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A capacitor of capacitance $C$ has charge $Q$ and stored energy is $W$. If the charge is increased to $2Q$, the stored energy will be

1. W/2
2. 2/W
3. 4W
4. 4/W

Correct answer: 4W

Solution

The energy stored in a capacitor is $W=\dfrac{Q^2}{2C}$. If the charge becomes $2Q$, the new energy is $W' = \dfrac{(2Q)^2}{2C} = 4\dfrac{Q^2}{2C} = 4W$.

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