The figure shows four charges at the corners of a square of side L. Assume q and Q are positive.

**Answer 1**

The formula for electrostatic attraction/repulsion is

F = k*q1*q2/r^2

where q1 = q, q2 = Q, and r = L. The direction is toward or away from the Q being considered.

Individually write expressions for the forces on q due to the charges at the other three corners. Then form the resultant of those 3 forces. (Make use of symmetry while forming the resultant.)

**Answer 2**

Start by drawing force vectors on charge q. You will notice that if the force vectors from both of the -Q charges are combined, the resultant will line up with the force vector from the +4Q charge.

So, use pythagorean theorem to find the resultant of the two -Q forces using Coulomb’s equation F = k*q1*q2/r^2, and then use this result to subtract from the force of the +4Q charge.

The final answer will look similar this:

F = (K|4Q∣∣q∣ / 2L^2) − (sqrt(2)*K|Q∣∣q∣∣ / L^2)

Source(s): Student