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[Phys-L] Re: work and heat



Carl,=20

I would like to make this as painless as possible and not bore you wi=
th engineering jargon; but, in my opinion, this reference to an Appen=
dix to my book (written with help from Dave Bowman) is very important=
. Also, it may serve to introduce you to http//tinyurl.com/, which s=
ee for exactly how to avoid those broken (long) hyperlinks. See http=
://tinyurl.com/dwyxd which is a reference that will last as long as I=
do. I would read all through the combined First and Second Laws and=
availability which is used to define exergy which is a measure of va=
lue of primary fuels and can be used to compute eMergies or embodied =
energy of manufactured goods so as to place a scientific value on goo=
ds and services to replace the failed monetary values. This is a big=
subject in energy policy and a necessary one.

Tom Wayburn, Houston

----- Original Message -----=20
=46rom: "Carl Mungan" <mungan@USNA.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Wednesday, February 01, 2006 10:09 AM
Subject: Re: work and heat


My replies will be delayed according to the digest postings. So sor=
ry
if I repeat something someone else says in the meantime or if I don=
't
seem to initially reply to something you write.
=20
It's darned useful to engineers and people who worry about engines,=
air
conditioning, and the like.
Jack
=20
I agree it's a useful distinction in some contexts. Just not *in ge=
neral*.
=20
John writes:
=20
That's a good question, but perhaps we would be wiser to precede
it with the question of *whether* we should spend class time trying
to define work and heat.
=20
I assume work was already treated in mechanics. I think it's helpfu=
l
to connect to that prior experience when thermo is introduced. I
agree one doesn't want to make to go overboard on work and heat
however.
=20
Today's textbooks are full of end-of-chapter problems involving
blah-blah-heat and blah-blah-work. But in the real world, if
you're trying to design something or analyze something, you are
vastly better off keeping track of the energy and the entropy.
=20
I hope I made it clear in my letter that I agree completely with
this. The real goal is figuring out the energy and entropy changes.
If heat and work help you get there, use them. If not, ditch them.
=20
Bottom line: We agree that this topic is important, and we largely
agree as to the nature of the problem. But my recommended solution
is different from Carl's.
=20
I didn't explicitly propose any solution. But implicitly I am
proposing that one start with categories of heat and work, and simp=
le
calculations thereof, as a way of introducing thermodynamic concept=
s.
Then (perhaps only in a dedicated course on thermo) progress to mor=
e
advanced ideas and ways of thinking and give up the earlier, weaker
scaffolding. I think this is what we do in physics teaching in
general.
=20
My recommendation: Spend the least possible class time on heat and
work. Concentrate on achieving a robust understanding of energy
and entropy.
=20
I agree with this. Perhaps the only slight difference between us (a=
nd
maybe you don't really disagree even on this point) is that I
recognize that heat and work have some utility. They can be helpful
when analyzing reversible processes. They have a place when thinkin=
g
about gases, particularly ideal gases. But I emphasize the qualifie=
r
*some*. Energy, entropy, and other state functions are of primary
importance; work and heat only secondarily so. See my remark about
Ref 3 in my letter for example.
=20
Tom wrote:
Heat and work appear in the energy balance, but only heat appears i=
n
the entropy balance.
=20
That sentence only appears valid for a reversible process. Joule's
experiment seems a prime example of a process in which the entropy =
of
the system of water changes when only work was done (by the paddles
which are part of the surroundings). No heat is involved.
=20
The equations I use are at http://dematerialism.net/baleqs31.htm .
=20
Please explain what L_CV "lost work" is and how to calculate it. Th=
en
we discuss this further.
--
Carl E. Mungan, Asst Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/

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