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A current loop of magnetic moment m is placed in a uniform field B so that m makes angle θ with B. The torque on the loop has magnitude:
Am B tan θ
Bm B cos θ
Cm B sin θ
Dm B
Answer & Solution
Correct answer: C. m B sin θ
1. The torque on a magnetic dipole is $\boldsymbol{\tau} = \mathbf{m}\times\mathbf{B}$.
2. The magnitude of a cross product is $\tau = mB\sin\theta$.
3. Here $\theta$ is the angle between m and B.
4. The torque is maximum at $\theta = 90^\circ$ and zero at $\theta = 0$, ruling out the cos θ form.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.156_
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