JEE Main & Advanced Physics Magnetism Question Bank Magnetic Equipments

  • question_answer
    The period of oscillations of a magnetic needle in a magnetic field is 1.0 sec. If the length of the needle is halved by cutting it, the time period will be    [MP PMT/PET 1998]

    A)            1.0 sec                                     

    B)            0.5 sec

    C)            0.25 sec                                  

    D)            2.0 sec

    Correct Answer: B

    Solution :

               \[T=2\pi \sqrt{\frac{I}{MB}}=2\pi \sqrt{\frac{\text{w}{{l}^{2}}/12}{\text{Pole}\ \text{strength}\times 2l\times B}}\] \[\therefore T\propto \sqrt{Wl}\] \[\therefore \,\frac{{{T}_{2}}}{{{T}_{1}}}=\sqrt{\frac{{{\text{w}}_{2}}}{{{\text{w}}_{1}}}\times \frac{{{l}_{2}}}{{{l}_{1}}}}=\sqrt{\frac{{{\text{w}}_{1}}/2}{{{\text{w}}_{1}}}\times \frac{{{l}_{1}}/2}{{{l}_{1}}}}=\frac{1}{2}\] \[\Rightarrow {{T}_{2}}=\frac{{{T}_{1}}}{2}=0.5\ s\]


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