nomenclature - from "what flows"

[Mark Sylvester <msylvest@spin.it>]

Standard traditional names:
U: internal energy
Q: heat

Problem: as discussed extensively, the word "heat" as a noun may be
conceptually and therefore pedagogically bad.  Jim Green's point seems to be
that it is furthermore desirable to keep a certain symmetry in how we think
about Q and W.

Using "heat" for both Q and U is, I hope, considered beyond the pale by all
concerned.

My first objection to "thermal energy" is that this term, too, is used for
both Q and U, not necessarily by the same people at the same time. Any new
nomenclature must use terms not already in use in a confusing way.

> 			delta Q = 0   (insulated)
> 			delta W = 0   (cylinder does not fail)
> 			delta U = 0   (the above, and first law)
> If the energy of the spark is negligible this is a completely closed 
> system, but clearly something has happened.  It has gotten hot, the 
> pressure of the gas has gone way up, and the chemistry of the gas inside 
> has been altered.  Are you going to tell me that it is not helpful from 
> the point of view of understanding to point out that although the 
> internal energy is the same, chemical energy has been converted to 
> thermal energy?  Unfortunately, I can't talk about it any other way.
> 					Barlow Newbolt 

I would distinguish between potential and kinetic energy of the particles. U
consists of both, but only the k.e. is perceived as temperature. In an ideal
gas there is no potential energy, so U is a function of temperature. I don't
think it helps  to introduce a term like "thermal energy".

What to do?

I would keep the word "heat" if the alternative is to have another confusing
term!

I've never used Jim's  "the heat done" for Q, but I have for years used "the
heating done" which is pretty close, and grammatically orthodox. Perhaps
changing the grammar here is a good idea, if in doing so we improve the
accuracy of the language. Once we've done the first law, I usually ask a
student who pronounces the noun "heat" if he/she means Q, deltaU or W, and
this tends to get the point across. The sad thing is that they then take an
examination in which these distinctions are not always clear. But this is
not the end of the story:

> BTW:  Jim, what do you call the quantity calculated  by   kA(delta)T/d  in
> conduction?  Many of us call this the heat flow across a boundary.

And this is why I wrote earlier that I find I have to teach some things in
terms of caloric theory, because that is how they were conceived. Thermal
conduction, from the viewpoint of temperature gradients, coefficients and
heat flows through sections uses the language of pure caloric theory, and
it's very cumbersome to try to change that. I don't, although we might later
discuss how to put it in thermodynamically correct terms.

So I guess I'm trying to keep clarity and logic in the way we formulate
concepts in class, while at same time maintaining the historical context of
the subject and preparing students for the fact that they may have to answer
questions framed in not-so-clear terminology, and deal with textbooks which
take a different view. I think it all adds up to education.

Mark.

Mark Sylvester
UWCAd, Duino, Trieste, Italy.