Question

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

Answer 1

Answer: (C)

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.