Re: Electrostatic shielding

[John Denker <jsd@MONMOUTH.COM>]

At 07:48 PM 1/31/01 -0500, Michael Edmiston wrote:
> If charges are outside a conductive hollow sphere, there will not be any
> electric field inside the sphere due to those charges regardless of whether
> the sphere is grounded or floating.

Right.

> If there are charges inside the sphere cavity, whether these cause electric
> fields outside the sphere does depend on whether the sphere is grounded or
> not.  If the sphere is not grounded there is electric field outside.  If the
> sphere is grounded there is no electric field outside.

Exactly so.

> What happens if the charges inside
> are not symmetrically distributed and are moving around?  Basically the same
> result as long as they are moving slowly enough that the charges from the
> earth have time to distribute themselves on the inner surface of the shell
> into the proper arrangement to keep the electric fields in the metal of the
> conductive shell at zero.

That's true as stated, but a stronger result also holds.  The phrase "from
the earth" could have been omitted.

Even for the UNGROUNDED shield, observers outside cannot learn anything
about where the charges are inside the shield.  (Yes, they can immediately
learn the total charge, but no, they cannot learn about the
distribution.)  The creatures on the inside cannot transmit any signals to
the outside by rearranging their charges, if they are restricted to moving
things slowly enough that the screening charges on the inside of the shield
have time to re-arrange themselves.

Usual disclaimer: None of the shielding ideas apply at high
frequencies;  at some point you have X-rays that can go through quite a bit
of shielding.