A capacitor C is charged to V and another capacitor 2C is charged to 2V. Batteries are removed and the capacitors are connected in parallel with positive plate of one to negative plate of the other. The final electrostatic energy is:
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Question

A capacitor C is charged to V and another capacitor 2C is charged to 2V. Batteries are removed and the capacitors are connected in parallel with positive plate of one to negative plate of the other. The final electrostatic energy is:

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G1 Physics · Accepted Answer

Initial charges are CV and 4CV with opposite connection, so net node charge magnitude is 3CV. Total capacitance is 3C, hence final voltage magnitude is V and Uf=½(3C)V²=3CV²/2. Correct answer: C.