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Re: [Phys-l] T dS versus dQ

But why the emphasis on reversible processes when most interesting processes in real life are irreversible? See my paper presenting an Ising model example in the general physics arXiv 0912.4068 (some errors will be corrected in a forthcoming version).

"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley

On Tue, 12 Jan 2010, Bob Sciamanda wrote:

Good night, John

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)

From: "John Denker" <>
Sent: Tuesday, January 12, 2010 11:09 PM
To: "Forum for Physics Educators" <>
Subject: Re: [Phys-l] T dS versus dQ

On 01/12/2010 07:54 PM, Bob Sciamanda wrote:
In order to construct the alternate, reversible process to connect the
initial and final states, I will have to introduce other interacting
entities in the system's environment. It is true that there will be zero
entropy change in the the total super-system during the alternate
process. However my system can change entropy, as can the new

For the alternate, reversible process to "replace" the Rumford boring, I
introduce a sequence of heat reservoirs going between the initial and
These reservoirs will lose entropy, while the cannon gains an equal
of entropy.

OK, let's call that the "fake entropy reservoir"
idea, used in conjunction with the "reversible-only
equilibrium-only" (ROEO) model of thermodynamics.

There are often multiple valid ways of modeling a
given bit of physics.

*IF* there were only one problem with the ROEO model,
and the problem could be solved using the fake entropy
reservoir, then I would say that ROEO thermodynamics
is arguably viable, but runs considerable pedagogical
risk. It risks setting the field back 200+ years, by
teaching a generation of students that entropy is --
according to the model -- a conserved quantity.
Heat = caloric. Student misconceptions in this area
would be left unchallenged, or reinforced, or indeed
introduced where no misconception previously existed.

Even leaving aside the pedagogical issues, I find the
ROEO model unnecessarily ugly and distasteful. I'm not
going to argue questions of taste, except to point out
that there exist much simpler models that successfully
handle irreversibility directly, without any need for
a fake entropy reservoir. That is to say, the fake
entropy reservoir is a laborious way of solving a
problem that should never have been a problem in the
first place, a problem that exists only within this
model, created by the model for no good reason AFAICT.

*AND* that's not the only problem with the ROEO model,
as previously pointed out.

Forum for Physics Educators

Forum for Physics Educators