CBSE 2024 · Region 1 · Set 2 · Q24 · 3 marks
A bar magnet of magnetic moment $\displaystyle 2 \cdot 5 \mathrm{JT}^{-1}$ lies aligned with the direction of a uniform magnetic field of $\displaystyle 0 \cdot 32 \mathrm{~T}$.(a)Find the amount of work done to turn the magnet so as to align its magnetic moment (i) normal to the field direction, and (ii) opposite to the field direction.(b)What is the torque on the magnet in above cases (i) and (ii) ?
A bar magnet of magnetic moment $\displaystyle 2 \cdot 5 \mathrm{JT}^{-1}$ lies aligned with the direction of a uniform magnetic field of $\displaystyle 0 \cdot 32 \mathrm{~T}$.
(a)
Find the amount of work done to turn the magnet so as to align its magnetic moment (i) normal to the field direction, and (ii) opposite to the field direction.
(b)
What is the torque on the magnet in above cases (i) and (ii) ?
Marking-scheme solution
(ii) \[\begin{aligned}
\mathrm{W} & =-\mathrm{mB}\left(\operatorname{Cos} 180^{\circ}-\operatorname{Cos} 0^{\circ}\right) \\
& =2 \mathrm{mB} \\
& =2 \times 0.8 \\
\mathrm{~W} & =1.6 \mathrm{~J}
\end{aligned}
\]
(b) (i)
\[\begin{aligned}
\tau & =\mathrm{mB} \sin \theta \\
& =0.8 \mathrm{Nm}
\end{aligned}
\]
(ii) \[\tau=0
\]
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CBSE Class 12 Physics past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.