Re: Electrostatic shielding

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Leigh's comment:
> ...
> Fields set up inside the cage have no effect whatever on fields outside
> the cage. It is impossible to place a net charge inside a cage. That is
> the message of Gauss's law! A closed surface entirely within the conductor
> surrounds the interior and has electric field identically zero everywhere.

This is misleading.  It is true that there is no net charge inside the
surface taken strictly in the bulk of the conductor. But there certainly
*can* be a net charge in the strict interior of the cage (i.e. strictly
inside the inner surface of the cage).  When this is the situation there
is a compensating opposite surface charge on the inner surface of the
cage so that there is no net charge in the interior of an enclosing
surface taken entirely in the metal of the cage where the surface never
reaches neither the inner nor outer surfaces of the cage.  There is also
a surface charge induced on the outer surface of the conducting cage
that is equal to the original charge that is strictly inside the cage
*if* the cage is isolated and has not been grounded or has had any of
this induced charge bled off of it.  If the cage *has* been grounded,
then the surface charge on the cage's outer surface adjusts itself
so that the cage always remains at the ground potential regardless
of the particular pattern of fields outside the cage.

David Bowman
David_Bowman@georgetowncollege.edu