Re: thermal energy

[Daniel Schroeder <DSCHROEDER@CC.WEBER.EDU>]

> I try not to use the term "thermal energy" because it was abused in high
> school and I want to expunge terms that might tend to be misconceived at
> the outset. Dan, is the latent heat associated with the change of state of
> a pure substance "thermal energy"? How about the latent heat associated
> with the change of state of an alloy? Both of these are very clearly
> components of the internal energy. Why mess with what works?
>
> Leigh

The "latent heat" associated with a phase change would be a change
in enthalpy, if it's measured under constant pressure as usual.
If we neglect the PV term in the enthalpy, or if the volume doesn't
change, then enthalpy reduces to energy so the question becomes
more focused.  Yes, my definition of thermal energy would have to
include this energy.  I would write U = U_thermal + U_static,
where U is the total energy of the system, U_thermal is a part of U
that includes all the T-dependence, and U_static is the difference
between the two.  For most everyday systems, it's convenient to take
U_thermal to be on the order of a few hundred joules per mole, while
U_static is enormous because it includes the rest energies of all the
particles in the system.  If you restrict yourself to the term
"internal energy", then technically your U is an inconveniently
large number.  Of course, you can refuse to talk about U itself
and mandate that we only discuss changes in U, but that makes
the subject more abstract for students.

My question for you, Leigh:  How do you write the equipartition
theorem as an equation that can be applied to an entire physical
system (as opposed to a single abstract "degree of freedom")?

Dan