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[Phys-l] Reversible versus quasi-static processes (was Re: PV question)

Leigh Palmer wrote:

I cannot follow this "thread". The metaphor is inappropriate for such a tangled mess. I want to make some disjoint comments that may or may not answer some of the questions that have arisen during this discussion. I hope that thinking about these will help dispel misconceptions and misuse of conventional terminology.

[perfectly reasonable commentary omitted]

I just did that to assure myself that I still knew my classical thermodynamics. If I've said anything that is incorrect I would appreciate correction.

In my judgment Leigh still knows his classical thermo and I certainly agree with him that this thread moved well away from the simple original question almost immediately. (Leigh may have been gone too long to remember that it's always like that around here!)

With regard to "reversibility," it's clear that there is a lot of confusion "out there" about what constitutes a reversible process and what the difference is between a quasi-static and reversible processes. For instance, Wikipedia, defines a reversible process as "a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy."

I take issue with this definition because it suggests that there are processes in which energy is "lost."

The Wikipedia article also provides an alternate definition: "a process that, after it has taken place, can be reversed and causes no change in either the system or its surroundings."

I'd be more comfortable with this definition if it read, "a process that could, in principle, be reversed leaving both the system and its surroundings in their initial states." But isn't it simpler and less ambiguous to define a reversible process as "a process that does not increase the total entropy of the system and its surroundings" or simply "a process that does not produce new entropy"? Personally, I prefer the even simpler, "a dissipationless process," as it focuses attention on the real issue--dissipation, which is THE mechanism by which new entropy is ALWAYS produced.

Unfortunately, Wikipedia muddies the water in its article on quasistatic processes by saying, "A quasistatic process often ensures that the system will go through a sequence of states that are infinitesimally close to equilibrium, in which case the process is typically reversible." Even if this were true, the statement is rendered essentially meaningless by the weasel words "often" and "typically" and it simply is not the case that a quasistatic process is "typically" reversible.

It seems to me that the confusion arises as a result of a very common failure to be clear about 1) the central role of dissipation and entropy production and 2) the system to be considered. I suspect there may be no universal agreement on definitions, but I'll propose two for target practice:


Definition: A system undergoes a quasistatic process if and only if no dissipation (or entropy production) occurs within the system.

Notes: The only way to do this is to ensure that the system's mechanical constraints (including things like ambient E, B, and g fields) change slowly AND that its physical boundaries are always held at a temperature that is close to that of the system itself. What is "slow" enough and "close" enough depends on how picky one wants to get about the absence of dissipation or the amount of new entropy that one is willing to tolerate. Any significant friction in a moving piston would disqualify the process IF the system under consideration includes the piston and/or container walls, but is completely irrelevant if the system under consideration is a gas confined to the container. In any event, note that the SYSTEM must be unambiguously defined.


Definition: A process is reversible if and only if no dissipation (or entropy production) occurs due to the process.

Notes: Implicit in the definition is that "the system" is "everything there is," i.e., "the universe." Thus, our attention is focused here only on the PROCESS. We do not question the reversibility of a Carnot cycle simply because a free expansion might be happening somewhere else in the universe at the same time. This is NOT because the free expansion isn't taking place within "the system;" it is because the free expansion isn't part of the PROCESS we are considering. If a process includes moving a frictional piston, then the process is not reversible. No real process is reversible, so the discussion of reversibility is always about principles, not experimental procedures or apparatus.

Fire away!

John Mallinckrodt
Cal Poly Pomona