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Re: [Phys-l] Isothermal and Internal Energy

On 8/23/10 4:07 PM, John Denker wrote:
On 08/23/2010 12:49 PM, James McLean wrote:
My question:
>>> Aside from phase transitions, are there other isothermal processes
that result in an internal energy change?<<<

a) Chemical reactions routinely violate that both ways:
dT=0 dU<>0
dT<>0 dU=0

OK, chemistry gets me thinking outside my box. Thanks.

b) Find a slightly "leaky" capacitor (or just attach a low-conductance
shunt). Charge it and leave it a thermally isolated container.

Should this be "leave it contact with a thermal reservoir"? I get that thermal insulation means dQ=dW=0, but that field energy has to go somewhere.

c) A high-Q mechanical oscillator in a box is another example.

I suppose this is another play on (b). I'm not entirely sure I would call that "internal" energy. If we introduce macroscopic degrees of freedom, then we open the whole pointless can of worms about where to draw the microscopic/macroscopic line.

=====================

Bottom line: temperature is not the same as energy. Really,
really not.
>
Sure. But then there are situations when they are really, really closely tied. Such as the ideal gas, the rose-colored glasses through which many students view all of thermodynamics. My question is how to most closely circumscribe those situations.

So let me up the ante:
Are there any isothermal processes in which there are no changes in
* macroscopic degrees of freedom,
* molecular structure (no chemistry or phase transitions), or
* magnetization (thanks BC),
and yet dU<>0?
I ask this knowing full well that what qualifies as "macroscopic" is vague.

Cheers,
-- James