(1) Leigh says some of us were not taking into consideration what net charge
the ungrounded shield/shell might have. This is true. I thought it was
given that the floating shell has no net charge.
Leigh might be saying this is impractical. Can I ground a sphere (in the
absence of other net charge) so there is no net charge on the shell, then
can I open a door and put a charge into the cavity, then exit and close the
door, all this without getting any net charge on the shell? Probably not if
I have to touch the door to open and close it. But I don't have to do that.
I'll build a big enough spherical shell to contain motors and a battery that
open the door by remote control. (If you're worried about how my remote
signal opens the door, forget that, just put the door opener on a timer.)
So the door is closed, no charges are near, I ground the shell, it has no
net charge. Then I un-ground the shell. The timer opens the door. I have
a positively-charge ball (that was previously hidden or very far away) that
I throw through the door and it lands perfectly on an insulating support
post, or it perfectly catches on a hook suspended by a nonconductive thread.
Then the timer closes the door. No charge has been transferred to the shell
because nothing external touched the shell.
Anyway, I assumed we were asking if the ungrounded shell with no net charge
on it would shield outside observers from observing an electric field caused
by net charges in the cavity. The answer to that question is no. Outside
observers will indeed observe a field. However, I agree with that the
outside observers will not be able to discern any motion of those charges
(assuming slow motions), nor will outside observers be able to distinguish
whether the field they observe is created by net charge inside the cavity
versus net charge on the surface of the sphere. The truth is, the field
they are observing IS originating (ending) on charges on the outer surface
of the shell.
(2) One of the problems people are having is understanding how the
outer-surface charge distribution stays constant even though the
inner-surface distribution is varying. As John and Bernard have pointed
out, this is based upon the idea that there is no electric field inside a
conductor after the initial polarization has taken place. I believe the
only way for the field to reappear (momentarily) within the metal of the
shell, would be to change the amount of charge in the cavity. Simply moving
the existing charge around inside the cavity will not do it.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817