At each of the four corners of a square of side A, a charge +q is placed freely. What charge should be placed at centre of square so that the whole system be in equilibrium?

Q. Four charges each equal to $Q$ are placed at the four corners of a square and a charge $q$ is placed at the centre of the square. If the system is in equilibrium, the value of $q$ is :

Q.

Four charges, each equal to -Q, are placed at the corners of a square and a charge +q placed at its centre. If the system is in equilibrium, the value of q is

Q. Four equal point charges, each of magnitude +Q, are to be placed in equilibrium at the corners of a square. What should be the magnitude and sign of the point charge that should be placed at the center of the square to do this job?