Q .     A pitch ball covered with a tin foil having a mass `m` kg hangs by a fine silk thread of length `l` metre in an electric field `E`. When the ball is given an electric charge of `q` coulomb, it stands out `d` metre apart from the vertical line. The magnitude of the electric force will be:

BITSAT 2013
A

`\frac{mgd}{\sqrt{l^2-d^2}}`

B

`\frac{mgd}{al}`

C

`\frac{mgl}{q\sqrt{l^2-d^2}}`

D

`\frac{mgl^2}{q(\sqrt{l^2-d^2})}`

HINT

Suppose that the thread makes angle `\theta` with the vertical. Then `tan\theta=\frac{d}{\sqrt{l^2-d^2}}`

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