Question

The $\text{B-H}$ curve for a ferromagnet is shown in the figure. The ferromagnet is placed inside a long solenoid with $1000\text{turns/cm}$. The current that should be passed in the solenoid to demagnetize a ferromagnet completely is:

• A. ${10}^{-1}\text{A}$
• B. ${10}^{-2}\text{A}$
• C. ${10}^{-3}\text{A}$
• D. ${10}^{-4}\text{A}$

Answer 1

The correct option is C ${10}^{-3}\text{A}$

We know that, the intensity of magnetisation required to demagnetise the magnet is equal to the coercivity.

$\text{Coercivity}=H=nI....\left(1\right)$

Here, it can be clearly seen that for the complete demagnetization of the ferromagnetic material, the $y-$axis component must be zero, i.e., $B=0$.

Clearly, at this point, the value of coercivity, $H$ is $100\text{A/m}$.

Putting this value of $H$ in equation $\left(1\right)$, we get:

$100=\frac{1000}{{10}^{-2}}\times I$

$\therefore I={10}^{-3}\text{A}$

So, the current that should be passed in the solenoid to demagnetise a ferromagnet completely is ${10}^{-3}\text{A}$.

Hence, option $\left(C\right)$ is correct.