Re: Electrostatic shielding

[William Beaty <billb@ESKIMO.COM>]

On Wed, 31 Jan 2001, Leigh Palmer wrote:

> > >  " . . . .  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."

<snip>

> > 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.
>
> Nonsense! "Conductor" => E==0 !

Something's wrong here.

Take a neutral hollow metal ball and place a positively charged object
inside. Is the field OUTSIDE of the hollow ball zero?  You say yes?  But
in order to shield the field of a charged object, you have to use an
OPPOSITELY charged object, and our hollow metal ball is neutral.
If a Faraday cage encloses a net charge, then as far as outside
measurements are concerned, the Faraday cage itself is charged.  A +1C
object within the cage induces a -1C charge on the inner surface of the
cage, but this leaves a +1C charge out the outer surface, and the field is
not stopped by the cage.  Yet the cage DOES produce a shielding effect,
because you can place the charged object in many different spots within
the cage, and the field outside will not change.

On the other hand, if the NET charge within the cage is zero, then the
cage acts as a perfect shield, and there will be no e-field outside, even
if the object inside the cage is composed of widely separated + and -
charges.


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