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# Re: Electrostatic shielding

At 01:56 AM 2/1/01 -0500, Bob Sciamanda wrote:

1) The charges will settle to an arrangement in which the conducting
material is an equipotential volume (E=0) under the combined effects of the
three items, the imposed charge, the inner surface charge and the outer
surface charge.

That's narrowly true, but not very useful, since the metal can freely make
essentially infinite electron/hole pairs and redistribute them, thereby
changing the inner-surface charge and the outer-surface charge; only the
sum of those two quantities is conserved.

2) In the case of the single imposed charge centered inside a hollow,
conducting sphere the imposed central charge and the inner surface charge
alone accomplish this as a pair, and the outer surface charge also does so
(contributes zero E and constant V throughout the conducting material).

I don't know what that is trying to say. I'll ignore it and proceed.

3) Two questions:

a) If the imposed charge moves off center will the inner surface charge
still settle so that it and the imposed charge still (as a pair) contribute
zero E in the conducting material.

Yes. The key idea is that there is no field inside the material. The
mobile charges guarantee it. At DC the conductor is an equipotential
throughout its bulk.

If so, the outer surface charge must
still contribute zero E (constant V) and so will not be re=arranged.

Right.

b) If a) is so for the sphere, is it true for the generally shaped hollow
conductor enclosing a fixed imposed charge.

Right.

==============================================================

Observers on the outside cannot tell the difference between

Si So
| |
| |
-Q=======Q -Q=====================Q (real)
| |
| |
metal

and

Si So
| |
-Q=======Q |
| -Q=====================Q (real)
| |
| |
metal

and

Si So
| |
| |
| -Q=====================Q (real)
-Q=======Q |
| |
metal

Remember that the -Q on the outer surface will move in response to electric
fields, and (in the DC limit) electric fields only, and there is no
electric field inside the material.