10prob - Physics 9B-C Assignment #10 Cole [20Q.1]...

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Physics 9B-C Assignment #10 Cole [20Q.1] Irreversible, because for a reversible process it must be possible for all things involved in the process must be returned to their initial states. Although the pot has been returned to it's initial state there is the entropy lost by the burner < Q T H 0 and entropy gained + Q T C by the kitchen at T C . Note the kitchen gains more entropy than the burner loses because the denomintor is smaller for the kitchen. [20Q.4] The coils on the low pressure side of the compressor get cold and absorb heat from the room the coils on the high pressure side get hot and exhaust heat to the room. No airconditioner is perfectly efficient (second law), so the amount of heat exhausted is larger than the amount absorbed. Thus, if both the cold coils and hot coils are inside the room, the room will actually get hotter, although there will be a local area near the cold coils that will be cooler. The only way to effect a net cooling of the room is to stick the hot coils outside. The cooler of ice would make the room cooler as heat flows from the room into the ice. The net entropy change is positive because the room loses heat at a higher temperature Î S room = < Q T H 0 , but the ice gains heat at a lower temperature, so Î S ice = + Q T C > | Î S | õ ÎS total > 0. Thus, this process would not violate the second law. [20Q.6] If a human being were created by all of the molecules coming together and one instant and spontaneously creating the person, yes, it would be most improbable! This is not the way it happens. The method is called stratified stability. Atoms bang into each other and at first form simple molecules. When the simple molecules become dense enough (they form a more complex stable level), it becomes probable that some will bang into eachother forming complex molecules (the next level). When these become dense enough, it becomes probable that the next higher level will evolve, ad infinitum. Hence biological evolution does not violate the statistical interpretation of entropy. Note that the thermodynamic version of the second law does not really apply because living biological systems are "far" from being in equilibrium. Biological processes are highly irreversible and produce far more entropy than the ordering would negate. [20.17] a) From the first law: QQ U W CH += + . Because the device is run in a cycle, U = 0 , and Q c = 5.00 kJ. õ WQ Q C H =+ . Use the second law. The entropy change of the device for one cycle is zero õ Q T Q T H H c c 0 õ Q T T Q H H c c = = () = 298 263 55 6 7 kJ kJ . . W = 500 . kJ - 5.67 kJ = - 0.67 kJ and it acts as a refrigerator b) Q H = = 273 263 1 9 kJ kJ .
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This note was uploaded on 05/19/2008 for the course PHY 009B taught by Professor Cole during the Fall '07 term at UC Davis.

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10prob - Physics 9B-C Assignment #10 Cole [20Q.1]...

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