The coercivity of a bar magnet is \( 140\,\mathrm{A\,m^{-1}} \). To demagnetize it, it is placed inside a solenoid of length 1.6 m and number of turns 112. What is the current flowing through the solenoid?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2 A
The coercivity is the value of the magnetising field H that must be applied in the reverse direction to bring the magnetisation of the material to zero. The solenoid supplies this field, and inside a solenoid with n turns per unit length carrying current I, \( H = nI \).
\( \displaystyle n = \frac{N}{l} = \frac{112}{1.6\,\mathrm{m}} = 70\,\mathrm{m^{-1}} \)
\( \displaystyle I = \frac{H}{n} = \frac{140\,\mathrm{A\,m^{-1}}}{70\,\mathrm{m^{-1}}} = 2\,\mathrm{A} \)