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A coil of inductance 1 H and resistance 100 Ω is connected to a battery of 6 V. Determine approximately: (a) The time elapsed before the current acquires half of its steady – state value (b) The energy stored in the magnetic field associated with the coil at an instant 15 ms after the circuit is switched on. (Given ln2 = 0.693, e –3/2 = 0.25)

1. t = 10 ms ; U = 2 mJ
2. t = 10 ms ; U = 1 mJ
3. t = 7 ms ; U = 1 mJ
4. t = 7 ms ; U = 2 mJ

Correct answer: t = 7 ms ; U = 1 mJ

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