JEE Main & Advanced Physics Magnetism Question Bank Magnet and its Properties

  • question_answer
    A magnetic needle lying parallel to a magnetic field requires W units of work to turn it through 60°. The torque required to maintain the needle in this position will be                                     [KCET 1994; MNR 1991; MP PET 1996; [AIEEE 2003; UPSEAT 2000; BHU 2004; Pb PET 2004]

    A)            \[\sqrt{3}\,W\]

    B)            W

    C)            \[\frac{\sqrt{3}}{2}W\]    

    D)            2W

    Correct Answer: A

    Solution :

                       \[W=MB(\cos {{\theta }_{1}}-\cos {{\theta }_{2}})=MB(\cos {{0}^{o}}-\cos {{60}^{o}})\]                    \[=MB\,\left( 1-\frac{1}{2} \right)=\frac{MB}{2}\]                    and \[\tau =MB\sin \theta =MB\sin {{60}^{o}}=MB\frac{\sqrt{3}}{2}\]            \[\therefore \ \tau =\left( \frac{MB}{2} \right)\sqrt{3}\Rightarrow \tau =\sqrt{3}\,W\]


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