Re: Some Thoughts

[Leigh Palmer <palmer@sfu.ca>]

Jim Green comments, *inter alia*:

> 4) The First Law is a good guide to this usage:  If W+Q=dU and a speaker
> wants to maintain consistent usage in a discussion, then just what meaning
> should be assigned to each term?  Well, there is the Second Law to deal with
> as well: if dS=Q/T, then those things assigned to Q should change S.  And a
> name for Q needs to be agreed upon.  Usually "Q" is named "heat" -- this is
> just fine, but then "heat" is what is done *to* the system -- and in the
> process changes the S of the system.  U is *inside* the system -- now what
> should U be called -- there is not much point in using *two* terms:
> "internal energy" *and* "thermal energy" -- at least without further reason
> to do so -- and that doesn't seem manifest.  I guess W should be called
> "work" but is that useful???

There is some need for clarification here. In the shorthand definition
dS=Q/T, Q is a hypothetical process, not a real one. dS is the change
in the entropy of a system which occurs in some process. The process
could certainly involve the performance of work on the system, even
work alone. Certainly Joule's stirring of liquid produced a change in
the entropy of the liquid. Q in this definition refers to the heat
required in a reversible process which would produce the same change
in state (and hence in entropy) as that produced by the process in
question. All reversible processes which produce the same change in
state require the same heat Q, so specification of a particular
process is not required. Thus sayeth the second law of thermodynamics.

> 5) There should be a sharp distinctions between "Q" and "W"; that
> distinction should be that "Q" changes S and "W" does not.  Some examples
> might help here: a  Bunsen burner does "Q", pdV does not, it therefore is
> "W".  An egg beater *does* give rise to "Q" ie it *does* change S, but there
> is no "dT" between the system and the universe.  Are we willing to call what
> the egg beater does "heat".  If we are being consistent with our First Law
> and Second Law criteria, we must.  If we do not want to be consistent, we
> just don't understand thermodynamics and our students will be mightily
> > confused.

This would clearly not be a good test. It suffices *only* for
reversible processes. The example that started this mess (the sliding
blocks) is an example of a system in which the transition form lower
to higher entropy occurs spontaneously without *either* heat or work
being done on the system. Joule raised the entropy of his system
while carefully avoiding heat; the increase in entropy in that case
could be expressed as dS=W'/T, where W' is the actual work done. That
is why the result is called the mechanical equivalent of heat. Egg
beaters don't "give rise to `Q'"; they do work on the system. That is
the custom, even if you don't like it.

> 7) In the equation W+Q=dU, we should mean that "W" and/or "Q" might be done
> *to* (or by) the system and thereby the "internal energy" of the system is
> changed. Just as we say that "F" is applied *to* an object and the
> *resultant* is that the "ma" *of* the object is changed.  If "Q" is done to
> the system the "S" will change; if "W" then "S" does not.  If at this point
> the word "heat" is used, that should be done with *great* care.  But if one
> wants to internalize the lessons of thermo, try not to use the word for
> awhile until the concepts sink in.

I mentioned my simplification some time ago. Don't use the words
"heat" and "work" as nouns when speaking thermodynamically. Use them
only as verbs! When used as nouns (and I do it myself) recognize that
there is likely some imprecision in their use.

> 6) And for the sake of students never say that "heat flows" -- unless you
> are talking to Carnot.

Right on! The noun alert rule applies here.

Leigh