Four point charges are located at the corners of a square with sides of length a. Two of the charges are +q, and two are -q. Find the magnitude and direction of the net electric force exerted on a charge +Q, located at the center of the square, for each of the following two arrangements of charge:

(a) The charges alternate in sign (+q, -q, +q, -q) as you go around the square;

(b) The two positive charges are on the top corners, and the two negative charges are on the bottom corners.