Re: Confused by a derivation.

[Bob Sciamanda <trebor@VELOCITY.NET>]

Michael wrote to me:
> If you are somehow getting these from E = sigma/2epsilon then I think
that
> is incorrect because that relation does not apply to this situation.
> Outside a conductor with surface charge density sigma the field is
> sigma/1epsilon.
And Michael wrote to John D:
>  Literally we only
> consider the charge on the near side because the electric field from the
far
> side does not penetrate the conductor.  So there is no field to "always
> always always" add from the far side of the conductor.  We only add the
> fields from the charges on the near side

These quotes show your error.  Coulomb's law says that each charge element
produces an inverse square field at every field point.  This does not
change if you bring conductors, insulators or whatever into the region.
When everything settles, Coulomb's law still applies to the final
configuration and the net field is the superposition of the inverse square
law fields of all of the elements of the resulting charge distributions.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor