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 dieletric 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 1/2CV²(1/K - 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 capaciotor decreases K times.

Answer: (b). Disconnected capacitor: charge Q = CV is conserved, so (b) is incorrect. C' = KC, V' = V/K: (c) is correct. U' = U/K, Δ U = 1/2CV²(1/K - 1): (a) and (d) are correct. So the incorrect statement is (b).

A parallel plate air capacitor of capacitance C is connected to a… | NEET 2015 (AIPMT, Cancelled Paper)

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Dielectrics

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 dieletric constant K, which can just fill the air gap of the capacitor, is now inserted in it. Which of the following is incorrect?

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Answer: (b)

Disconnected capacitor: charge $Q = CV$ is conserved, so (b) is incorrect.
$C' = KC$ , $V' = \frac{V}{K}$ : (c) is correct.
$U' = \frac{U}{K}$ , $\Delta U = \frac{1}{2}CV^2\left(\frac{1}{K} - 1\right)$ : (a) and (d) are correct.
So the incorrect statement is (b).

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Dielectrics question from NEET 2015 (AIPMT, Cancelled Paper)

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