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Question:

In the following diagrams, a particle with small charge −q is free to move up or down, but not sideways near a larger fixed charge Q. The small charge is in equilibrium because in the positions shown, the electrical upward force is equal to the weight of the particle. Which statement is true?

In Fig. (a), −q is in stable equilibrium

In Fig. (a), −q is in neutral equilibrium

In Fig. (b), −q is in stable equilibrium

Neither in fig (a) nor in fig (b) −q is in stable equilibrium

Solution:

In figure 1, charge -q is in unstable equilibrium because if it is pulled down the electrostatic force is lesser than the downward gravitational pull and hence it will accelerate downward. Now in other case If charge -q is displaced upward then it will come down due to gravitational force and if it is pulled downward then also it will tend to move to its original position hence it is the case of stable equilibrium.