Re: Electrostatic shielding

[John Denker <jsd@MONMOUTH.COM>]

At 03:21 PM 1/31/01 -0600, Lemmerhirt, Fred wrote:
> Can anyone share a reference to a favorite explanation of "electrostatic
> shielding" (at the level of calculus-based introductory physics)?

For a shield with no holes in it, in the DC limit the shield is an
equipotential and the result follows by inspection.

For a shield with holes in it, a suitable pedagogical example is a
semi-infinite cylinder with one open end.  Treat it as a waveguide beyond
cutoff, or otherwise solve the Maxwell equations to show that the applied
field falls off exponentially as a function of distance from the end.

Another pedagogical example is to consider a shield with LOTS of holes in
it, i.e. a grid.  Even simpler is to start with a regular array of vertical
bars and solve the equations using Fourier techniques.  Again the field
falls off exponentially if you're not too close to the holes.  An example
is worked out in Feynman;  he considers the far-field of an array of
fluorescent tubes, but the math is about the same for DC.

> " . . . . 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?

Obviously not correct for a shield with holes in it, if you look closely
enough.  Also obviously not correct for AC.  You can _SEE_ through the
holes, after all.

> (Maybe the answer depends on whether the
> cage is "grounded" or isolated?)

No.

Charge is conserved.  An isolated cage can't suddenly acquire a monopole
electric charge, which is the only thing you could hope to remove with a
grounding strap.