compressed gas, but weight of tank exceeds weight of hydrogen o Obtained by electrolysis or breaking apart methane gas Takes energy Thus, hydrogen is a means for transporting energy It is not a source of energy Breaking apart methane gas releases carbon dioxide Possibilities o Can be used for buses and tucks, or possibly airplanes o “hydrogen economy”—hydrogen as more widely used fuel o 2.6 times more chemical energy per gram than gasoline o Only waste product is water o Unlike battery, when you run out of fuel all you have to do is provide more fuel SOLAR POWER Difficulties o Inefficiency of solar cells Cheap solar cell converts 15% power: 150 watts per square meter Expensive solar cell converts 30% power: 300 watts per square meter o Prohibitive cost Installation costs $10 per square foot at least o Solar-powered automobiles 1 square m sunlight = 1 kilowatt ≈ 1 horsepower Expensive solar cells 30% efficient: 300 watts for 1 square m Automobiles usually use 100-400 horsepower! Possibilities o Energy of sunlight is 1 kilowatt per square meter 1 kilowatt ≈ 1 horsepower Energy needed for one house o “renewable” Sunlight keeps coming as long as the sun shines Can be converted to electricity through silicon solar cells o It is not impractical A gigawatt, what a nuclear power plant produces, would take up 7 square kilometers California’s typical peak power use is 50 gigawatts of electrical power This would require 350 square kilometers of solar cells That is 1/1000 of the area of California o It’s not just available during the day Hydrogen fuel cells could store solar energy for use at night
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Physics for Future Presidents Possible Essays REFRIGERATORS & HEAT PUMPS Temperature difference is required to extract useful energy from heat (2 nd law thermodynamics) Heat engine requires temperature difference o Something hot to provide the energy o Something cold for the heat to flow to o Reversing this creates a refrigerator or heat pump Mechanical force to reduce pressure o Temperature of gas will decrease by gas law equation o Gas law equation: P = constant x T o Cool gas can then be used to freeze ice or cool room Mechanical force must push piston against room air pressure o This slightly heats the air o Thus, one side is being heated while the other is being cooled
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This note was uploaded on 04/01/2008 for the course L&S 70 taught by Professor Mueller during the Spring '08 term at Berkeley.

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PffpPossibleEssay - Physics for Future Presidents Possible...

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