Question

A bar magnet has coercivity $4\times {10}^{3}A{m}^{-1}$. It is placed in a solenoid having $40$ turns per cm and is $1.2m$ long. The current carried by the solenoid to demagnetize the bar magnet is :

• A. $6A$
• B. $8A$
• C. $5.78A$
• D. none of these

Answer 1

The correct option is D none of these

$H=4\ast {10}^{3}A/m$

$B=H\ast l=4\ast {10}^{3}A/m\ast 1.2=48\ast {10}^{2}A$

${\mu }_0=4\pi \ast {10}^{-7}Tm/a$

$48\ast {10}^2={\mu }_{o}NI=4\pi \ast {10}^{-7}\ast 40\ast I$

$I=\frac{48\ast {10}^2}{4\pi \ast {10}^{-7}\ast 40}=0.095\ast {10}^{9}A$