- #1

Smith987

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## Homework Statement

I have been reading electricity and magnetism by Duffin in order to understand my first year EM course. So far it has been straightforward- however, I've hit a wall when it derives the force between 2 conductors (p.117 section 5.7 if you have the book!). The book says:

## Homework Equations

Force along x = - dU/dx with Q held constant, where U=electrical energy stored(I accept this)

Elementary increment dU= 0.5QdV (i accept)

=0.5 Q^2 * d(1/c) (I accept)

=-0.5 V^2 * dC.

This I do not accept, as in the first equality, it implies V is changing (dV) yet in the last equality V is constant? I'm also unsure as to where the minus sign cam from but I assume it was a misprint.

## The Attempt at a Solution

The only answer I can come up with is that the equation was meant to be

dU= 0.5 d[ V^2 * C]. But if I use this for the example of a parallel plate capacitor I get a sign error.

Please help!!