Question

In an isolated parallel-plate capacitor of capacitance C, the four surfaces have charges Q₁, Q₂, Q₃ and Q₄, as shown. The potential difference between the plates is

(a) Q₁ + Q₂ + Q₃ + Q₄ / 2C

(b) Q₂ + Q₃ / 2C

(c) Q₂ - Q₃ / 2C

(d) Q₁ + Q₂ / 2C

Answer 1

Correct Answer is: (c) Q₂ - Q₃ / 2C

The charge on a capacitor means the charge on the inner surface of the positive plate—in this case, Q₂.

Potential difference between the plates

= charge on the capacitor ÷ capacitance.

∴ potential difference = Q₂/C = 2Q₂/2C = Q₂(-Q₂)/2C = Q₂ - Q₃ / 2C.