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A capacitor is fully charged by a battery, and then the battery is disconnected. When a dielectric slab is inserted between the capacitor plates, what is the effect?

1. The charge on the plates decreases and the potential difference increases
2. The potential difference increases, stored energy reduces, but charge on the plates is unchanged
3. The potential difference decreases, stored energy decreases, but charge on the plates is unchanged
4. None of the above

Correct answer: The potential difference decreases, stored energy decreases, but charge on the plates is unchanged

Solution

Once the battery is disconnected, the capacitor becomes isolated, so its charge remains constant. Inserting a dielectric increases the capacitance, which lowers the potential difference and also reduces the stored energy.

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