A dielectric disc of radius R and uniform positive surface charge density `sigma` is placed on the grup with its axis vertical. A particle of mass m and positive charge q is dropped, along the axis of the disc from a height H with dropped, along the axis of the disc from a height H with zero intial velocity. The charge -mass ratio of the particle is `(q)/(m)=(4epsion_(0)g)/(sigma)`.
(i). Find the value of H if the particle just reaches the disc.
(ii). Sketch the potential energy of the particle as a function of its height and find its height in equilibrium position.

Answer

(i) `H=(4)/(3)R`
(ii). `H=(R)/(sqrt(3))`