A small metallic bob of mass `m` of a simple pendulum of length `l` is suspended from a silk threas between two parallel charged plates.The electric field intensity `E` act vertically downwards . If bob is given a charge `+ q`,time period of oscillation
(i) `T = 2 pi sqrt((l)/((g - (qE)/(m))))` , if the lower plate is charged positively
(ii) `T = 2 pi sqrt((l)/((g - (qE)/(m))))`,if the lower plate is charged negatively
(iii) `T = 2 pi sqrt((l)/((g + (qE)/(m))))`,if the lower plate is charged positively
(iv) `T = 2 pi sqrt((l)/((g + (qE)/(m))))` , if the lower plate is charged negatively

Options

A. `(i) ,(ii)`
B. `(i),(iii)`
C. `(i),(iv)`
D. all

Answer

C

Explanation

Time -period of simple pendulum
`T = 2pi sqrt((L)/(a)) ,a` : effective acceleration
(i)
`a = (mg + qE)/(m) = g - (qE)/(m)`
(iv)
`a = (mg + qE)/(m) = g + (qE)/(m)`