Re: Thermodynamics essay

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

It appears to me that there may be some talking past each other
going on in the Rauber/Denker exchange.

If I understand it correctly (and I think I do), John D wants to
characterize energy on either "side" of the transfer as either
thermal or mechanical and argues that one can't, therefore,
properly characterize the transfer itself as "Q" or "W" except
(possibly) in those cases in which it starts *and* ends as
"thermal energy" or those in which it starts *and* ends as
"mechanical energy."

I think it is fair to say that this is at odds with the standard
interpretation of Q and W as characterizing the *mechanism* of the
*transfer itself* rather than the form of the energy before and/or
after the transfer.  Furthermore, I think it's not so easy to
define "thermal energy" and to clearly distinguish it from
"mechanical energy."  Nevertheless, I still agree with John D's
thrust in the sense that it is also tricky to define what one
means by a "thermal transfer of energy" and to carefully
distinguish it from a "mechanical transfer of energy."

(Note:  It isn't at all tricky if one confines one's self to
quasistatic processes.  Indeed, the clear distinction between Q
and W in those ideal cases is of fundamental importance in one of
the more common ways of calculating entropy differences between
arbitrary thermal equilibrium states.  But it is tricky, I'd say
impossible, in real processes.  Fortunately, it is also completely
unnecessary.  See, for instance, Gordon M Barrow, "Thermodynamics
should be built on energy - not on heat and work," J. Chem. Ed.
65, 122-125 (1988).)

Joel R, on the other hand, seems to be a bit of a fence straddler
as hinted at in his statement:

> E.g. 3 units of energy are transfered from system A in thermal form and
> arrive at B in nonthermal form.
>
> Then Delta E = +3 and W = 0 and Q =+3, the structure allowed it.

This would seem to indicate that, for Joel R, the "transfer" is Q
if the energy *started out* in "thermal form" regardless of how it
ended up.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm