[Phys-l] Physics Challenge for Teachers and Students (Feb08)

[Carl Mungan <mungan@usna.edu>]

> Perhaps most of you agree with the solution provided by J. Iñiguez
> from Universidad de Salamanca.

I assume you mean the final expression for the electric force, which is indeed correct. John D has already pointed out the error in the derivation "fortuitously" giving the right answer.

I would merely point out that one can more easily get the answer by saying: F = QE where Q = charge on one plate and E=Q/2*A*eps0 is the field due to the other plate.

It would be wrong (by a factor of 2) to use the total E due to both plates. One plate does not exert a force on itself. -Carl