Question

Assertion: A parallel plate capacitor is connected across battery through a key. A dielectric slab of dielectric constant K is introduced between the plates. The energy which is stored becomes K times.
Reason: The surface density of charge on the plate remains constant or unchanged.

• A. If both assertion and reason are true but the reason is the correct explanation of assertion.
• B. If both assertion and reason are true but the reason is not the correct explanation of assertion.
• C. If assertion is true but reason is false.
• D. If both the assertion and reason are false.
• E. If reason is true but assertion is false.

Answer 1

The correct option is C If assertion is true but reason is false.

Energy stored in the capacitor $U=\frac{1}{2}C{V}^2$

When a dielectric slab of dielectric constant K is introduced between the plates of the condenser, then

it becomes KC

So energy stored will become K times.
Since Q =CV , So Q will become K times
∴ Surface charge density

$\sigma =\frac{KQ}{A}K{\sigma }_o$