Re: thermal energy

[Leigh Palmer <palmer@SFU.CA>]

> > 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.

That is true for any system. I thought you stated that thermal energy
was that energy which is associated with a temperature change of the
system.

> 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.

The problem you envision never arises with me because I refer only to
changes in internal energy, not to internal energy itself. Where does
one use internal energy? It would seem to me that the absolute value
is difficult to define.

> 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")?

I haven't a clue. Since the equipartition theorem applies to a model
with quadratic degrees of freedom I guess I would say that the system
internal energy has equal magnitude contributions associated with
each quadratic degree of freedom, together with the caveat that one
must be far from the temperature where quantum phenomena are important.
There's still lots of room for other terms. Remember, I'm teaching a
course in astrophysics right now. We deal with stars. There are lots
of non-quadratic energy terms in a plasma.

Leigh

Leigh