A simple pendulum consists of a small sphere of mass `m` suspended by a thread of length `l`. the sphere carries a positive charge `q`. The pendulum is placed in a uniform electric field of strength `E` directed vertically upwards. With what period will pendulum oscillate if the electrostatic force acting on the sphere is less than the gravitational force ?

Answer

B

Explanation

The two forces acting on the bob are shown in figure

`g_(eff)` inthis case will be `(w - F_(e))/(m)`
or `g_(eff) = (mg - qE)/(m)`
`:. T = 2pi sqrt ((l)/(g_(eff)))`
`= 2pi sqrt ((l)/(g-(qE)/(m))`