A coil having ${N}$ turns is wound tightly in the form of a spiral with inner and outer radii $'a'$ and $'b'$ respectively. Find the magnetic field at centre, when a current $I$ passes through coil :
- [JEE MAIN 2021]
- A$\frac{\mu_{0} I N}{2(b-a)} \log _{e}\left(\frac{b}{a}\right)$
- B$\frac{\mu_{0} I}{8}\left[\frac{a+b}{a-b}\right]$
- C$\frac{\mu_{0} I}{4(a-b)}\left[\frac{1}{a}-\frac{1}{b}\right]$
- D$\frac{\mu_{0} I}{8}\left(\frac{a-b}{a+b}\right)$
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- [JEE MAIN 2018]
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