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A small magnet has a coercivity of 3 × 10³ A m⁻¹. What current must flow through a solenoid of length 10 cm with 100 turns so that the magnetic field inside it just demagnetizes the magnet placed within the solenoid?

1. 30 mA
2. 60 mA
3. 3 A
4. 6 A

Correct answer: 3 A

Solution

Turns per metre n = 100/0.10 = 1000 /m. To demagnetize, the solenoid field H must equal the coercivity: n*I = 3e3, so I = 3000/1000 = 3 A.

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