Point charges +4q, -q and +4q are kept on the X-axis at points x=0, x=a and x=2a respectively. Then

only -q is in static equilibrium
none of the charges is in equilibrium
all the charges are in unstable equilibrium
all the charges are in stable equilibrium

Correct answer: only -q is in static equilibrium

Solution

The charge -q is in static equilibrium because the forces due to the two +4q charges on either side of it are equal in magnitude and opposite in direction, canceling each other out. The other charges are not in equilibrium as there is a net force acting on them.

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