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(it looks just like it's described. The middle plate is positive sigma, the outer plates are both negative sigma.)

## Homework Statement

A small hole passes through the middle plane. At t=0 an electron emerges from the hole moving perpendicular to the planes with speed v

_{o}. Assuming v

_{o}is small enough that the electron does not collide with the negative plate, show that its motion is periodic, and find the period.

## Homework Equations

## The Attempt at a Solution

It's obvious that it will be periodic since there is no force to pull it off the x-axis, and there is no friction to rob it of energy.

I have a feeling it's like a pendulum, where the amplitude does not affect the period. But we haven't had any examples of periodic motion yet in E&M.

From the pendulum, we have a small amplitude formula for period of T=2pi sqrt(L/g). I imagine charge is somewhat analogus to gravity, and perhaps distance between the plates is somewhat analgous to L (length of pendulum). Where do I begin?