Question

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

• A. 8 A
• B. 10 A
• C. 12 A
• D. 14 A

Answer 1

Answer: (A)

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