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Re: ENERGY WITH Q



Wissenschaftspiel?

bc

I fond Computer play, but not science play (science games tho.)

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Tom Wayburn wrote:

----- Original Message -----
From: "Bernard Cleyet" <anngeorg@PACBELL.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, November 28, 2001 11:40 AM
Subject: Re: ENERGY WITH Q

Carl et al.!

I filter by subject, after reading several, not sender, and manually.

bc


"Carl E. Mungan" wrote:

I should probably begin by apologizing if I was too strong in my
last
message. No one has yet written to me to say I was too strong but
just in case, I should start with hoping no one has added me to
their
"Filter to Trash" list.
************************************************************************
*******
Hi All esp. Carl,.

I believe I am being challenged to back up my statement that work and
heat are obviously distinct as work does not appear in the Entropy
Balance, which is the version of the Second Law employed by The Lost
Work School of Classical Thermodynamics to which I subscribe.
I accept the challenge, but I have many things that must be done before
I can indulge myself in Scienceplay. (I bet Scienceplay would sound
good in German.) I have time now only to accept the challenge and read
the remaining postings - quickly. I hope you all like the equation I
shall use. I have a feeling that many of us have never seen it.
Perhaps, I'll throw in the Combined First and Second Law Equation as
well. It is my favorite.

But, to correct classical economics along the lines introduced by the
great Howard Odem, I have to employ exergy as well as emergy. The
result is economics for Reasonable People. Thus, Milton Friedman (sp?)
is immediately excused along with the rest of the U. of Chicago mob;
and, for that matter, every economist who calls himself an economist in
the Free World, where Free World, as pointed out by Noam Chomsky, like
every term employing the modifier "free", is Politically Correct speech
for World Where Tyranny Is Unrestrained.

Regards / Tom Wayburn
************************************************************************
*****

Tom W wrote:

Work and heat are obviously quite distinct because the Entropy
Balance has Q but not W in it.

This is only true (for def. 1 of heat) for a reversible process. A
sliding block is an irreversible process. So in my opinion, the
entropy term *does* have W in it. You can however find an
*equivalent* process which brings the block and table to the same
final state. Namely, sweep a hot plate at just the right
temperatures
and speeds across just the right places on the table and block
surfaces. This equivalent process has only Q in it and can be done
reversibly if the hot plate temperature is always just barely higher
than that of the contacting object. For this equivalent process, the
entropy change does have Q in it only. BUT for the original sliding
process, there are both Q and W terms in general and thus both
clearly contribute to the entropy.

I think if you look in a thermo text, you will find that dQ = TdS
*only* applies to a reversible process.

So I shoot back again, W and Q are not so distinct as you make them
out. I think this may be one of the reasons why John D wants to
throw
this mess out. But I think that's throwing the baby with the
bathwater.

I agree: The words "A block slides along a table" are insufficient
to
understand what the problem is.

What's unclear? Let me try again another way:

A block slides to rest along a level rough table. The only net force
on the block is kinetic friction. The table is clamped to the floor
and hence to the earth. We approximate the earth as being an
inertial
system. The observer is at rest with respect to the table. I hope
this is clear. Now for the question:

HOW MUCH WORK DID THE BLOCK DO ON THE TABLE?

Since I only read the digest, I may as well provide a grading key
right now. If your answer was zero, congratulations, you are a
believer in pseudowork. If your answer was positive, you are
consistent in your use of thermodynamic work. A new text you may
like
is Halliday, Resnick, and Krane 5th ed. (See the top left hand
column
on page 283.)

Please do not alter the spirit of the question. You may *not* try to
put both the block and table in the system. The system is the table
(plus earth). You may *not* instead tell me about heat; my question
is only about the work. You may *not* answer by invoking a special
magical formula; the whole point of this question is to illuminate
how you think about work in general--the specifics of this example
are just there to give us something concrete to discuss.

Only three people that I know about are allowed to choose both
answers: they are John M, Joel R, and Gene M. John D has lots of
support here:
* all of the industrial researchers he polled - say 15 people
* all other active PHYS-L members interested in this thread - say 50
people
* the remainder of my dept - 35 faculty
So not only are industrial researchers unwilling to use pseudowork,
so are academic scholars. From this limited survey, we can say that
about 97% of all physicists are *hostile* to pseudowork. I chose the
highlighted word purposefully. As I have gone around my dept here at
the Academy for example, my colleagues tell me they not only don't
teach pseudowork, they believe it's a confusing, heretical term in
league with cold fusion, astrology, and such ilk. Here's a sample
quote of just this kind from the list:

Hey don't get me involved with a spurious thing called
"pseudowork". I
disavow ever using the term out loud. There is enough nonsense in
physics
teaching without introducing yet more.

Ludwik wrote:

Recognizing that work and pseudowork are two different
concepts is useful. But why not to say that F*ds is work
and W from the first law is "pseudowork?" We teach about
F*ds before we teach the First Law. It would be silly to
introduce the name "pseudowork" to students who are not
familiar with the concept of work.

Again you're hung up on the name. In fact, I don't particularly like
the name either. As I have made clear elsewhere, there are 3 other
names I use in class and elsewhere:

* center-of-mass work - this is a good and clear name but only
*after* the concept of center of mass has been discussed; not all
texts put this before the work chapter

* mechanical work - this name is somewhat misleading, but as a
contrast to thermodynamic work it has some merits

* just plain work - Ludwik you are right that this is *exactly* what
I do! Re-read my pdf document (which is badly needing some updating
but will have to wait a while longer). The reason I do this is
because... this is what most texts do. Certainly all texts I have
ever used for general physics (Jones & Childers, Cutnell & Johnson,
Serway & Faughn, Giancoli, Serway) *all* do. There is an easy test
to
see whether *your* text is doing this too. Look up the work-energy
theorem. The "work" term in that equation is actually pseudowork.
Check the derivation of the equation if you don't believe me - I bet
it comes from Newton's second law (N2). Only pseudowork can be
derived from N2; thermodynamic work appears in the first law which
*cannot* be rigorously derived from N2, although I most definitely
*do* think there's a connection and that beginning with work-energy
is a useful artifice to build a bridge to the first law which I
consider to be a more advanced (subtle) topic. Most kids I know
would
rather do mechanics problems than thermo problems!

I have more to say, but I better stop here.
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/