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Re: A "heat" question



Jim Green wrote:

In Carnot's cylinder the difference between Q and W (by moving the piston)
is that moving the piston doesn't change the entropy. I don't know of any
other situation where this occurs. All other forms of Q and W actions all
change S. (I would be very appreciative of hearing of a counter
example.)

This is shooting from the hip, but let me at least throw out some
possibilities for discussion:

1. Absorption and emission of radiation by a two-level system.
(Doesn't radiation have energy? Doesn't it travel from place to
place?)

2. The Ising model on which an externally applied magnetic field does
work. (Really the same as 1 in a thin disguise.)

3. A mass on an undamped spring on which an external driver does work.

Notice that all of these are ideal. So presumably is the gas in Jim's
cylinder. Otherwise we're back to my example of striking a bell,
which *initially* is also pure work that does not change the entropy.
Carl

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.