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



Hi everyone,

Some of you've participated in the Physics Challenge for Teachers and Students.
Perhaps most of you agree with the solution provided by J. Iñiguez from Universidad de Salamanca.

However, I have had an interesting discussion with my friend on one "problem".
http://scitation.aip.org/journals/doc/PHTEAH-home/challenges/feb2008.pdf

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, since C = Q/V. So to change x while keeping V constant, we will also have to move the charges, and some of the change in energy stored has to be used to do this.

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.

Look forward to any insights from the List...


Alphonsus