Re: Why do we care about heat?

[Bob Sciamanda <trebor@VELOCITY.NET>]

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