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

  • question_answer
    The distance of two points on the axis of a magnet from its centre is 10 cm and 20 cm respectively. The ratio of magnetic intensity at these points is 12.5 : 1. The length of the magnet will be

    A)            5 cm                                         

    B)            25 cm

    C)            10 cm                                      

    D)            20 cm

    Correct Answer: C

    Solution :

                       \[\frac{{{B}_{1}}}{{{B}_{2}}}=\frac{{{d}_{1}}}{{{d}_{2}}}{{\left( \frac{d_{2}^{2}-{{l}^{2}}}{d_{1}^{2}-{{l}^{2}}} \right)}^{2}}\Rightarrow \frac{12.5}{1}=\frac{10}{20}{{\left( \frac{400-{{l}^{2}}}{100-{{l}^{2}}} \right)}^{2}}\]                    \[\Rightarrow l=5\ cm\]            Hence length of magnet \[=2l=10\ cm\]


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