[Phys-L] Re: work and heat

[Carl Mungan <mungan@USNA.EDU>]

My replies will be delayed according to the digest postings. So sorry
if I repeat something someone else says in the meantime or if I don't
seem to initially reply to something you write.

> It's darned useful to engineers and people who worry about engines, air
> conditioning, and the like.
>                                 Jack

I agree it's a useful distinction in some contexts. Just not *in general*.

John writes:

> 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.

I assume work was already treated in mechanics. I think it's helpful
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.

> 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.

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.

> 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.

I didn't explicitly propose any solution. But implicitly I am
proposing that one start with categories of heat and work, and simple
calculations thereof, as a way of introducing thermodynamic concepts.
Then (perhaps only in a dedicated course on thermo) progress to more
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.

> My recommendation:  Spend the least possible class time on heat and
> work.  Concentrate on achieving a robust understanding of energy
> and entropy.

I agree with this. Perhaps the only slight difference between us (and
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 thinking
about gases, particularly ideal gases. But I emphasize the qualifier
*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.

Tom wrote:
> Heat and work appear in the energy balance, but only heat appears in
> the entropy balance.

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.

> The equations I use are at http://dematerialism.net/baleqs31.htm .

Please explain what L_CV "lost work" is and how to calculate it. Then
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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