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# [Phys-l] meaning(s) of "adiabatic" (was: T dS versus dQ)

• From: John Denker <jsd@av8n.com>
• Date: Thu, 14 Jan 2010 08:55:26 -0700

As we began to discuss in a previous note:

In the quantum mechanics literature the slow and gradual
case is conventionally called the "adiabatic"
approximation in contrast to the "sudden" approximation.
This usage is quite firmly established ... even though
it conflicts with the also well-established convention
in other branches of physics where "adiabatic" means
thermally insulated (see examples below). I refuse to
take sides in holy wars over definitions. I ordinarily
just avoid using the term "adiabatic". My recommendation:
If you mean isentropic say "isentropic". If you mean
thermally insulated say "thermally insulated".

On 01/13/2010 12:14 PM, Bernard Cleyet wrote in part:
And I thought the rapid compression ...

That reminds me of a funny story from graduate school.
Professor A thought Professor B was crazy for thinking
that adiabatic meant "slow". Conversely Professor B
thought Professor A was crazy for thinking that adiabatic
meant "fast". The poor grad students ran around in
circles trying to figure out which professor was right.
I reckon both professors were crazy. It would have been
better if they had talked with each other and worked
out their differences.

Again: The meaning depends on context: Examples:
*) adiabatic = slow = nondissipative
-- example: quantum mechanics: adiabatic approximation
as opposed to sudden approximation. adiabatic = slow
enough that perturbation produces transitions to
corresponding states only
-- example: low temperature physics: adiabatic
demagnetization is done quite slowly
*) adiabatic = fast = thermally insulated
-- example: NMR: adiabatic fast passage
-- example: adiabatic as opposed to isothermal
compression of a gas, as in BC's example.
-- example: adiabatic demagnetization should not
be done toooo slowly.
-- note: this is probably closer to the original
meaning, as suggested by the etymology:
α + δια + βατoς = not passing across