A) \[\text{ }\!\!\theta\!\!\text{ }\,\text{=}\,{{\text{0}}^{\text{o}}}\]
B) \[\text{ }\!\!\theta\!\!\text{ }\,\text{=}\,{{45}^{0}}\]
C) \[\text{ }\!\!\theta\!\!\text{ }\,\text{=}\,{{60}^{0}}\]
D) \[\text{ }\!\!\theta\!\!\text{ }\,\text{=}\,{{90}^{0}}\]
Correct Answer: A
Solution :
Torque acting on a bar magnet suspended - magnetic field of induction B is \[\tau =MB\,\sin \theta \] On differentiation we get \[\frac{d\tau }{d\theta }=MB\,\cos \theta \] \[\therefore \] Rate of change of torque \[\Rightarrow \] \[\frac{dz}{d\theta }=MB\,\cos \theta \] Obviously, for maximum value of \[\frac{d\tau }{d\theta }\] \[\cos \,\theta =1\] or \[\theta ={{0}^{o}}\]
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