Re: Isobaric expansion

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

> Subject: Re: PHYS-L Digest - 17 Mar 2000 - Special issue (#2000-95)
> Date: Fri, 17 Mar 2000 23:34:28 -0800
> From: Leigh Palmer <palmer@SFU.CA>

<snip>

> 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?

Equilibrium thermodynamics does fail when applied to real
thermodynamic processes, but to different degrees.  Relatively slow
processes (slow compared to the relaxation time of the system) are
described very well by equilibrium thermodynamics.  Fast processes
such as the free expansion of a gas are not described well at all by
equilibrium thermodynamics.  To show this, consider a gas at
equilibrium in half a volume separated from the other half by a
partition.  Now, remove the partition.  Use equilibrium thermodynamics
to calculate the pressure of the gas throughout the expansion (not
just at the end).  It cannot be done.

> 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.

I'm curious, then.  How do you define "dynamic equilibrium"?  Is a
reversible chemical reaction at equilibrium static or dynamic?

<snip>

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