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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