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



Quoting Carl Mungan <mungan@usna.edu>:

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

Thanks for the insights from Carl Mungan, John Denker and Bernard Cleyet.
Perhaps, someone from the list can inform The Physics Teacher such that this error would not be printed in the coming Apr 08 issue...


Alphonsus