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A, B and C are three large, parallel conducting plates, placed horizontally. A and C are rigidly fixed and earthed. B is given some charge. Under electrostatic and gravitational forces, B may be (a) in equilibrium midway between A and C (b) in equilibrium if it is closer to A than to C (c) in equilibrium if it is closer to C than to A (d) B can never be in stable equilibrium |
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Answer» Correct Answer is: (b, d) 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 CAB is > CBC. 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. |
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