The principle of superposition in electrostatics depends on more
than the notion of vector addition. There is the explicit assumption that
the sum is only over the "two-body" terms, I.e., that there are no
"many-body" forces present when you have more than two charges present.
This is an EXPERIMENTAL result....., which tells us about the nature of
electrostatic forces for isolated, static point charges. It cannot be
derived....
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866