- #1

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## Homework Statement

I'm going over some EM notes and I found a derivation for the potential energy of a dipole in an E field which first found the torque on the dipole and then integrated.

I had a go at a derivation that goes the other way, starting from the potential of the 2 charges. I've ran into a bit of a problem midway through and I can't see how to get past it.

## Homework Equations

__Want to show:__U = -

**p**.

**E**

## The Attempt at a Solution

U = q(V

_{B}-V

_{A})

now I don't know how to get from here to what I want to show. Both of the potentials look like [itex]\frac{q}{4πε

_{0}r}[/itex] , where the r is different in either potential.

The only thing I can think of is to use an approximation to r, but that doesn't give an acosθ term in the numerator.