A small charged ball is in state of equilibrium at a height h above a large horizontal uniformly charged dielectric plate having surface charge density of `sigma C//m^2`. (a) Find the acceleration of the ball if a disc of radius r (ltlt h) is removed from the plate directly underneath the ball. (b) Find the terminal speed `(V_0)` acquired by the falling ball. Assume that mass of the ball is m, its radius is x and coefficient of viscosity of air is `eta`. Neglect buoyancy and assume that the ball acquires terminal speed within a short distance of its fall.

Answer

(a). `(gr^(2))/(2h^(2))`
(b). `V_(0)=(mgr^(2))/(12 pi eta xh^(2))`