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Re: [Phys-l] PV question

I've been following this thread and now recently find an issue of vocabulary that I want to ask about: I thought that isobaric and isochoric changes were NOT considered "reversible", whether done fast or slow. I think we need another word for what they are, perhaps "undoable but not time-reversible". I thought that "reversible" meant more than that you can return your cylinder to its previous state but that the interaction between the system and its environment could be time-reversed without breaking the second law. If isochoric or isobaric changes were reversible in that sense then there would be nothing special about the Carnot cycle. Do I have this wrong? And as long as I am asking questions, say you have a gas in a cylinder. Is any process reversible other than isothermal and adiabatic expansion/compression?
From: [] On Behalf Of John Mallinckrodt []
Sent: Friday, January 22, 2010 6:56 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] PV question

Biran Whatcott wrote:

If one specifies a straight line on such a PV graph, then the curve
describing the pressure volume and temperature change during its
compression will necessarily cut some isotherm (actually many of

Not so. Not in the case of an isobaric or isochoric process or, for
that matter, many other processes. Consider, for instance, any
process for which P(V) = P_o + C (V - V_o) with C a positive
constant. In fact, depending on starting and ending volumes, many
processes with negative C also satisfy the requirement. All of these
processes can be carried out reversibly.

The implication (for me) is that this straight line is not a
of a reversible adiabatic. process.

That's *certainly* true, but you don't need to and, in fact, can't
deduce that result from any of the foregoing observations. A
reversible adiabatic process is described by a curved line on a PV
graph, period.

John Mallinckrodt
Cal Poly Pomona
Forum for Physics Educators