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Re: W+Q deprecated



Jim queried:

>Okay. I've got a gas of magnetic atoms. I slowly change the current
which flows both through a set of Helmholtz coils around the gas and
an electrical heater immersed in the gas.

Is the deltaE due to Q or W? How would you know?

Is deltaS=Q/T? How would you know?

Would you agree that it takes work W (exertion of a torque over an
angle) to flip a group of spins in an organized manner by the coils?
This doesn't look like heat to me. The fact that the flipping is done
coherently is the key idea that entropy isn't changed.

Would you agree however that increased temperature can jiggle the
spins around into random directions? I would sure like to say that
the heater does Q. The fact that the energy is added in a highly
disorganized manner is what links this to entropy.

Do I think that such a neat division is always possible? No. Back to
the block sliding on a rough table. I'm exciting phonons; I'm
differentially warming up different patches of the block and table;
I'm accelerating asperities sticking out of the block and table.
Clearly everything is so far out of equilibrium both mechanically and
thermally, and the energy transfer involves both disorganized
(jiggling molecules at the interfaces) and organized (the whole block
comes to rest) aspects that I'm at quite a loss to decide what part
of delta(E) is Q and what part is W. I concede that irreversible
processes, with the exception of "trick" cases like a free expansion
of an ideal gas, are not conducible to W+Q and one had better stick
with calculating changes in E and S another way. Carl
--
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/