Re: intermolecular forces

[Richard Bowman <rbowman@BRIDGEWATER.EDU>]

Thanks to John S. Denker for mentioning his note of Monday. I had missed
that. And I do agree that the quantum description does give us some useful
handles. However, as a physics teacher and molecular researcher, I like to
remind myself and my students of the very basic underlying concepts (even
if we don't get involved with them at the level of detailed calculations).

So, here I simply go back to Schrodinger's equation and remember that for
atoms (and molecules, at some level of approximation) we solve the energy
equation for the behavior of an electron in the presense of the Coulombic
(electrostatic) potential.

And whether we talk about (what chemists call) Leonard-Jones potentials
or van der Waals forces or whatever model, we are still modeling the
electrostatic interactions between particles.

John also wrote today:
> Similarly, to return to the uncharged gas in a piston that I mentioned at
> 09:16 AM 2/11/01 -0500, you could argue that "normally" the gas particles
> interact with the piston via electrostatic forces at impact.  But what
> happens if I coat the surface of the piston with some hypothetical material
> that repels the gas via some magnetic interaction, or some nuclear
> interaction, or whatever?  The measured pressure is unchanged.  The
> pressure does not depend on the nature of the interaction.  It only depends
> on the kinetic energy via the quantum statistics.

But in fact this is what I referred to as the "hard sphere"
approximation. At its basic level, all such interactions are in fact not
totally elastic. Some energy is lost in the interaction with the walls and
the kinetic energy (temperature) of the gas decreases with time. Which is
what happens dramatically in magnetic confinement in fusion reactors, I
believe.

I may be applying too fine a distinction to this discussion, but I thought
that was what Tucker Hiatt was asking about when he questioned whether
there were only four basic forces in our universe or not.

Richard Bowman
Bridgewater College