question_answer

Point charges + 4q, - q and + 4q are kept on the \[x\]-axis at points \[x=0,x-a\] and \[x=2a\] respectively, then

A) only - q is in stable equilibrium

B) all the charges are in stable equilibrium

C) all the charges are in unstable equilibrium

D) none of the above

Correct Answer: C

Solution :

As net force on each charge is zero, therefore all the charges are in equilibrium. If we were to  displace\[-q\]to the right, net force of attraction will be to the right which will displace it further. Therefore, equilibrium is unstable.