Re: How many volts ?

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Mon, 24 Mar 1997, Mark Shapiro wrote:

> 	I agree with John's answer, but I arrived at it using a slightly
> different line of reasoning....
>
> 	The 100 nC charge will spread equally over both surfaces of one
> plate, while the -10 nC charge will spread equally over both surfaces of
> the other plate.  Thus the surface charge density on one plate will be 50
> C/m^2 and -5C/m^2 on the other.  Using superposition, the E field between
> the plates is 55 C/m^2/epsilon0, ignoring edge effects.  Integrating this
> field from one plate to the other gives the potential difference (62.1 V).

Just a point of clarification:  This analysis gets the field between the
plates right as will *any* assumed distribution of the charges on the
plates. But the field between the plates requires +/-55 nC on the inner
surfaces and, therefore, +45 nC on each outer surface.  Furthermore, only
*this* distribution gives E = 0 within the plates. 

John
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