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. . .
Now back to charges which are distributed on the outer surface
of a metallic sphere. What keeps them at rest?
. . .
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.
. . .
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.