Re: KE & temperature (was: Newton's 3rd law? ...)

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

I have to strongly disagree with David here.  The derivation of absolute
temperature (as a measure of the average translational kinetic energy per
molecule due to random motion) IS, IMO, very accessible to AP level students
and IS a very good example of how a concept such as temperature that seems
far afield from their basic kinematic and dynamic studies can be shown to
arise from just such studies.  To be sure, the Algebra text derivations do
not consider any of David's subtleties and usually starts with an assumption
of the Universal Gas Law as part of the derivation.  But that being said,
looking at the wall collisions as a source of the pressure, the change in
momentum in such collisions, the time of flight between ends of the box,
etc., etc. is a very important exercise for this level of student to work
through.  It really shows how to work out new physics from old, basic
principles.

Rick






----- Original Message -----
From: David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, October 25, 1999 1:54 PM
Subject: KE & temperature (was: Newton's 3rd law? ...)


> Regarding where Ed Schweber wrote:
> > ...
> >   BTW: two years ago I had a particularly bright AP class which was
> > disappointed that I didn't really prove that average KE per molecule was
> > proportional to Kelvin temperature and we had an extended and (I think)
> > productive discussion about how the microscopic and macroscopic
> > interpretations have an entirely different vocabulary that cannot be
equated
> > through equations but only by interpretation. This is what physics is (at
> > least in my opinion).
>
> It seems to me you were wise to not attempt to prove for your students
> that the (thermal equilibrium) avg. translational KE of the center of
> mass degrees of freedom of the particles (treated classically) was
> proportional to the absolute temperature--*not* because such a proof is
> not available, but because the proof goes way beyond the level
> appropriate for a HS AP class.  I don't think foregoing the proof is as
> much "what physics is", as it is an exercise in good pedagogical
> judgment