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



Strictly speaking, a system at equilibrium cannot change; hence, even
speaking theoretically, isobaric, isochoric, isothermal, isentropic
processes are nonequilibrium processes even in the ideal. 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.

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?

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.

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

Subject: Re: isobaric expansion
Date: Fri, 17 Mar 2000 10:00:16 -0800
From: Leigh Palmer <palmer@SFU.CA>

At 9:05 AM -0800 3/17/00, Savinainen Antti wrote:
Hello,

I have a question on isobaric expansion (this is taken from the
paper
published by Rozier and Viennot):
îAn ideal gas is heated at constant pressure. Its volume and
temperature
both increase. Why?î

The question is quite easy to answer from the point of view of
energy.
Heat transferred to the system is partially used to mechanical work
(=
P*deltaV) and partially to increase kinetic energy of the molecules
and
hence temperature.

But what happens in a molecular level? The molecules bounce from a
piston
and exert a force on it. When temperature increases the molecules
have
greater momentum which results to greater force in collisions.
Increase in
volume decreases number of collisions per time unit. The overall
effect is
that average force per area, pressure, remains constant.

Is there momentarily greater pressure exerted on the piston for
instance
in the beginning of the process? A student of mine asked this and I
couldnít convince him that pressure is constant all the time
(thatís why
it is called isobaric!). Have I given the correct argument or am I
missing
something?

Regards,

Antti Savinainen
Kuopio Lyseo High School/IB
Finland

If you consider the details of the process then the pressure must
differ
any time the piston is accelerating. Your explanation (which is very
clear
and should be understood by any student) pertains to equilibrium
states
through which the system passes during the process of isobaric
expansion
or, alternatively, to comparison of two static equilibrium states.
If the
piston is accelerating then clearly thermodynamic equilibrium does
not
describe the system.

Leigh