A particle of charge – q1 is at the origin of an x axis.
(a) At what location on the axis should a particle of charge – 4q1 be placed so that the net electric field is zero at x = 2.0 mm on the axis?
(b) If instead a particle of charge + 4q1 is placed at that location, what is the direction (relative to the positive direction of the x axis) of the net electric field at x = 2.0 mm?