$A$ toroid is a long coil of wire ($N$ turns) wound over a circular core. The coefficient of self-induction of the toroid is [The magnetic field in it is uniform and $R >> r$,where $r=$ radius of wire,$R=$ radius of coil] ($\mu_0=$ permeability of free space).

  • A
    $\frac{\mu_0 N^2 R^2}{2 r}$
  • B
    $\frac{\mu_0 N r}{2 R}$
  • C
    $\frac{\mu_0 N^2 r^2}{R}$
  • D
    $\frac{\mu_0 N^2 r^2}{2 R}$

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