Two positive charges +Q are held fixed a distance d apart. A particle of negative charge -q is placed midway between them, perpendicular to the line joining them .At what point along perpendicular bisector will it experience maximum force

If the -ve charge is kept at the mid point of the line joining the two charges the net force will be 0, bcoz the force experienced by the -ve charge due to the two +ve charges will be equal and in the opposite direction.

It will experience max force only when it is kept in mid of the charges and on the line joining them....as if you keep them anywhere else on tne bisector..... the parallel components will always cancel.... and you'll get only the cos components..........