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## Homework Statement

I have a dipole in the electric field of a parallel plate capacitor. The plates are circles with radii r and they are at plus or minus d on the z axis. The coordinate system thus has the origin directly below/above the centres of the plates with x and y axes. I have to deduce the variation of force on the dipole as it moves along the y axis from y=-(3/2)r to y=(3/2)r for situations in which the dipole is parallel to each of the three axes.

## Homework Equations

U=-

**p.E**

**F**=-

**∇U**

## The Attempt at a Solution

I'm a little bemused by the movement from -(3/2)r to (3/2)r. Is the factor of 3/2 significant or is it chosen so that the force will safely be zero that far out from the plates? Anyway, the force would always be zero but for edge effects, so I guess they should be considered.

I should probably let the upper plate be at a higher potential than the top. If the dipole moment is parallel to the x axis, then I believe we would never have any net force, only a torque. Parallel to the z axis, we would have zero force, then increasing to some net force towards the origin as we get to the edges of the field, then dropping to zero as we enter the field, then increasing again at the other edge acting towards the origin, then back to zero again. I'm not too sure about those two anyway, but at least have an idea. Then when parallel to the y axis, I guess it would be similar to when parallel to the z axis. Anyway I'm not too sure so if somebody could have a check that would be nice :)