Re: Why do we care about heat?

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 31 Oct 1999, John S. Denker wrote:

> At 10:19 AM 10/31/99 -0800, John Mallinckrodt wrote:
> >  Macroscopic
> > processes in the real world are almost without exception irreversible ...
>
> To my ears that statement is not just unorthodox -- it is bizarre.  I'm
> trying to figure out where JM is coming from but I'm just stumped.  Totally.
> If I haul a weight from height H=1m to height H=2m that is the epitome of
> macroscopic work -- and there is no reason why it should not be reversible
> (to an exceedingly good approximation).

Sigh.  I must confess that I am growing more than a little weary of your
unrelenting need to find virtually everything I say totally bizarre,
unacceptable, not the way it is done, or just plain crazy.

Look.  If *you* haul a weight from height H = 1m to H = 2m in the real
world *you* have just performed an irreversible process for a number of
reasons.  But I know that you are a smart (if, unfortunately,
preternaturally hostile) guy, so I'm not going to pretend that I think you
don't know that. I know that you were simply speaking a little sloppily
and were actually thinking about the fact that the entropy of the weight
itself does not increase very much in the process and that it can be
returned to its original level with a similarly small additional increase
in its entropy.

One can point to lots of real world processes where some subsystem has a
small and sometimes even vanishing increase of entropy, but I'm quite
certain that you understand the basic truth of the statement that
"Macroscopic processes in the real world are almost without exception
irreversible."  Strictly speaking they are and I know you know it.

Sheesh.  Why *is* it *so* hard for you simply to acknowledge the point of
what I am saying?

> > We calculate the change in entropy of a system by devising an imaginary
> > *reversible* thermodynamic path from the initial to the final state.
>
> What do you mean "we", Kemosabe?  Does that mean if I have gas confined to
> half a container and suddenly pull out the divider, "we" cannot calculate
> the change in entropy produced by the non-adiabatic, irreversible
> expansion?  I bet I can.

How *do* you make that interpretation of what I said above?  Of course you
can.  I never doubted your ability to do that for a moment.  So can I.  I
can do it using precisely the method I spoke of.  For an ideal gas (not a
real world substance) I can also do it using statistical methods. Is there
a point here beyond simply trying to jerk my chain?

> The notion that heating has to be reversible is just so foreign to
> real-world thermodynamics that it makes my head spin.

I apologize for being *so* completely nutty.

> > 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.
>
> That statement is consistent with the previous bizarre statement,

What would you expect from a total lunatic?

> and is
> consistent with some of JM's other incomprehensible viewpoints, such as
> (10:30 AM 10/31/99 -0800):
> > Furthermore, I think that the modern perspective of virtually all
> > textbooks is that the short and long term result in the Joule experiment
> > is increased internal energy, not heat.

Perhaps you should take the time to look at some modern textbooks.

> The conventional name of that experiment is "the mechanical equivalent of
> heat".  The notion that the result of this experiment was "not heat" is
> beyond unorthodox.  It's beyond unconventional.  It's bizarre.

Can I help?  "Wild, crazy, freakish, abberant, so nutty it makes me want
to put on Chopin and dance the Macarena."

> > 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.
>
> What about inductive heating?  Suppose I shine a high-power microwave beam
> onto a chunk of butter.  I think it heats the butter.  Does anybody really
> think this should be described as work not heat?

I do.  But you'd be wise to consider the source.

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