Re: Electrostatic shielding

[William Beaty <billb@ESKIMO.COM>]

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

> Also, in the process of seeking a satisfactory explanation, I tried A. D.
> Moore's little book on electrostatics.  In Moore's discussion of Faraday's
> cage experiment, he says:
>
> " . . . .  He proved that any electric field he might set up outside of the
> cage had no effect whatever on detection instruments placed inside.
> Likewise, fields set up inside had no effect outside."
>
> Is that last statement correct?  (Maybe the answer depends on whether the
> cage is "grounded" or isolated?)

It's wrong.  I've seen other authors saying the same thing.

Only if the static fields within a faraday cage are generated by a
distribution of charges having ZERO net charge, do zero fields appear
outside the cage.

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.


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