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

  • question_answer
    A circular coil of radius a carries an electric current. The magnetic field due the coil at a point on the axis of the coil located at a distance r from centre of the coil, such that r>>a varies:

    A) \[\frac{1}{r}\]                           

    B) \[\frac{1}{{{r}^{2}}}\]

    C) \[\frac{1}{{{r}^{3}}}\]                        

    D) \[\frac{1}{{{r}^{3/2}}}\]

    Correct Answer: C

    Solution :

    (c) \[\frac{1}{{{r}^{3}}}\] \[B=\frac{{{\mu }_{0}}l{{a}^{2}}}{2{{({{a}^{2}}+{{r}^{2}})}^{3/2}}}=\frac{{{\mu }_{0}}l{{a}^{2}}}{2{{r}^{3}}}\,\text{for}\,{{r}^{2}}+{{a}^{2}}\approx {{r}^{2}}\].


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