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An air-cored solenoid has length 30 cm, cross-sectional area 25 cm² and 500 turns, carrying a current of 2.5 A. The current is switched off in 10⁻³ s. Ignoring end effects, find the average back-emf induced across the open switch.

1. 6.5 V
2. 0.65 V
3. 65 V
4. 650 V

Correct answer: 6.5 V

Solution

Compute the solenoid's self-inductance from L = mu0*N²*A/l, then the average emf during switch-off is L times the rate of current change (2.5 A in 10⁻³ s).

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