Re: [Phys-L] phase change

[Carl Mungan <mungan@usna.edu>]

Sorry, two typos corrected below and one comment added. Disregard my previous message.

> > 1. Why is there a definite sublimation temperature T?
>
>
> The idea only works at constant /pressure/, with the solid
> nearly in equilibrium with its vapor.
>
> With that added requirement, the explanation is pretty simple.
> All physics is /locall/.  Look at what is going on locally at
> the surface.  The surface doesn't know how much solid is
> behind it or how much vapor is in front of it;  all it knows
> is the local temperature, pressure, density, et cetera.
> None of that changes as the reaction progresses.


I interpret this idea to mean: Since the gas and solid are in thermal contact (at the surface) and in (at least quasi) equilibrium, they have to be at the same temperature. Further the surface doesn’t know how big overall it is, so there can only be one local sublimation temperature (at any given pressure).

Fair enough, I can’t object to that. I now revise my question 1 to:

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

With regards to 1(a), I still wonder if folks think the animation on the following page is basically okay or not:

> > http://zonalandeducation.com/mstm/physics/mechanics/energy/heatAndTemperature/changesOfPhase/changeOfState.html <http://zonalandeducation.com/mstm/physics/mechanics/energy/heatAndTemperature/changesOfPhase/changeOfState.html> 

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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 <mailto:mungan@usna.edu>     http://usna.edu/Users/physics/mungan/ <http://usna.edu/Users/physics/mungan/>