In Fig. 21-18, a central particle of charge -2q is surrounded by a square array of charged particles, separated by either distance d or d/2 along the perimeter of the square.  What are the magnitude and direction of the net electrostatic force on the central particle due to the other particle s?

 

-79 +4q +2q -3q -5q +3q -5 -39 +29 +4q -79 FIG. 21-18 Question 6.