Question

The drawing shows a positive point charge +q1, a second point charge q2 that may be positive or negative, and a spot labeled P, all on the same straight line. The distance d between the two charges is the same as the distance between q1 and the spot P. With q2 present, the magnitude of the net electric field at P is twice what it is when q1 is present alone. Given that q1 = +2.86 µC, determine the magnitude |q2| when q2 is the following.

Answer 1

E=kq/r^2

so if r=P + P for the farther charge

then q2 must be four times as large as q1, as the distance is twice as far.