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Re: Re IONS/metals pedagogy



I've been watching all this, being too busy to get involved, but perhaps
I can help. Let me say first that my background is surfaces, and in
particular the interaction of electrons with surfaces. I do experimental
work so I am mostly an informed observed when it comes to theory.
Let me suggest a few ideas, they may have been mentioned before, since I
haven't read all of the charge on metal messages.

From a classical point of view, image that the electrons in a metal are
of two groups, one set localized to their atoms and another set very free
to move around, almost like an ideal gas. What happens if we put a
charged object on the surface of that metal, say for example another
electron, so that metal now has a net charge. 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. Now lets add a second electron. You might think that since the
metal already carries a negative charge, the next electron would be
repelled. It is, as long as the electron is not too near the surface.
By not too near I mean far enough away not to induce a redistribution of
the electrons in the metal. However when the electron gets close enough,
it repels the electrons in the metal just as the first one did, and
creating a positive region which attracts the second electron. Even
though the metal carries a charge, the induced interaction is
stronger than the coulomb one, and the electron is bound to the surface.
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.

I suppose I have two questions, first do you think a child who
understands the Coulomb force would understand this model, and does it
fit with your understanding of phyiscs. Note that it is a one
dimensional model, perpendicular to the surface, but as long as we avoid
molecules that is probably ok.

cheers

1~On Thu, 8 Oct 1998, Ludwik Kowalski wrote:

What are we arguing about? Certainly not about superiority
of quantum physics. We are talking about a conceptual
dilemma a teacher of elementary physics faces while
interpreting most basic electric demonstrations. A piece of
metal is touched with an electrified rod and electrons
distribute themselves over the outer surface.

Here is a gedanken experiment. Two likely electrified pith balls
are placed in the middle of a sealed tube (which has no air) and
released. They repel each other and travel toward the opposite
ends of the tube. There is no bouncing and they remain as far
as possible from each other. Each pith ball is at rest because the
net force acting on it is zero (Fnet=Fcoul+Freact). If there were
no reaction forces at the tube's endings the balls would travel
to infinity.

Now back to charges which are distributed on the outer surface
of a metallic sphere. What keeps them at rest? Unless we say
that Fnet=m*a does not apply to electrons on the sphere (a=0
means Fnet=0) we MUST invent a force. Some say this can only
be done by using QM. And then they say that the concept of F
does not belong to the arsenal of its tools. Even if the concept
of F ("Pauli F") was acceptable, the QM can not be used to explain
things before students learn it.

Something is not right somewhere. There must be an attractive
force acting on each bunch of electrons; it must be equal and
opposite to the repulsive force exerted on it by other bunches.
Admitting our inability to explain this force in terms of what we
already know (in a given course) is much better than saying that
a=0 when Fnet is not zero.

I never saw a textbook telling students that electrostatic facts
demonstrate that mechanics has nothing to do with electricity.
On the contrary, we use newtonian mechanics to design mass
spectrometers and cyclotrons. Textbooks talk about limitations
of classical physics while discussing Bohr's model of atoms
(orbiting electrons radiate in synchrotrons but not in atoms).
Why don't they talk about this while discussing electrostatics?

It looks like we must teach QM before teaching electricity in
order to be preserve logical consistency. Teaching classical
kinematics before relativistic kinematics is OK but teaching
electricity before QM is not OK. I am not yet ready to take this
position. Perhaps there is a way to identify electric surface
forces classically. Please help.