Three identical spheres, each having a charge q and radius R. are kept in such a way that each touches the other two. The magnitude of the electric force on any sphere due to the other two is

Explanation

Given three identical spheres each having a charge q and radius r are kept as shown.

For any external point, sphere behaves like a point charge. So it becomes a triangle having point charges at its corners.
`|vec(F)_(q q)|=(kq^(2))/(4R^(2))`

So, net force `(F)=2. (kq^(2))/(4R^(2)) . "cos "60/2=2. (kq^(2))/(4R^(2)) sqrt(3)/2=sqrt(3)/4 . (kq^(2))/R^(2)`