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JEE Main & Advanced Physics Magnetism Question Bank Self Evaluation Test - Magnetism and Matter

  • question_answer
    A freely suspended magnet oscillates with period T in earth's horizontal magnetic field. When a bar magnet is brought near it, such that the magnetic field created by bar magnet is in same direction as earth's horizontal magnetic field, the period decreases to T/2. The ratio of the field of the magnet F to the earth's magnetic field (H) is

    A) 1 : 3     

    B) 1 : 1  

    C) 3 : 1     

    D) 9 : 1

    Correct Answer: C

    Solution :

    [c] \[{{T}_{1}}=2\pi \sqrt{\frac{I}{HM}}\], and \[{{T}_{2}}=2\pi \sqrt{\frac{I}{(H+F)M}}\] \[\frac{T_{1}^{2}}{T_{2}^{2}}=\frac{H+F}{H}=\frac{{{T}^{2}}}{{{T}^{2}}/4}=\frac{4}{1}\Rightarrow H+F=4H\] \[\Rightarrow \,\,3H=F\]


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