Re: PHYS-L Digest - 17 Mar 2000 - Special issue (#2000-95)

[Leigh Palmer <palmer@SFU.CA>]

At 2:34 PM -0800 3/17/00, Glenn A. Carlson wrote:
> Strictly speaking, a system at equilibrium cannot change; hence, even
> speaking theoretically, isobaric, isochoric, isothermal, isentropic
> processes are nonequilibrium processes even in the ideal.

That said, he continues:

> But they
> can be described "as if" passing through a series of "equilibrium
> states" if they are assumed to be infinitesimally close to equilibrium
> throughout the process.  In reality this would necessitate that they
> occur infinitely slowly.  Real processes do not occur infinitely
> slowly and, thus, are not infinitesimally close to equilibrium.
> However, real gases achieve equilibrium very quickly, which is why
> equilibrium thermodynamics describes such nonequilibrium processes so
> well.

What does "ideal" mean to you, Glenn? Surely equilibrium thermodynamics
cannot be said to fail when applied to real thermodynamic processes.
If it isn't of any use in those cases then what is it good for?

> To test your understanding of equilibrium thermodynamics, consider
> this question:  Water is supposed to boil at 100 deg C at 1 atmosphere
> pressure.  But, if so, can you explain to a student why water
> evaporates at room temperature?

This would appear to be a nonsequitur. The words "boil" and "evaporate"
pertain to different processes. Whom do you presume to be testing? Do
you suspect that I do not understand elementary thermodynamics?

> Finally, to pick a nit, thermodynamic states are not static
> equilibrium states.  Even exactly at equilibrium, they are quite
> dynamic -- exchanging energy between molecules, transferring energy
> and molecules from one part of the volume to another.  Even the
> average values of entropy, pressure, density, internal energy, etc.
> are not "static".  They fluctuate about average values, and the
> magnitudes of the fluctuations are inversely proportional to the
> number of particles.

Thermodynamic equilibrium states subsume static equilibrium states.
A box of gas in an unchanging state of thermodynamic equilibrium is
in static equilibrium by conventional definition.

What I did not state explicitly in my explanation, and what I
expect Antti assumed, was that the system under consideration is
the standard isobaric expansion system, a vertical cylinder
containing a gas by means of a frictionless piston which is held
at a level at which the gas pressure times the piston area is
equal to the weight of the piston. When the gas in the cylinder
is heated it expands isobarically.

The reason I respond here is that I thought I did a good job of
answering Antti's question. Glenn's response should not be taken
as clarifying mine, but I fear that that is what he intended, and
somehow I don't think it helped one bit.

I ask, please, that Glenn *only* reply to my questions; I would
not like to see a mob battle over a point which is only important
to me. (I would also like to hear if Antti found my explanation
to be lacking something. For him I would gladly correct any error
I may have made.)

Leigh