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# Lect31 - Physics 101 Lecture 31 Thermodynamics part 2 q q q...

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Physics 101: Lecture 31, Pg 1 Physics 101: Physics 101: Lecture 31 Lecture 31 Thermodynamics, part 2 Thermodynamics, part 2 Review of 1st law of thermodynamics 2nd Law of Thermodynamics Engines and Refrigerators The Carnot Cycle

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Physics 101: Lecture 31, Pg 2 Quick Review Quick Review 1st Law of Thermodynamics: energy conservation Q   U + W Heat flow  into  (or out of) system Increase  (or decrease) in internal energy of system Work done by  (or on) system V P   U depends only on T  (U = 3nRT/2 = 3PV/2)  Point on P-V plot completely specifies     state of system  (PV = nRT)  work done is area under curve  for complete cycle U=0   Q=W
Physics 101: Lecture 31, Pg 3 Second Law of Thermodynamics Second Law of Thermodynamics Not all processes that are allowed by energy conservation occur in nature. Why ? Example: Stone falls from height h:  mgh ->  ½  m v 2  (just before impact) -> heat  (contact with floor) This process is consistent with energy conservation. The reversed process:  Stone lying on floor cools down and moves upward to height h,  has never been observed in nature, although it is also allowed by  energy conservation:  Q->1/2 mv 2 ->mgh  Or: Ice melts but water does not spontaneously freeze, heat flows from hot to cold but never from cold to hot.  We need a new concept which makes these (reversed) processes       highly unlikely.

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Physics 101: Lecture 31, Pg 4 New concept: Entropy (S) New concept: Entropy (S) A measure of “disorder” or probability of state of a system. A property of a system (=state function, just like P, V, T, U) related to number of different “states” of system Examples of increasing entropy: ice cube melts gases expand into vacuum Change in entropy: S = Q/T  (T in K !)     SI unit: [J/K] » >0 if heat flows into system (Q>0) » <0 if heat flows out of system (Q<0)
Physics 101: Lecture 31, Pg 5 Reversible vs. Irreversible changes in a thermodynamic system: Reversible vs. Irreversible changes in a thermodynamic system: Reversible changes are conceived to be those that would occur very

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Lect31 - Physics 101 Lecture 31 Thermodynamics part 2 q q q...

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