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Re: Electrostatic shielding

The macroscopic answer is that charge is conserved, therefore, the same charge
(amount and sign) that is inside is on the outside. If the inside charge
contacts the inside of the sphere it neutralizes the charge induced on the inside
and leaves the outside charge unchanged. The lack of "communication" is true
only in the static case. The point is a conductor conducts until the field is
zero (within the conductor) by rearrangement of charges. Since there is no field
with in the conductor it is a constant (equi) potential surface and, therefore,
the charge outside is uniform by symmetry (The field due to the charge must be
normal to the sphere, otherwise there'd be a flow-flow.) The fields due to the q
inside the sphere and the q on the inside of the sphere cancel otherwise there
would be electron flow-flow (assuming it's a metal).

Now Leigh can tell me if I've got it.


P.s. I'm rather certain the q inside the sphere will experience a (net) force if
it's not in the center

William Beaty wrote:

On Wed, 31 Jan 2001, Lemmerhirt, Fred wrote:

Now when the
point charge moves way off center, how do we know that only the negative
charge on the inner surface redistributes itself to compensate? I can't
seem to come up with the condition that requires the outer positive charge
to remain uniformly distributed.

Aha! I suddenly see Leigh's point. I'm blinded by empirical behavior of
hollow conductive objects: when you place a charged object into a
conductive shell, the charge "effectively" migrates to the outer surface
of the shell, and then redistributes itself regardless of the charge
distribution within the shell. The field outside the shell tells us how
much charge we've placed inside.

On the other hand, since the field within the metal of the hollow shell is
zero, there is no continuity between the flux inside and the flux outside.

Hmmm. Perhaps the zero flux within the metal of the shell is maintained
in a manner similar to the zero g-field within a hollow spherical shell of
mass. It's a set of very strong forces which sum to zero.

When we place a charged object inside a conductive shell, it induces an
opposite charge on the inner surface. The reason for this is clear. But
HOW does it also induce an alike charge on the outer surface, since there
can be no communication between the inside and the outside through the
thick layer of metal? Yet there MUST be communication, otherwise the
charge on the outside surface wouldn't "know" that it's supposed to stay
there, and it would recombine with the charge on the inner surface.
During the time when the charged object is being inserted (through a hole
in the shell?), that's when the transient currents and the "communication"
must take place; when the charges on the inner and outer surface come to
an agreement of how to distribute themselves and yet maintain zero field
within the metal.

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