Re: Isobaric expansion

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

> > "Perfectly well"? Equilibrium thermodynamics cannot describe the
> > states of a gas *during* a free expansion.
>
> Good grief! Leigh was talking about "slow" processes ie slow
compared to
> the relaxation time of the system -- A free expansion can hardly be
"slow"
> in this regard!!!
>
> Jim Green
Even if "they" in the statement "One might say they are described
perfectly well" refers only to what I describe as "slow" processes,
the statement is still inaccurate since even slow processes occur at a
finite rate.  A thermodynamic process which occurs at a finite rate
can only be approximated by a series of equilibrium thermodynamic
states.  The equilibrium description is then an approximation, not
perfection.

My distinction between "slow" and "fast" thermodynamic processes was a
clarification to the statement "Surely equilibrium thermodynamics
cannot be said to fail when applied to real thermodynamic processes"
which is also not accurate since there are many real thermodynamic
processes which are not well described by equilibrium thermodynamics.
And, more importantly, I described parameters (time scale for the
process and relaxation time) which can be used to determine when
equilibrium thermodynamics works well and when it doesn't.

Even when my point is recognized that equilibrium thermodynamics only
approximates nonequilibrium processes ("[E]quilibrium thermodynamics
can be applied to nonequilibrium systems, though the conclusions may
only yield approximations."), there is a swing to the other extreme by
the overstatement that equilibrium thermodynamics "is of no use in
describing free expansion."  "[N]o use" is also inaccurate.  Mr.
McInnes correctly paraphrases the point I made repeatedly that
equilibrium thermodynamics works for nonequilibrium processes but only
with limitations (e.g., inititial and final states of a free
expansion).  My contribution to the discussion was to describe what
those limitations are and how we can determine whether a particular
process is within or without those limitations.

I'm merely clarifying some inaccuracies and expanding on the topic of
discussion.  I do, though, continue to challenge the use of the term
"static equilibrium" to describe gases in thermodynamic equilibrium.
I still have yet hear an example of a system in equilibrium which is
not static.

Glenn A. Carlson, P.E.
GCarlson@mail.win.org

> Glenn, to your question
> > Why does equilibrium thermodynamics work very well in one
> > situation and very poorly in another?
>
> I would answer that equilibrium thermodynamics works well in both
> cases you describe. In both cases it does what it claims to do;
give
> a complete description of the initial and final equilibrium states
of
> the systems.
>
> It won't tell you anything of any non-equilibrium intermediate
> process. In an irreversible process, such as a free expansion, many
> thermodynamic parameters of the system such as pressure and
> temperature are undefined and indefinable. In an (artificial)
> quasi-static process, all intermediate states would be equilibrium
> states, so I guess thermodynamics will tell you all about those
states
> but that wouldn't be much use, would it? Nevertheless such
artificial
> processes are the strength of thermodynamics since they allow you
to
> tread a path through natural irreversible changes from a real
> equilibrium initial state to a real initial equilibrium final
state.
> Brian McInnes