Question

Four equal positive charges, each of charge Q are arranged at the corners of a square of side l. A unit positive charge of mass m is placed at point P at height h above the centre of the square. Calculate Q, so that the charge is in equilibrium.

Answer 1

Correct option: (b)

Explanation: 

The force due to one charge on the charge at P is shown here. When all the forces are taken in to account, then the sine component cancel each other. So, 

For equilibrium of m, this F_net must balance the weight of the particle mg (downwards)