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I just wanted to clarify what is probably a very obvious idea.

Suppose I'm holding a a piston separating two containers at different pressures. If I release the piston, it will naturally move, probably quite quickly, to a new position, so the pressures in both containers are equal.

This is obviously an irreversible process.

However, people often talk of a 'reversible' process as one that occurs 'very slowly', so that each step can be reversed. But I think that's ridiculous!

I see two possible ways I could slow this process down, so that in the limit it's moving arbitrarily slowly.

In the first case, I could add gas to one container, so the pressures are arbitrarily close, and thus make the piston move arbitrarily slowly. If I kept slowly removing gas I could run the whole process, and I think this would be reversible.

On the other hand, I could keep the pressures different, but make my piston heavier and heavier - I could make it out of neutron stars! In this limit I could also make the process arbitrarily slow, but it wouldn't be reversible - I can calculate how the entropy will increase!

The analogy with temperature is obvious (I could slow heat flow by reducing $\Delta T$ or by adding more insulation), and indeed question is really this:

In thermodynamics, is it the case that reversible processes are not processes which occur arbitrarily slowly (as I've been taught and read many-a-time) but instead are processes with arbitrarily small driving forces at each stage? On a side-note, is this difference ever discussed in the literature? :)

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  • $\begingroup$ I erased my answer because it didn't answer your specific question. $\endgroup$ – Wolphram jonny Oct 23 '14 at 16:46
  • $\begingroup$ I think that part of the problem here is that you're dealing with an unconstrained piston. At the start you've got a force on the piston due to the pressure and you're applying a restoring force to keep it in place. This is an equilibrium condition. If you suddenly remove that force then your system is suddenly not at equilibrium (irreversible territory). What you'd need to do is slowly reduce the force and let the piston come to an infinite progression of equilibrium states. $\endgroup$ – user3823992 Oct 24 '14 at 2:28
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You are right. The definition of a reversible process is that the process can be reversed by an arbitrarily small change in the external parameters describing the system (see for example the wiki page) That the change takes place quasi-statically is indeed a necessary but not a sufficient condition for reversibility. The fact that the condition on the time take for the process is not enough to define a thermodynamically interesting property of the process should not be surprising as thermodynamics never says anything about the rates at which processes occur.

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Even if you are trying to increase resistance by increasing mass of the piston, you are having a definite pressure gradient resulting in acceleration of the piston, making it an irreversible process.

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