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A parallel plate condenser with a dielectric of dielectric constant K between the plates has a capacity C and is charged to a potential V volt. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in this process is

1. zero
2. 1/2 (K − 1) CV²
3. CV² (K − 1)/K
4. (K − 1) CV²

Correct answer: zero

Solution

Removing the dielectric and then reinserting it brings the capacitor back to exactly its original configuration (same charge/voltage and stored energy). Over the complete cycle the net work done is zero.

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