JEE Main & Advanced Sample Paper JEE Main - Mock Test - 28

  • question_answer
    A magnetic needle suspended parallel to a magnetic field requires \[\sqrt{3}\,J\] of work to turn it through \[60{}^\circ \]. The torque needed to maintain the needle in this position will be:

    A) \[2\sqrt{3}\,J\]               

    B) 3J

    C) \[\sqrt{3}\,J\]    

    D)        \[\frac{3}{2}\,J\]

    Correct Answer: B

    Solution :

    According to work energy theorem \[W={{U}_{final}}-{{U}_{initial}}=MB\,(cos0-\cos 60{}^\circ )\] \[W=\frac{MB}{2}=\sqrt{3}\,J\] \[\tau =\vec{M}\times \vec{B}=\,\,MB\,\,sin\,\,60{}^\circ =\,\,\left( \frac{MB\sqrt{3}}{2} \right)\]   ... (ii) From equation (i) and (ii) \[\tau \,\,=\,\,\frac{2\sqrt{3}\times \sqrt{3}}{2}=3\,J\]


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