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A parallel plate air capacitor of capacitance C is connected to a cell of emf V and then disconnected from it. A dielectric slab of dielectric constant K, which can just fill the gap of the capacitor, is now inserted in it. Which of the following is incorrect?

1. The energy stored in the capacitor decreases K times.
2. The change in energy stored is \( \frac{1}{2} CV^2 \left( \frac{1}{K} - 1 \right) \).
3. The charge on the capacitor is not conserved.
4. The potential difference between the plates decreases K times.

Correct answer: The charge on the capacitor is not conserved.

Solution

When the capacitor is disconnected from the cell, the charge on the capacitor remains conserved because there is no external circuit for charge to flow. Therefore, option C is incorrect.

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