A point charge is placed at each corner of a square whose length is a. All charges have magnitude q. a) Two charges with a negative sign are located at x=a/2, y=a/2 and x=-a/2, y=a/2. The other charges are positive. What is the direction and the magnitude of the electric field located at the center of the square in terms of q and a? b) If the magnitude of the negative charge at x=a/2, y=a/2 is doubled repeat the calculation.