My remark was intended to show that the charge distribution
producing the E-field in a limited region of space need not be unique.
By using charge layers (without regard to whether they are on
insulating or conducting sheets) and superposition you can arrive at the
appropriate fields everywhere....a la Faraday.... Students sometimes find
that instructive, and it helps them to understand where those +45 nC
layers come from.....
Cheers,
Mark
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866