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+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. none of the charges are in equilibrium
C. all the charges are in unstable equilibrium
D. all the charges are in stable equilibrium.
Sol: At the present position all the charges are in equilibrium. But when they displaced slightly from their present position, they do not return back. So, they are in unstable equilibrium position.