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Perhaps most of you agree with ...
http://scitation.aip.org/journals/doc/PHTEAH-home/challenges/feb2008.pdf
However, I have had an interesting discussion ....
The "problem" lies with differentiating "1/2 CV squared" with respect
to x and claiming this to be the electric force. The "problem" is that
V also depends on x. In order to keep V constant while we change x
(and hence change the capacitance), we will have to change the amount
of charge stored in the capacitor, ....
The energy change in changing x does not completely go towards moving
the plates. It also goes partly to moving the charges. So
"differentiating "1/2 CV squared" with respect to x" is NOT equal to
the electric force between the plates.