A metal foil of thickness \( (3/4)d \) is introduced between two plates of a capacitor of capacitance C. The distance between the plates of the capacitor is d. The new capacitance of the capacitor will be
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4 C
The metal foil is a conductor, so the electric field inside it is zero. The potential difference builds up only across the remaining air gap \( d - t \), and the capacitor behaves as one with a smaller effective plate separation.
\( \displaystyle C = \frac{\varepsilon_0 A}{d} \)
\( \displaystyle C' = \frac{\varepsilon_0 A}{d - t} = \frac{\varepsilon_0 A}{d - \frac{3}{4}d} = \frac{\varepsilon_0 A}{d/4} \)
\( \displaystyle C' = 4\,\frac{\varepsilon_0 A}{d} = 4C \)