Question

A parallel plate air capacitor has capacity 'C' farad, potential 'V' volt and energy 'E' joule. When the gap between the plates is completely filled with dielectric

• A. V increases, E decreases.
• B. V decreases, E increases.
• C. both V and E increase.
• D. both V and E decrease.

Hint:

For an isolated charged capacitor the charge stays constant, so a larger capacitance lowers both V and E.

Answer 1

The Correct Option is D

Step 1: Constant quantity
With no battery connected, the charge $Q$ stays fixed.

Step 2: Capacitance rises
Capacitance is $C = \dfrac{K\varepsilon_0A}{d}$, which grows by the factor $K$.

Step 3: Dependent quantities
$V = Q/C$ and $E = Q^2/(2C)$ both vary inversely with $C$, so both fall by the factor $K$.

Step 4: Answer
Option (D).

Final Answer:
Both V and E decrease. This is option (D). \[ \boxed{\text{(D) }\text{Both V and E decrease}} \]