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Re: [Phys-L] phase change



1. Do the particles in the gas and in the solid have the same (a) average kinetic energy?

Classically, yes, by the equipartition theorem. Quantum mechanically, no, unless you're reasonably close to the high-T limit where the equipartition theorem applies. At lower temperatures there's an exponential suppression of vibrational modes with hf >~ kT.

I mean for gas and solid coexisting at the sublimation point and I mean for a real material. You may pick any real material you like.

(b) average total energy?

Absolutely not. The average potential energy per particle in the solid is negative, compared to the gas.

So equipartition doesn’t apply?
To be more fair, are there two kinds of PE: vibrational PE (as used in the Dulong-Petit model for instance) and binding PE (which is what I assume you’re referring to)?

Carl, I still get the impression that you're looking for a purely mechanical (rather than thermodynamic, or statistical, or kinetic) explanation here. Is that a fair statement?


I’m not sure I know what you mean. I’ll accept any thoughts or ideas that would fly in an introductory majors physics class. This is the thermo part of their first course. So anything you want to say at that level, I’m happy to go with.

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Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9b, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/