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

Okay, this seems to be a valid point. You do hear people describe a slow leak as a quasi-static process. On the other hand, you also hear people offer the opinion that, in a quasi-static process, "the system" remains at or near thermodynamic equilibrium. Clearly, if "the system" consists of all gas on either side of the porous plug, then "the system" is nowhere near thermodynamic equilibrium. Maybe we need to alter the definition to allow systems that are in piecewise thermodynamic equilibrium? Not completely sure how you'd do that.

So I wonder if anything important would be lost if we refined these ideas to say that the gases on either side of the porous plug undergo quasi-static processes, but that the gas as a whole doesn't.

I do recognize that this raises an important question about "where" the dissipation is taking place and I'm not particularly comfortable saying that it happens in or "near" the porous plug. Another problem has to do with the consideration of an open system that gains or loses particles. For instance, in the case of the slow leak, the gas on the the gas on the low pressure side gains entropy even though I might want to say that there is no dissipation going on "within it." And the gas on the high pressure side loses entropy. Just thinking as I type here!

So I'm tempted to say "Nevermind!," but maybe I'll just sit over here in my corner and think on it a little longer.

John Mallinckrodt
Cal Poly Pomona

On Jan 24, 2010, at 12:42 PM, John Denker wrote:

On 01/24/2010 11:43 AM, John Mallinckrodt wrote:


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

I don't buy it.

"Static" has a well-established technical meaning.

"Quasistatic" has heretofore meant "almost static"
... which seems sensible and unsurprising. This
definition has always worked for me, and I cannot
imagine why anybody would propose any change, let
alone a highly counterintuitive change.

If you want an example of a physical process that
is quasistatic but clearly dissipative, consider
Joule-Thomson expansion through a porous plug.