A) \[\theta =0{}^\circ \]
B) \[\theta =45{}^\circ \]
C) \[\theta =60{}^\circ \]
D) \[\theta =90{}^\circ \]
Correct Answer: A
Solution :
Torque acting on a magnet of magnetic moment M placed in uniform field B is: \[\tau \,\,=\,\,M\,\,\times \,\,B\,\,=\,\,MB\text{ }sin\theta \] So \[\frac{d\tau }{d\theta }\] = Rate of change of torque with displacement \[(\theta )\] \[\frac{d\tau }{d\theta }\,\,=\,\,MB\,\,\cos \theta \] \[\frac{d\tau }{d\theta }\] will be maximum if \[\cos \theta \] = Maximum = cos 0 or \[0=0{}^\circ \] Hence, choice [a] is correct
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