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Two identical capacitors are joined in parallel, charged to a potential difference V, then separated, and then connected in series with the positive plate of one connected to the negative plate of the other.

1. The charges on the free plates connected together are destroyed
2. The charges on the free plates are enhanced
3. The energy stored in the system increases
4. The potential difference across the free plates becomes V/2

Correct answer: The potential difference across the free plates becomes V/2

Solution

Each identical capacitor charged in parallel to potential V gets charge CV. After reconnecting in series with opposite plates joined, the charges on the joined plates cancel in effect and the outer free plates remain at potentials that differ by V/2. Thus the potential difference across the free plates becomes V/2.

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