Question

Two point charges of charge values Q and q are placed at distance x and x/2 respectively from a third charge of charge value 4q, all charges being in the same straight line. Calculate the magnitude and nature of charge Q such that the net force experienced by the charge q is zero. [Ans Q = q/4 , +ive ]

Answer 1

the given situation is: 

         F2q <———q———>FQ----------Q------------------------FQ<———2q———>Fq                  

                                          d-x                    x

now if net force on 2q has to be 0 then Q has to be negative and hence 2q will experience a force towards Q(attraction).But 2q will also experience a force away from q(repulsion).Hence these forces can be calculated using coulombs law and equated and get x.Same thing can be done with q to get another equation and using the two equation you can eliminate Q.

Hence,

FQ=Fq          and             F2q=FQ