Re: intermolecular forces

["John S. Denker" <jsd@MONMOUTH.COM>]

At 09:16 AM 2/11/01 -0500, I wrote:

> > "The repulsive [intermolecular] force has to do with
> > kinetic energy, and it's not electrostatic.  Think of an uncharged particle
> > in a box.  As you decrease the size of the box, the wavefunctions get more
> > wiggles per unit length, so their kinetic energy goes up.  This causes a
> > repulsive force, i.e. a pressure on the piston."

Then at 06:45 AM 2/20/01 -0500, Richard Bowman wrote:

> In fact at the most fundamental level, this is as a result of an
> electrostatic force.

I really don't think that's the most fundamental view.

> What John is referring to might be called the "hard
> sphere approximation" to gas behavior.

No, not at all.  Suppose my uncharged gas particles are _not_ hard
spheres.  That doesn't affect anything I said about them.

> But we also should ask what is
> causing one gas molecule to recoil from another one? In fact the
> electrostatic repulsion of the electrons of the gas molecules is the
> source of the recoil.

There is no reason to assume that.  Indeed, at
  http://mailgate.nau.edu/cgi-bin/wa?A2=ind0102&L=phys-l&F=&S=&P=52285
you can find a note that I posted yesterday (Monday), describing in some
detail the origin of a molecular field that is quite reasonably described
as being due to quantum effects AND NOTHING ELSE.  To be sure, there are
electrostatic interactions and other interactions lurking in the
background, but...
  a) the background interactions are incapable of creating a field with
the observed symmetry and magnitude.
  b) you don't need to know whether the background forces are
electrostatic, or magnetic, or nuclear, or whatever, in order to predict
the character of the molecular field.

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.

So I persist in believing that quantum mechanics has something to say that
is essentially independent of the electrostatic (or other) interparticle
potentials.  Trying to understand "degeneracy pressure" in terms of
potential energy is a losing strategy.  It's much better to think of it in
terms of kinetic energy.

> To a good measure at one atmosphere and near room
> temperature, we can model this behavior using the concepts of conservation
> of linear momentum and conservation of kinetic energy.

Yes, momentum and kinetic energy.  That supports my point.  That's very
different from electrostatic energy, which is usually considered potential
energy.