A bar magnet has coercivity $4 \mathrm{x} {10}^3 {\mathrm{Am}}^{-1}$. It is desired to demagnetise it by inserting it inside a solenoid 12 cm long and having 60 turns. The current that should be sent through the solenoid is

The bar magnet coercivity is $4 \mathrm{x} {10}^3 {\mathrm{Am}}^{-1}$ i.e., it requires a magnetic intensity $\mathrm{H} = 4 \mathrm{x} {10}^3 {\mathrm{Am}}^{-1}$ to get demagnetized. Let i be the current carried by solenoid having n number of turns per metre length, then by definition H = ni. Here $\mathrm{H}=4 \mathrm{x} {10}^3 {\mathrm{Am}}^{-1}$

$\mathrm{n}=\frac{\mathrm{N}}{\mathrm{l}}=\frac{60}{0.12}=500 \mathrm{turns} {\mathrm{metre}}^{-1}$

$\Rightarrow \mathrm{i}=\frac{\mathrm{H}}{\mathrm{n}}=\frac{4 \mathrm{x} {10}^3}{500}=8.0 \mathrm{A}$