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## Homework Statement

Find the force of interaction per unit length between the upper and bottom halves of a cylinder of uniform charge density rho.

## Homework Equations

Solution should be [itex]\rho^2R^3/3\epsilon_0[/itex]

## The Attempt at a Solution

Frustrated cause I've solved this problem in the past and don't recall how to do it.

There's no analytic expression for the field of half a cylinder yes? How would I start to set the problem up? Would I have to find the interaction energy first then take its gradient?