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Re: Thermal Energy



Michael Edmiston's post is right on the mark as far as I am
concerned. Rotational KE is troublesome because it really SHOULD
be considered "internal" for some purposes and really should NOT
be consider "internal" for others. The difference often has to do
with whether or not we are doing thermodynamics. Indeed, I like
to use rotational energy as a poster child for the difference
between "internal" and "thermal" energy. (And BTW I do know that
JG and others don't approve of thermal energy, I understand why,
and I even partially agree. Nevertheless, I think thermal energy
is a useful term. In fact, I have changed my own thinking in this
regard at least in part due to conversations with Dan Schroeder,
whether he realizes it or not.)

The rotational energy of a macroscopic body is easily populated
with far more than its "thermal share" of energy and it usually
does not readily trade that energy back and forth with other
internal modes of energy storage. That is, it is not subject to
"thermalization" on reasonable time scales. Thus the energy may
be internal, but it is not thermal. Consider a satellite.

On the other hand the rotational energy of a diatomic gas molecule
cannot be easily populated with more than its "thermal share" of
energy and, even if it is, it tends rapidly to lose that energy
via collisions with other molecules.

John