JEE Main & Advanced Physics Magnetism Question Bank Critical Thinking

  • question_answer
    A vibration magnetometer consists of two identical bar magnets placed one over the other such that they are perpendicular and bisect each other. The time period of oscillation in a horizontal magnetic field is \[{{2}^{5/4}}\]seconds. One of the magnets is removed and if the other magnet oscillates in the same field, then the time period in seconds is                                                                              [EAMCET (Med.) 2003]

    A)            \[{{2}^{1/4}}\]                     

    B)            \[{{2}^{1/2}}\] 

    C)            2    

    D)            \[{{2}^{3/4}}\]

    Correct Answer: C

    Solution :

                       Initially magnetic moment of system                    \[{{M}_{1}}=\sqrt{{{M}^{2}}+{{M}^{2}}}=2M\] and moment of inertia                    \[{{I}_{1}}=I+I=2I.\]                    Finally when one of the magnet is removed then                    \[{{M}_{2}}=M\]and \[{{I}_{2}}=I\]                    So \[T=2\pi \sqrt{\frac{I}{M\ {{B}_{H}}}}\]            \[\frac{{{T}_{1}}}{{{T}_{2}}}=\sqrt{\frac{{{I}_{1}}}{{{I}_{2}}}\times \frac{{{M}_{2}}}{{{M}_{1}}}}=\sqrt{\frac{2I}{I}\times \frac{M}{\sqrt{2}M}}\]\[\Rightarrow {{T}_{2}}=\frac{{{2}^{5/4}}}{{{2}^{1/4}}}=2\ \sec .\]


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