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# 445lec8 - PHYS 445 Lecture 8 Interacting systems Lecture 8...

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PHYS 445 Lecture 8 - Interacting systems 8 - 1 © 2001 by David Boal, Simon Fraser University. All rights reserved; further resale or copying is strictly prohibited. Lecture 8 - Interacting systems What's Important: equilibrium and reversibility systems in interaction Text : Reif Constraints and (Reif, Sec. 3.1; not covered in class) Suppose that two systems are brought in contact, and a constraint that isolated them is removed. How do we describe theoretically what happens? For example, consider what happens when a thermal barrier between two gases is removed, but the volume of their container is not altered. Once the barrier has been removed, the number of accessible states must be the same, or larger: f i . That is, removing a constraint cannot decrease the number of accessible states. As the probability of the system being in a particular state is proportional to , then the probability of a system returning to its initial state is i / f . Reversible and irreversible processes (Reif, Sec. 3.2; not covered in class) If the number of accessible states doesn't change during a process, that is f = i , then equilibrium can be maintained during the process and it is reversible. If the number of accessible states increases during a process, f i , then the process is irreversible. For example, if a gas expands to occupy a larger volume by removing a barrier, then it will not go back to its original state if the barrier is reinserted: Remove the barrier

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PHYS 445 Lecture 8 - Interacting systems 8 - 2 © 2001 by David Boal, Simon Fraser University. All rights reserved; further resale or copying is strictly prohibited. Systems in interaction
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## This note was uploaded on 09/07/2009 for the course PHYS 445 taught by Professor Davidboal during the Spring '08 term at Simon Fraser.

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445lec8 - PHYS 445 Lecture 8 Interacting systems Lecture 8...

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