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 into it. Which of the following is INCORRECT?
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Question

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 into it. Which of the following is INCORRECT?

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G1 Physics · Accepted Answer

Step 1: Battery removed implies free charge Q = CV is conserved. Step 2: Dielectric raises capacitance to C′ = KC and lowers voltage V′ = V/K. Step 3: Stored energy falls from ½CV² to ½CV²/K, change in energy = ½CV²(1 − 1/K). Step 4: Therefore saying that the charge is not conserved is incorrect.