Q 12-05-078JEE MainJEE Main 2018 (15 Apr, Shift 1)Medium
The $B-H$ curve for a ferromagnet is shown in the figure. The ferromagnet is placed inside a long solenoid with $1000\ \text{turns cm}^{-1}$. The current that should be passed in the solenoid to demagnetize the ferromagnet completely is,
Answer: (C) $1\ \text{mA}$
To demagnetize the ferromagnet completely, the applied field $H$ must equal the coercivity, where the loop cuts the $H$-axis:
$$H_c = 100\ \text{A m}^{-1}$$
For a solenoid, $H = nI$ with $n = 1000\ \text{cm}^{-1} = 10^5\ \text{m}^{-1}$:
$$I = \frac{H_c}{n} = \frac{100}{10^5} = 10^{-3}\ \text{A} = 1\ \text{mA}$$
Solution by Sreeraj P, M.Sc Physics