Two simiilar 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 expirience a force in magnitude and direction as

Dear pallavi

 let the initial distance is d

 F= k qq/d²

 if a third sphere is placed between then  sphere having charge q repel it and spehere having charge -q attract it.

 force by q charged sphere =F1=k qq/(d/2)²

                                                             =4k qq/d2 =4F ( from +q to -q)

force by -q charged sphere =F2 =k qq/(d/2)²

                                                            =4k qq/d2 =4F (from +q to -q)

so net force  = 4F + 4F =8F

Please feel free to post as many doubts on our discussion forum as you can.
 If you find any question Difficult to understand - post it here and we will get you the answer and detailed solution very quickly.
 We are all IITians and here to help you in your IIT JEE  & AIEEE preparation.

 All the best.
 
Regards,
Askiitians Experts
Badiuddin