A current loop $ABCD$ is held fixed on the plane of the paper as shown in the figure. The arcs $ BC$ (radius $= b$) and $DA $ (radius $= a$) of the loop are joined by two straight wires $AB $  and $CD$. A steady current $I$ is flowing in the loop. Angle made by $AB$ and $CD$ at the origin $O$ is $30^o $. Another straight thin wire with steady current $I_1$ flowing out of the plane of the paper is kept at the origin.

The magnitude of the magnetic field $(B)$ due to the loop $ABCD$ at the origin $(O)$ is :

131-236

  • [AIEEE 2009]
  • A

    $0$

  • B

    $\frac{{{\mu _0}I(b - a)}}{{24ab}}$

  • C

    $\frac{{{\mu _0}I}}{{4\pi }}\left[ {\frac{{b - a}}{{ab}}} \right]$

  • D

    $\frac{{{\mu _0}I}}{{4\pi }}\left[ {2(b - a) + \frac{{\pi (a + b)}}{3}} \right]$

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