Re: Explanation of def. 4 of Q (long)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

Carl Mungan wrote:

> John M wrote:
>
> > (Actually, I think any reasonably practical model of the inner
> > wall of his toroidal box *would* end up coupling the rotational
> > energy to the other modes and would, as a result, change the
> > occupation numbers of those modes, the temperature, and the
> > entropy of the gas.  Thus, I would classify the energy change as
> > heat.  But this is all beside the point.)
>
> I am very interested in understanding different viewpoints. Please
> clarify for me two questions:
>
> 1. You refer to an "energy change" above. Does this mean you
> do not necessarily consider heat to be an "energy transfer"
> from one system to another? It can be a thermalization process
> entirely internal to a system?

I didn't mean to give that impression although I see how what I
wrote could be interpreted that way.  What I meant was that, if
something is done to give the gas in the toroidal box bulk
rotational KE and if that bulk KE is subsequently thermalized,
then the net result is that of a purely thermal energy transfer to
the system (i.e., "heat").  I know some will probably object that
the process that gave the rotational energy to the gas looks more
like "work" and I won't argue against that viewpoint.  I'm simply
saying that it doesn't matter and that the net result is
indistinguishable from that of a process that involves *only* heat
and *no* work.

> 2. Yes or no: Is "pure irreversible work" a
> self-contradiction? If any entropy change is associated with
> heat, then an irreversible energy transfer can never be solely
> work. Therefore turbulent paddle wheels, sliding friction, etc
> can never be pure work. Do you agree or disagree?

I guess I would disagree; I'd like to be able to talk about
"irreversible work."  But fundamentally I don't think it matters
what we call the energy change in an irreversible process.  I'd be
just as happy calling it "irreversible energy transfer."  The
*only* time that it *matters* what we call "heat" and what we call
"work" is when we are calculating entropy changes and for that we
usually construct equivalent reversible paths between the initial
and final thermal equilibrium states.

> ... I am getting the sinking feeling that no one agrees with
> defn 1 of heat, despite the fact that it's in most textbooks.

I don't have much trouble with definition 1--certainly not as much
as I do with definitions 2 and 3--except that it isn't completely
unambiguous.  It will do just fine for a first pass through
thermodynamics. I do think, however, that undergraduate physics
majors ought not to graduate (as I did!) without having understood
the more precise definition offered by David or, in simplified
form, by myself.

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