Re: Electrostatic shielding

[Leigh Palmer <palmer@SFU.CA>]

Michael Edmiston, Bill Beaty, et al are confusing themselves. They are
talking about a process in which the cage has no net charge, a condition
which is never mentioned in the description of the Faraday cage shielding
effect. Grounding the cage has no relation to the net charge on the cage.

Bill said:

> If you put a coulomb of charge inside a faraday cage, the cage itself
> seems to be charged with 1c, as if the flux lines go right through the
> walls of the cage.  However, if you move your 1c of charge to different
> places within the cage, the field outside the cage does not change.  The
> cage does prevent us from seeing the charge distribution from outside.
> But it doesn't prevent us from seeing the flux from any net charges which
> we place inside.

If you put a coulomb of charge inside a Faraday cage I'll be impressed,
but the charge on the cage itself will depend upon other factors as well,
particularly it initial charge and its state of isolation from the rest
of the world. It is very important to specify these things! There are no
default values. Bill's picture is almost correct if the cage is isolated
and initially uncharged. It is a real stretch to say that we are "seeing
the flux from any net charges" inside, one I certainly would not make.

David Bowman is correct; I should have made it clear that my "inside" and
"outside" referred to the gaussian surface, and that inside included the
inner surface of the cage.

Leigh