Four charges of magnitude +q are placed at the corners of a square whose sides have a length d. what is the magnitude of the total force exerted by the four charges on a charge q located a distance b along a line perpendicular to the plane of the square and equidistant from the four charges?

Explanation:

Since all the four charges are equidistant from the position of Q

so here we can assume this charge distribution to be uniform same as that of a ring

so here electric field due to ring on its axis is given as

here we have

x = b

and the radius of equivalent ring is given as the distance of each corner to the center of square

now we have

so the force on the charge is given as