JEE Main · 2014 · 06 Apr 2014 · Offline · Magnetic Properties of Matter

The coercivity of a small magnet where the ferromagnet gets demagnetized is

3 \times 1 0^{3} 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)3 A✓ Correct
(B)6 A
(C)30 mA
(D)60 mA

Answer: (A)

Solution

Inside a long solenoid the magnetising field is

H = ni

, with

n

the turns per unit length. The sample is demagnetised when

H

reaches the coercivity,

3 \times 1 0^{3}

A/m. Here

n = \frac{100}{0.1} = 1000

turns per metre, so

3 \times 1 0^{3} = 1000 i

, giving

i = 3

JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option

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