Question

A bar magnet has coercivity $4\times 10^{3}A{m}^{-1}$. It is desired to demagnetise it by inserting it inside a solenoid $12cm$ 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 $4\times 10^{3}A{m}^{-1}$
ie., it requires a magnetic intensity $H=4\times 10^{3}A{m}^{-1}$ to get demagentised.
Let $i$ be the current carried by solenoid having $n$ number of turns per metre length, then
$H=ni$
Here $H=4\times 10^{3}A{m}^{-1}$
and $n=\frac{N}{l}=\frac{60}{0.12}=500$ turn $/m$
$\therefore i=\frac{H}{n}=\frac{4\times 10^3}{500}$
$=8.0A$