A small positively charged ball of mass m is suspended by an insulating thread of length L. This ball remains in equilibrium with string horizontal when another small charged ball is placed exactly at a distance L below the point of suspension of the first ball. The second ball is slowly moved away from the first ball to a far away point. (The second ball is moved horizontally so that the first ball does not accelerate). As a result the first ball lowers down to the original position of the second ball and the string become vertical. Find the work done by the external agent in removing the second ball.

Answer

`W_(ext)=-3mgL`