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A 10 Ω, 20 mH coil carrying a constant current is connected to a battery of 20 V through a switch. Now after the switch is opened the current becomes zero in 100 μs. The average e.m.f. induced in the coil is _____ V.

Asked in JEE Main 26th June 1st Shift 2022 · Self-inductance and self-induced EMF

Answer: 400

Step-by-step solution

Steady current I=(20)/(10)=2 A.

ε=L(Δ I)/(Δ t)=20×10⁻³×2/(100×10⁻⁶).

ε=400 V. This large back emf is why a switch sparks.

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