12th Class Physics Magnetism Question Bank MCQ - Moving Charges and Magnetism

  • question_answer
    A solid cylindrical wire of radius R carries a current l. The ratio of magnetic field at points which are located at R/2 and 2R distance away from the axis of the wire is:

    A) 1 : 1    

    B) 1 : 2    

    C) 2 : 1                             

    D) 1 : 4

    Correct Answer: A

    Solution :

    (a) 1 : 1. We know that, \[{{B}_{out}}=\frac{{{\mu }_{0}}l}{2\pi r}(r>R)\] Here,                 \[r=2R\] \[\therefore {{B}_{out}}=\frac{{{\mu }_{0}}l}{2\pi \times 2R}=\frac{{{\mu }_{0}}l}{4\pi R}\]     …(i) Also.     \[{{B}_{in}}=\frac{{{\mu }_{0}}lr}{2\pi {{R}^{2}}}(r<R)\] Here,     \[r=R/2\] \[\therefore {{B}_{in}}=\frac{{{\mu }_{0}}lR/2}{2\pi {{R}^{2}}}=\frac{{{\mu }_{0}}l}{4\pi R}\]          …(ii) Required ratio, \[\frac{{{B}_{in}}}{{{B}_{out}}}=\frac{1}{1}.\]


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