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.

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If assertion is true but reason is false.

If both assertion and reason are true but the reason is the correct explanation of assertion.

If both assertion and reason are true but the reason is not the correct explanation of assertion.

If both the assertion and reason are false.

answer is C.

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Detailed Solution

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
$σ = \frac{K Q}{A} = K σ_{0}$