Re: Isobaric expansion

["Glenn A. Carlson" <gcarlson@MAIL.WIN.ORG>]

My original point was to clarify why equilibrium thermodynamics works
very well in one situation and very poorly in another?  It's because
of the relative size of the time scale for the process and relaxation
time for the system.  Between the extremes is a continuum of
situations for which the description based on equilibrium
thermodynamics ranges from very good to very poor.

> Subject: Re: Isobaric expansion
> Date: Sat, 18 Mar 2000 23:24:26 -0800
> From: Leigh Palmer <palmer@SFU.CA>

> Equilibrium thermodynamics can't
> explain what happens during supernova core collapse either.

Very good.  Equilibrium thermodynamics doesn't explain what happens
during supernova core collapse.  (I don't know this is the case, but
let's accept it for now.)  Now, *why* doesn't it explain what
happens?  If, in fact, it doesn't, it is not because equilibrium
thermodynamics is a failure, but, rather, because (among other things)
the time scale for supernova core collapse is much less than the
relaxation time for the matter and energy in the supernova to achieve
equilibrium.

> Please be aware that I've read and reread Reif several times during
> my 34 years as a physics professor. I first prescribed the book in
> 1967 when I first taught the course at SFU. Don't you realize that
> you are telling me nothing I don't know?

I cannot speak to what you know or don't know.  But, I wonder whether
your students know what the "equilibrium" in equilibrium
thermodynamics means; why equilibrium thermodynamics describes well
the slow expansion of a gas but describes poorly the fast expansion of
a gas; and how to decide what is "slow" and "fast."

And how long one has taught something is not an argument for why it is
correct.

<snip>
> In Reif chemical equilibrium is treated as a case of equilibrium
> thermodynamics (Section 8.9, p. 317). The terminology I use is
> that of Reif, the conventional physical terminology.
<snip>
> > Even accepting Leigh's definition of "static
> > equilibrium" to include motion or not, the use of the qualifier
> > "static" carries no meaning at all.  Otherwise, what, then, would be
> > an example of equilibrium which is not static?
>
> I can only reiterate.

And I reiterate my question.  Using the definition of "static
equilibrium" to include systems in motion, what would be an example of
equilibrium which is not static?

You are understandably hard pressed to give an example of an
equilibrium which is not static. Almost as hard pressed I would be if
I defined an "even number" as one "evenly or not evenly divisible by
two," and was, then, asked to give an example of a number which is
"not even."

Responding with "Oh -- well -- it has the conventional meaning" is no
response at all.  Reading your nonresponse, I am reminded of Humpty
Dumpty talking to Alice:

        "When I use a word," Humpty Dumpty said, in rather a scornful
tone,
"it means just I choose it to mean -- neither more nor less."
        "The question is," said Alice, "whether you can make words
mean so
many different things."
        "The question is," said Humpty Dumpty, "which is to be master
--
that's all."

<snip>
> As
> you have noted yourself equilibrium thermodynamics can be
> applied to nonequilibrium systems, though the conclusions
> may only yield approximations.

Well, we have come quite far from "described perfectly well" to "may
only yield approximations."  Which brings us back again to my original
point:  Why does equilibrium thermodynamics work very well in one
situation and very poorly in another?

Glenn A. Carlson, P.E.
St. Charles, MO