A spherical shell having charge +Q (uniformly distributed) and a point charge `+ q_(0)` are placed as shown. Find the force between shell and the point charge `(r gt gt R)`.

(i) Force on the point charge `+q_(0)` due to the shell `=q vec(E)_("0 shell")=(q_(0)) ((KQ)/r^(2))hat(r)=(KQq_(0))/r^(2) hat(r)` where `hat(r)`. is unit vector along OP.
From action - reaction principle, force on the shell due to the point charge will also be : `F_("shell")=(KQq_(0))/r^(2) (-hat(r))`
Conslusion : To find the force on a hollow sphere due to outside charges, we can replace the sphere by a point charge kept at centre.