The coercivity of a small magnet where the ferromagnet gets demagnetized is 3 × 10³ Am⁻¹. The current required to be passed in a solenoid of length 10 cm and number of turns 100, so that the magnet gets demagnetized when inside the solenoid, is: (a) 30 mA (b) 60 mA (c) 3 A (d) 6 A

Answer: (c). The solenoid's H = nI must equal the coercivity: I = H/n. n = N/L = (100)/(0.1) = 1000 m⁻¹ I = 3 × 10³1000 = 3 A So the answer is (c) 3 A.

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Q17JEE Main 2014Easy

Hysteresis, Soft Iron vs Steel & Shielding

The coercivity of a small magnet where the ferromagnet gets demagnetized is $3 \times 10^{3}\text{ Am}^{-1}.$ The current required to be passed in a solenoid of length $10\text{ cm}$ and number of turns $100,$ so that the magnet gets demagnetized when inside the solenoid, is:

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Answer: (c)

The solenoid's $H = nI$ must equal the coercivity: $I = \dfrac{H}{n}$ .
$n = \dfrac{N}{L} = \dfrac{100}{0.1} = 1000\text{ m}^{-1}$
$I = \dfrac{3 \times 10^{3}}{1000} = 3\text{ A}$
So the answer is (c) 3 A.

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