Two similar spheres having +q and -q charge are kept at a certain distance. F force acts between the two. If in the middle of two spheres, another similar sphere having +q charge is kept, then it experiences a force whose magnitude and direction are
Initially, force betwen A and C F=KQ2r2
When a similar sphere B having charge +Q is kept at the mid point of line joining A and
C, then Net force on B is
Fnet=FA+FC=KQ2(r2)+KQ2(r2)2=8KQ2r2=8F.
(Direction is shown in figure)