BHU PMT BHU PMT (Screening) Solved Paper-2008

  • question_answer
    Two identical wires A and B have the same length L and carry the same current\[I\]. Wire A is bent into a circle of radius R and wire B is bent to form a square of side a. If\[{{B}_{1}}\]and\[{{B}_{2}}\] are the values of magnetic induction at the centre of the circle and the centre of the square respectively, then the ratio\[{{B}_{1}}/{{B}_{2}}\]is

    A)  \[({{\pi }^{2}}/8)\]                         

    B)  \[({{\pi }^{2}}/8\sqrt{2})\]

    C)  \[({{\pi }^{2}}/16)\]                       

    D)  \[({{\pi }^{2}}/16\sqrt{2})\]

    Correct Answer: B

    Solution :

                     \[{{B}_{1}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2\pi i}{R}\] \[=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2\pi i\times 2\pi }{L}\]                                 \[(\because L=2\pi R,\]for circular loop) \[{{B}_{2}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{i}{(a/2)}\]         \[[\sin {{45}^{o}}+\sin {{45}^{o}}]\times 4\] where \[a=L/4\] \[\therefore \]\[{{B}_{2}}=\frac{{{\mu }_{0}}i}{4\pi L}\times 8\times 4\times \left[ \frac{1}{\sqrt{2}}+\frac{1}{\sqrt{2}} \right]\]                 \[=\frac{{{\mu }_{0}}i}{4\pi L}\times \frac{64}{\sqrt{2}}\]                                   ...(ii) Hence,  \[\frac{{{B}_{1}}}{{{B}_{2}}}=\left( \frac{{{\mu }_{0}}}{4\pi } \right)\times {\frac{4{{\pi }^{2}}i}{L}}/{\frac{{{\mu }_{0}}i}{4\pi L}}\;\times \frac{64}{\sqrt{2}}\] Or           \[\frac{{{B}_{1}}}{{{B}_{2}}}=\frac{{{\pi }^{2}}}{8\sqrt{2}}\]


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