Re: Surface charge distribution

["Spagna Jr., George" <gspagna@RMC.EDU>]

Hasan writes:

-----Original Message-----
From: Fakhruddin, Hasanbhai [mailto:hfakhrud@BSU.EDU]
Sent: Wednesday, December 04, 2002 2:23 PM

> 1. If the point charge inside is +q will the total charge on the outer
> surface be +q (and the charge on the cavity wall be -q)?

Yes - charge must be conserved.

> 2.  Assume that the point charge is close to the cavity surface.  Now
> if we increase the radius of the sphere (keeping the point charge at the
> same distance from the cavity surface) the charge density on the outer
> surface will decrease (right?) When the radius becomes infinity the part
> of conducting sphere close to the point charge becomes a conducting
> plate.  The E-field on the other side of the plate will become ZERO
> because the charge density on the plate will be zero!   Is that correct?

Essentially, yes.  I'd quibble on the language and suggest that as the shell
radius gets very large, the outer surface charge density approaches zero. 
My students are so enamored with numbers they can enter on their calculators
that abstractions like "infinity" slip right by them.


********************************************
  "My idea of an agreeable person is a 
    person who agrees with me." 
                        --Benjamin Disraeli  
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George Spagna
Department of Physics
Randolph-Macon College
P.O. Box 5005
Ashland, VA 23005-5505
phone: (804) 752-7344     
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e-mail: gspagna@rmc.edu
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