A charge Q is brocken in two parts and placed at some distance.Find that the force between them is maximum.

let parts are sepereted at a distance of d

so repulsion force F= k q(Q-q)/d²

     here Q and d are constant so for maximum force differentiate the force w.r.t q

 dF/dq = k/d² [Q -2q]

 for maximum force dF/dq=0

                             Q-2q=0

                              Q/q =2