Question

at each of the four corners of a square of side a, a charge +q is placed freely. what charge should be placed at the centre of the square so that the whole system be in equilibrium. plzzzzz fast it's urgent

Answer 1

Since all the charges placed at the corners are positive, so to maintain equilibrium a negative charge must be placed at the center of the square.

Now what remains is to calculate the magnitude of this negative charge. To calculate, you can use the coulumb's law for force. Since the system is in equilibrium, net force on any charged body in the system will be zero. So to calculate the center charge, take the net force on a corner charge due to all other charges to be zero.

Answer 2

Since all the charges placed at the corners are positive, so to maintain equilibrium a negative charge must be placed at the center of the square.

Now what remains is to calculate the magnitude of this negative charge. To calculate, you can use the coulumb's law for force. Since the system is in equilibrium, net force on any charged body in the system will be zero. So to calculate the center charge, take the net force on a corner charge due to all other charges to be zero.