Lecture_26 - increases kinetic energy, which increases...

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Thermodynamics Engines and Entropy
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Thermodynamic Devices Engines use heat to work. Most efficient engine (perpetual motion machine) will convert all heat input to work. Carnot showed this is impossible. All engines expel some heat. W = Q H Q C e = W Q H e = 1 Q C Q H
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Carnot's Engine Q C Q H = T C T H e = 1 T C T H Alternative formulation of the 2 nd Law: No heat engine can have greater efficiency than this
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Examples Combustion engine, Stirling engines, drinking bird, perpetual motion (impossible)
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Refrigerator Carnot engine is reversible, can run it backwards Heat flows from cool to warm, but this doesn't violate the 2 nd law since it doesn't occur spontaneously W + Q C = Q T e fridge = Q C W = T C T H 1 T C T H Want this to be low but this means low efficiency
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Heat Pump Thermodynamically identical to the refrigerator e heat pump = Q H W = 1 1 T C T H W pump = Q H ( 1 T C T H ) Can be greater than 1!
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Entropy Entropy, S, is a measure of the disorder in a system Entropy is a macroscopic property of a system Increasing temperature
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Unformatted text preview: increases kinetic energy, which increases disorder Adding heat always changes entropy: S = Q T Entropy S is 0 for a reversible process. S > 0 for irreversible processes 2 nd Law can be stated: S Energy Conversion It is possible to convert all kinetic or potential energy of a system to heat energy (100% efficiency). 2 nd Law says it's not possible to do the reverse. Impossible because of the disorder (entropy) of the system. Entropy and Probability 3 rd Law Since , and it is impossible to have negative kinetic energy, it is implies that temperatures under T = 0K are not possible. 2 nd Law tells us that all heat engines must expel heat to a cold reservoir, to a cold reservoir with T = 0K is not possible It is impossible for the temperature of a system to reach absolute zero. KE = 3 2 N k B T...
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This note was uploaded on 12/07/2011 for the course PHYS 220 taught by Professor Chang during the Fall '09 term at Purdue University-West Lafayette.

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Lecture_26 - increases kinetic energy, which increases...

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