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Questions  

A parallel plate air capacitor of capacitance C is connected to a cell of emf V and then disconnected from it. A dielectric slab of dielectric constant K, which can just fill the air gap of the capacitor, is now inserted in it. Which of the following is incorrect?

a
The change in energy stored is  12CV21K-1 .
b
The charge on the capacitor is not conserved.
c
The potential difference between the plates decreases K times.
d
The energy stored in the capacitor decreases  K times.

detailed solution

Correct option is B

Due to dielectric insertion, new capacitance C2=CK Initial energy stored in capacitor, U1=q22C Final energy stored in capacitor, U2=q22KCChange in energy stored, ΔU=U2-U1ΔU=q22C1K-1=12CV21K-1New potential difference between plates V'=qCK=VK

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