Question

A bar magnet has coercivity of $4 \times 10^3 A m ^{-1}$. 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

• A. 2 A
• B. 4 A
• C. 6 A
• D. 8 A

Answer 1

Answer: (D)

The bar magnet's coercivity means that it requires a magnetic intensity $H=4 \times 10^3 Am ^{-1}$ to get demagnetised.
Let $i$ be the current carried by solenoid having $n$ number of turns per metre length, then by definition $H=n i$.
Here $n=$ turn $/$ metre
$n=\frac{N}{l}=\frac{60}{0.12}=500$ turn metre ${ }^{-1}$
$\Rightarrow i=\frac{H}{n}=\frac{4 \times 10^3}{500}=8.0\, A$