One end of a 10 cm long silk thread is fixed to a large vertical surface of a charged non conducting plate and the other end is fastened to a small ball having a mass of 10 g and charge of `4.0 xx 10^ C`. In equilibrium, the thread make an angle of `60^0` with the vertical. Find the surface charge density on the plate.

Explanation

(a) Tension in the string in equilibrium
`T cos 60^@ = mg `
` T = (10^-1) xx 2 = 0.20 N.`
(b) Straightening the same fig, now the
resultant force 'R' induces the accln.on the
pendulum
` T = 2 pi (sqrt(i/g))`
` = 2 pi (sqrt(10 xx (10^-2)/9.8))`
` = 0.45 sec.` .