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Re: Why do we care about heat?



The bottom line is that it does not fundamentally matter how you
(legitimately) divide up dw and dQ, as far as the answer to any real
physics/engineering question is concerned - as you illustrated in the
various interpretations of your inelastic collision example. Rutherford
would say that these arguments are about stamp collecting (taxonomy), not
physics. I think this flexibility should be part of our teaching, just as
a history teacher should admit of another "ratio divisionis" (other than
his) of civil war causes, to his students.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Sunday, October 31, 1999 1:19 PM
Subject: Why do we care about heat?


I don't know if anyone out there in PHYS-L land is still following this
thread, but I know that for me personally it has been a learning
experience. John Denker's posts in particular have forced me to think
more deeply about why I feel the way I do about terminology issues. I
had
a bit of an "Ah ha moment" this morning in thinking about the difference
between work and heat. Here's what I think defines my attitude about
heat: (I say "think" because I haven't "thought" this for long and
certainly haven't "thought it through." I *think* I like what I am
about
to say, but I am prepared to have my mind changed.)

Heat is related to the second law intimately; work is not. Macroscopic
processes in the real world are almost without exception irreversible
and
cannot be used to calculate the change in entropy so it doesn't really
matter what we call work or heat as far as they are concerned. We
calculate the change in entropy of a system by devising an imaginary
*reversible* thermodynamic path from the initial to the final state. If
no heat is required, the change in entropy is zero. If it is, we make
use
of it to calculate the entropy change. In any event, the heat we are
talking about here is *always* a quasistatic exchange of energy between
two systems that occurs specifically as a result of an infinitesimal
difference in temperature.

I'd like to reserve the word heat to mean *essentially* this same thing
in
all circumstances. I say "essentially" because I am willing to soften
my
definition to include *nonquasistatic* exchanges between systems that
occur as a result of *finite* temperature differences. Otherwise, I'd
just as soon call everything else work to make a clear distinction
between
work--which can be arbitrarily distinguished from heat for use in the
first law--and heat--which must conform to a far more rigid definition
for
use in the second law.

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