445lec10 - PHYS 445 Lecture 10 Systems in thermal contact...

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PHYS 445 Lecture 10 - Systems in thermal contact 10 - 1 © 2001 by David Boal, Simon Fraser University. All rights reserved; further resale or copying is strictly prohibited. Lecture 10 - Systems in thermal contact What's Important: heat flow heat reservoirs thermal contact mechanical contact Text : Reif Heat flow (not covered in class) This result is rather intuitive, but is worthwhile establishing nevertheless. Suppose that only an infinitesimally small amount of heat is exchanged between two systems A and A' . Then, the condition of increasing entropy is S + S ' 0 ln ( E ) ( 29 +∆ ln '( E ') ( 29 0 ln ( E ) ( 29 E •∆ E + ln '( E ') ( 29 E ' •∆ E ' 0 ß •∆ E + ß ' E ' 0 For small heat flows, E ~ Q E' ~ - Q such that ßQ - ß'Q 0 or ( ß - ß' ) Q 0. (10.1) What does this tell us about heat flow? If A is colder than A' , then ß ß' and ( ß - ß' ) 0. Eq. (10.1) then says that Q is positive for this situation: heat flows from A' (hot) to A (cold). Heat reservoirs We now consider the situation in which our system of interest A is much smaller (fewer accessible states) than system A' . This is the same as saying E ' >> E , so any exchange of energy Q has only a negligible effect on E '. System A' is called a heat reservoir; this strongly asymmetric situation is a very common one in everyday life.
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PHYS 445 Lecture 10 - Systems in thermal contact 10 - 2 © 2001 by David Boal, Simon Fraser University. All rights reserved; further resale or copying is strictly prohibited. At the reservoir:
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445lec10 - PHYS 445 Lecture 10 Systems in thermal contact...

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