X-Message-Number: 1989
Date: 19 Mar 93 06:15:21 EST
From: "Steven B. Harris" <>
Subject: CRYONICS Thermodynamics II

    PS.   While we're at the thankless task of explaining thermo-
dynamics on Cryonet, by the way <yawn-- read this just before
bed...> I might as well address Mike D.'s confusion about
temperature and phase change, as he has requested.  All this is a
matter of heat or kinetic energy disappearing into potential
energy.  We're all familiar with this.  You can put energy into
picking a hockey puck (say) off the floor and putting it up on a
table, and find that although you've put energy into the object,
its velocity hasn't changed at all.  This doesn't happen if you
put that energy into pushing the puck horizontally along on the
ice-- there you get a velocity change for your energy addition. 
When you *lift* the puck, however, the push energy goes not into
velocity (kinetic energy) but into "gravitational potential" --
it's stored in the system itself (somewhere-- don't ask me
exactly where) because of a change in object position in a force
field (in this case the gravitational field).  

   The same thing happens with molecules leaving the surface of a
liquid-- they're overcoming Van der Waals attraction forces
between molecules (these are London forces for nitrogen and both
London and dipole forces for water) and these are all something
like gravity, except that the field is electromagnetic.  The
upshot, in any case, is that you can put energy into a molecule
in changing its position in an electromagnetic field--- pulling
it away from the surface of the liquid which is pulling it back--
without changing its velocity (temperature).  Again, it's like
picking something up or rolling a truck up a mountain to a flat
plain beyond.  You put energy (heat) in, but the temperature
(velocity of the molecules) does not necessarily have to change. 
Gas molecules above a liquid then are special because of their
position and larger potential energy (like a truck on top of a
hill), not necessarily because of any larger kinetic energy
(velocity or temperature).  Their temperature may be just the
same.

   By the way, note that this is the general rule in all phase
changes.  When you put heat (energy) into a cube of ice slowly
and it melts, the temperature does not change at all while it is
melting.  The heat energy is going into potential energy, not
kinetic energy (i.e., it's not going into velocity of molecules,
which is measured as temperature).  The water molecules in ice
are pulled very close to each other in electromagnetic inter-
actions, and when you melt ice you're pulling about 12% of those
interactions apart, just like lifting something up on a table and
pulling it away from the Earth.  It's energy money in the bank,
but there's no obvious molecular zip at the end to go with it.


                                         Steve

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