Date: Tue, 17 Jan 2012 11:57:00 -0700
From: John Denker <jsd@av8n.com>
Reply-To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Temperture profile in a graviational field
> ...
> Here's a status report on my thinking: I'm sticking with the
> proposition that thermodynamic equilibrium is isothermal.
Normally I choose not to say anything because John is usually right.
Today I'll break my silence.
The general principle is that any system in mass transport equilibrium
has a uniform chemical potential. If there are no external forces then
that means the temperature will be uniform. Here, with an imposed
gravitational potential, air at the top of a column will be colder than
at the bottom.
No wind needed. No external energy source needed. All that is needed
is that the air is free to move to equilibrate whatever it needs to
equilibrate. Looking at it microscopically, if air molecules can
diffuse up and down, doing so will change their speeds because of the
work that the gravitational force does on them, which macroscopically
we say makes them colder or hotter respectively. If, on the other hand
you prevent diffusion, then the air is no longer in mass transport
equilibrium and consequently, in equilibrium temperature would then be
uniform, even in the presence of a gravitational field -- but that
wasn't the original query.
Chris
_________________
Christopher M. Gould 213-740-1101 Dept. of Physics & Astronomy
gould@usc.edu Univ. of Southern California http://physics.usc.edu/~gould/ Los Angeles, CA 90089-0484