$A$ toroid is a long coil of wire wound over a circular core. If $r$ and $R$ are the radii of the coil and toroid respectively,the coefficient of self-induction of the toroid is (The magnetic field in it is uniform and $R >> r$). ($N =$ number of turns of the coil and $\mu_{0} =$ permeability of free space)

  • A
    $\frac{2 \mu_{0} r^{2}}{N^{2} R}$
  • B
    $\frac{\mu_{0} N^{2} R^{2}}{2 r}$
  • C
    $\frac{\mu_{0} N^{2} r^{2}}{2 R}$
  • D
    $\frac{\mu_{0} R}{2 N^{2} r^{2}}$

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