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Re: [Phys-L] heat content



On 02/10/2014 01:33 PM, Jeffrey Schnick wrote:

[300 words snipped]

I am open to change, but at present, I would call that atomic-level
energy that the object had, but later got radiated away, "thermal
energy".

Whenever I say there is no such thing as thermal energy, I try
to be careful to add the proviso: "except in trivial cases".

In particular, if the situation is so cramped that you can define
a notion of "thermal energy" aka "heat content", then it is so
cramped that you could not possibly build a heat engine.

You certainly can construct such situations ... but they are
trivial.

In particular, subject to modest restrictions we can write the
useful equation

dE = - P dV + T dS [1]

*If* you cobble up a situation where the P dV term is guaranteed to
be zero, *then* we can say two things:

A) The "heat" term i.e. T dS is the gradient of a potential.
You can integrate it and thereby /define/ a notion of
"heat content" aka "caloric" aka "thermal energy" as a
function of state.

B) This is a trivial victory, because in this situation, integrating
T dS is identical to integrating dE. In other words, the "thermal
energy" is just the /energy/ (to within a meaningless constant of
integration).

=========

I say again: You certainly can construct such situations. Indeed,
such situations are common and familiar. You don't need a water
pistol in outer space. If you warm up a baby bottle and then let
it cool down, there is no P dV term. The T dS term tells the whole
story. Then:
A) In the short run, in this narrow situation, you have not much
to lose by talking about the "thermal energy" aka "caloric" aka
"heat energy" in the baby bottle.
B) OTOH you would do at least as well to talk about the plain
old /energy/, in this situation and every other.

The advantage of talking about energy rather than "thermal energy" is
that when we expand the scope of discussion to include P dV terms, to
include heat engines, to include physics and chemistry and engineering
and all that stuff, we get to keep the notion of energy. In contrast,
the notion of "thermal energy" is dead on arrival.

To repeat: when heating and cooling baby bottles, the idea of "thermal
energy" is not wrong; it's just trivial. It confers no advantage over
plain old /energy/.

In contrast, the idea of /energy/ is a keeper. You've got nothing to
lose and much to gain by focusing attention on the energy.