Re IONS/metal pedagogy

[Ludwik Kowalski <kowalskiL@Mail.Montclair.edu>]

John Sample wrote:
> It seems to me that if you are looking for a qualitative
> understanding of why excess charge (up to some limit)
> does not fly off a charged metal surface, then the
> explanation is relatively simple and can be understood
> in purely classical terms. ....

I tend to agree. This approach, already suggested by Joe,
may not answer all questions but it is good enough for an
introductory physics course. Quantum physics must build
on that course, not the other way around, if possible.

The work function can be explained without difficulties
for a neutral metal. Electrons deep below the surface are
surrounded by positive ions but electrons at the surface
have positive ions only on one side. Therefore they do not
escape unless kinetic energies are high.

Now comes a sphere which has a net negative charge. Here
electrons above the surface are surrounded by a region which
is electrically neutral. But that region is polarized by the
layer of negative charge and this is sufficient to generate
an attractive force. Not very different from attraction of
a neutral pith ball by an electrified rod.

Joe wrote:
> ... The electrons in the metal near that charge will be
> repelled leaving a positive region in the metal which
> serves to attract that electron and keep it from drifting
> off into space. ... This happens because the Coulomb force
> falls off as the square of the distance, so the positive
> region near charge on the surface has a much greater
> influence than the distributed charge further away.
> Of course as I add more and more electrons, the coulomb
> repulsion gets bigger and bigger, and eventually other
> things happen.

Why should this not be acceptable? The conflict with what
is predicted by  Earnshow's theorem is intriguing. As alluded
by Bob, one may ask "how does mother nature defy the
theorem"? Let experts address this issue. They will find a way
out (for a very complex system), one way or another, probably
by using QM. I think that tolerating the dilemma emphasized
by Bob will create less trouble (in an introductory physics
course) than abandoning F=m*a.

Ludwik Kowalski