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A process is reversible if and only if it's always at equilibrium during the process. Why? I have heard several specific example of this, such as adding weight gradually to a piston to compress the air inside reversibly, by why should it be true in general? EDIT: Here is something that would firmly convince me of this: Suppose I have a reversible process that is not always in equilibrium. Describe a mechanism for exploiting this process to create a perpetual motion machine. |
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I guess the simplest answer is just to carefully read you own words again. A reversible process is the one that can be made flow backwards. It is intuitive to think that it can be made flow backwards at any time we wish. But if the system were in a non-equilibrium state, one would need to wait a bit until it goes to equilibrium before trying to drive it back. So, it does not satisfy our desire to have the system under the control at any time. |
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Because a reversible process must not increase the entropy in the system. Any change due to lack of equilibrium will lead to an increase in entropy, and thus irreversibility. You can visualize this by thinking of two bottles, one full of gas and one with vacuum. If the bottles were to be connected, atoms would randomly migrate between the two bottles leading to a system with the gas divided between the containers with equal pressure. Thus, the disorder of the system has increased, because the atoms that were all sorted in one bottle initially are now subdivided in two partitions of a much larger volume. In other words, the lack of equilibrium in pressure leads to an uniformization of the same, and to an irreversible increase in entropy. |
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A process is reversible if and only if there is not production of entropy. If you perform a process quasi-statically, you are minimizing the production of entropy (e.g. by adding infinitesimal weight to a piston to compress the air) and the whole process can be considered, at one good approximation, reversible |
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