Question

A bar Magnet of pole strength $2Am$ is kept in a magnetic field of induction $4\times {10}^{-5}Wb/m^2$ such that the axis of magnet makes an angle ${30}^o$ with the direction of the field. The couple acting on the magnet is found to be $80\times {10}^{-7}Nm$ . Then the distance between the two poles of the magnet is :

• A. $20m$
• B. $2m$
• C. $3cm$
• D. $20cm$

Answer 1

Answer: (D)

$20cm$

As shown in the figure, A bar Magnet kept in a magnetic field of induction
such that the axis of magnet makes an angle ${30}^0$30o with the direction of the field.

The couple acting on the magnet $=\overrightarrow{M}\times \overrightarrow{B}=mlB\sin \theta =80\times {10}^{-7}$

$\Rightarrow 2\times l\times 4\times {10}^{-5}\times \sin {30}^0=80\times {10}^{-7}$

$\Rightarrow 4\times {10}^{-5}\times l=80\times {10}^{-7}$

$\Rightarrow l=20\times {10}^{-2}m=20cm$

Therefore, D is the correct option