A 2 μF capacitor is connected to an ideal 20 V cell. The plate separation is slowly doubled. The work done by the external agent is:
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Question

A 2 μF capacitor is connected to an ideal 20 V cell. The plate separation is slowly doubled. The work done by the external agent is:

G1 Physics · Accepted Answer

With the battery connected, V remains 20 V. Doubling separation halves capacitance from 2 μF to 1 μF. The electrostatic attraction must be overcome; integrating the force or using the battery-energy balance gives external work +200 μJ. Correct answer: B.