Re: How many volts ?

[Mark Shapiro <mshapiro@sputter2.fullerton.edu>]

John,

	I agree with your statements regarding the final surface charges
on each of the plates.  However, your statement that +/- 55 nC are
required on the inner surfaces to produce the required field between the
plates is not quite accurate.  For example an insulating sheet with -5
nC/m^2 on each side opposite another insulating sheet with +50 nC/m^2 on
each side will produce the same field between both insulating sheets.
Your statement that +45 nC/m^2 is required on each of the outer plates of
the capacitor in order to produce ZERO field in the plates themselves is
correct.  This can be seen by noting that charge must be induced to
terminate field lines produced by the excess charges on both plates.
Consider the following diagram:

	Excess charge:  -5  -5           +50  +50
	Induced charge: +50 -50           +5  -5
        Total charge:   +45 -55          +55  +45

Cheers,

Mark 

Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866

Phone: ++ (714) 773-3884
Fax:   ++ (714) 449-5810
e-mail: mshapiro@fullerton.edu