The discussion of shielding has been very helpful, but there is still one
part that for some reason is troubling me. Suppose we start with the
positive point charge at the center of the spherical conducting shell. The
shell has a uniformly distributed negative charge on its inner surface and a
uniformly distributed positive charge on its outer surface. Now when the
point charge moves way off center, how do we know that only the negative
charge on the inner surface redistributes itself to compensate? I can't
seem to come up with the condition that requires the outer positive charge
to remain uniformly distributed. (I suppose when I come back in the
morning, I'll wonder why I wrote this. Maybe it's just that it's 9:20PM
and I've been here since 7:20AM.)
______________________________________
Fred Lemmerhirt
Waubonsee Community College
Sugar Grove, Illinois
flemmerhirt@mail.wcc.cc.il.us <mailto:flemmerhirt@mail.wcc.cc.il.us> http://chat.wcc.cc.il.us/~flemmerh/physics.html
<http://chat.wcc.cc.il.us/~flemmerh/physics.html>
-----Original Message-----
From: John Denker [SMTP:jsd@MONMOUTH.COM]
Sent: Wednesday, January 31, 2001 7:15 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Electrostatic shielding
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.