Question

A, B and C are three large, parallel conducting plates, placed horizontally. A and C are tightly fixed and earthed. Plate B is given some charge. Under electrostatic and gravitational forces, B may be:

• A. In equilibrium if it is closer to A than to C.
• B. In equilibrium midway between A and C.
• C. B can never be in stable equilibrium.
• D. In equilibrium if it is closer to C than to A.

Answer 1

Answer: (A), (C)

The correct options are
A In equilibrium if it is closer to A than to C.
C B can never be in stable equilibrium.

As A and C are earthed, they are connected to each other. Hence, ‘A + B’ and ‘B + C’ are two capacitors with the same potential difference. If B is closer to A than to C, then the capacitance $C_{AB}>C_{BC}$. The upper surface of B will have greater charge than the lower surface. As the force of attraction between the plates of a capacitor is proportional to $Q^2$, there will be a net upward force on B. This can balance its weight.