Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: reversible heat and irreversible weight-lifting



On Mon, 1 Nov 1999, John Denker wrote:

At 10:19 AM 10/31/99 -0800, John Mallinckrodt wrote:

[certain]
processes in the real world are almost without exception irreversible and
cannot be used to calculate the change in entropy ...
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.

And in this context at 04:04 PM 10/31/99 -0800, John Mallinckrodt wrote:

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

That's because several of the things JM is saying are closer to being
diametrically wrong than they are to being correct.

In the real world, irreversible processes are precisely the ones that
*must* be considered when calculating the change in entropy.

In the real world, many forms of heating are irreversible. In contrast,
restricting the definition of JM-heat to quasistatic quasi-isothermal heat
flows guarantees that JM-heating is reversible.

Once again John D. has butchered the context of my remarks and twisted the
meaning of my words making it sound as if I don't think one can calculate
changes of entropy in irreversible processes or, perhaps, that I think
that it is impossible to "heat" (using the colloquial/historic rather than
the modern/conventional meaning of the word) objects irreversibly.
Because I have already set this record straight once, I am dismayed to see
it popping up again. I can see only two possible explanations for John's
behavior and I don't much like either one.

Here is what I actually said:

"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."

Within the full context of my remarks, I had hoped it was clear that I was
distinguishing between what actually happens in the real world and what we
do in classical thermodynamics to calculate the change in entropy that
results. In particular when I said, "The heat we are talking about here
...", I thought it was pretty clear that the "here" meant "in
*calculating* the change in entropy."

My point remains that for use in the first law, it makes no difference how
we interpret heat and work; the first law essentially says "When an amount
of energy is added to a system its energy content changes by that amount."
(Oh boy, I can already see the trouble I am going to get in for that
statement, but I am confident that intelligent readers including John D.
understand my meaning.) For quantitative use in the second law, however,
we need a clear distinction between heat and work. My proposed--and still
somewhat loose--definition of heat is intended to preserve to a large
extent, the meaning of heat as it *is* used in entropy calculations.

At 04:04 PM 10/31/99 -0800, John Mallinckrodt wrote:

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.

So in a single day we are told that the important thing about JM-heating is
its reversibility, and the important thing about weight-lifting is its
irreversibility!??!?!!???

I defer to brian's apropos response to this item.

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