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

  • question_answer
    An electric current passes through a long straight wire. At a distance of 4 cm from the wire, the magnetic field is B. The magnetic field at 12 cm from the wire would be:

    A) B/6                              

    B) B/3

    C) B/4                              

    D) B/2            

    Correct Answer: B

    Solution :

    (b) B/3                                              In a long straight wire,             \[B=\frac{{{\mu }_{0}}l}{2\pi r}\Rightarrow B\propto \frac{1}{r}\] \[\therefore \frac{B}{{{B}_{1}}}=\frac{{{r}_{2}}}{{{r}_{1}}}\Rightarrow \frac{B}{{{B}_{1}}}=\frac{12}{4}=3\] \[\Rightarrow B=3{{B}_{1}}\] or \[{{B}_{1}}=B/3\].     


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