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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.air
=20
It's darned useful to engineers and people who worry about engines,=
neral*.conditioning, and the like.=20
Jack
I agree it's a useful distinction in some contexts. Just not *in ge=
=20l
John writes:
=20
That's a good question, but perhaps we would be wiser to precede=20
it with the question of *whether* we should spend class time trying
to define work and heat.
I assume work was already treated in mechanics. I think it's helpfu=
to connect to that prior experience when thermo is introduced. Ile
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=20
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.
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=20
agree as to the nature of the problem. But my recommended solution
is different from Carl's.
I didn't explicitly propose any solution. But implicitly I am
proposing that one start with categories of heat and work, and simp=
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, weakernd
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=20
work. Concentrate on achieving a robust understanding of energy
and entropy.
I agree with this. Perhaps the only slight difference between us (a=
maybe you don't really disagree even on this point) is that Ig
recognize that heat and work have some utility. They can be helpful
when analyzing reversible processes. They have a place when thinkin=
about gases, particularly ideal gases. But I emphasize the qualifie=r
*some*. Energy, entropy, and other state functions are of primaryn
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=
ofthe 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 =
the system of water changes when only work was done (by the paddlesen
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=
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/