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Question: Answered & Verified by Expert
A bar magnet has a magnetic moment of $200 \mathrm{Am}^{2}$. The magnet is suspended in a magnetic field of $0.30 \mathrm{NA}^{-1} \mathrm{m}^{-1}$. The torque required to rotate the magnet from its equilibrium position through an angle of $30^{\circ},$ will be
PhysicsMagnetic Effects of CurrentWBJEEWBJEE 2013
Options:
  • A $30 \mathrm{N}$ -m
  • B $30 \sqrt{3} \mathrm{N}-\mathrm{m}$
  • C $60 \mathrm{N}$ -m
  • D $60 \sqrt{3} \mathrm{N} \cdot \mathrm{m}$
Solution:
2907 Upvotes Verified Answer
The correct answer is: $30 \mathrm{N}$ -m
Given, $\mathrm{M}=200 \mathrm{A} \mathrm{m}^{2}$
$\mathrm{B}=0.30 \mathrm{NA}^{-} \mathrm{M}^{-1}$
and $\theta=30^{\circ}$
We know that the Torque,
$$
\begin{aligned}
\tau &=M \times B \\
\Rightarrow \quad|t| &=M B \sin \theta=200 \times 0.3 \times \frac{1}{2} \\
&=100 \times 0.3=30 \mathrm{N} \mathrm{m}
\end{aligned}
$$

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