A non-conducting disc of radius $\displaystyle a$ and uniform positive charge density $\displaystyle \sigma$ is placed on the ground, with its axis vertical. A particle of mass $\displaystyle m$ and positive charge $\displaystyle q$ is dropped, along the axis of the disc, from a height $\displaystyle H$ with zero initial velocity. The particle has $\displaystyle \frac{q}{m} = \frac{4\epsilon_0 g}{\sigma}$

A) Find the value of H if the particle just reaches the disc.

Answer:Spoiler:

B) Sketch the potential energy of the particle as a function of its height and find its equilibrium position.

How to do it?