JEE Main & Advanced JEE Main Paper Phase-I (Held on 09-1-2020 Morning)

  • question_answer
    A long straight wire of radius a carries a current distributed uniformly over its cross-section. The ratio of the magnetic fields due to the wire at distance \[\frac{a}{3}\] and 2a, respectively from the axis of the wire is [JEE MAIN Held on 09-01-2020 Morning]

    A) \[\frac{1}{2}\]

    B) \[\frac{3}{2}\]

    C) \[2\]

    D) \[\frac{2}{3}\]

    Correct Answer: D

    Solution :

    [d] \[B=\frac{\left( {{\mu }_{0}} \right)I}{2\pi r}=\frac{\left( {{\mu }_{0}} \right)\left( J \right)\pi {{r}^{2}}}{2\pi r}=\frac{{{\mu }_{0}}Jr}{2}\] \[{{B}_{1}}\left( at\frac{a}{3} \right)=\frac{{{\mu }_{0}}Ja}{6}\] \[{{B}_{2}}\left( at\,2a \right)=\frac{\left( {{\mu }_{0}} \right)\left( J \right)\pi {{a}^{2}}}{2\pi \left( 2a \right)}=\frac{{{\mu }_{0}}Ja}{4}\] \[\Rightarrow \frac{{{B}_{1}}}{{{B}_{2}}}=\frac{2}{3}\]


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