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Q.
A circular coil of radius 40 mm consists of 250 turns of wire in which the current is 20mA. The magnetic field in the center of the coil is $\mu =4\pi×10^{-7}Hm^{-1}$
In a circular coil of n turns, magnetic field is
$B=\frac{\mu_0nI}{2r}=\frac{4\pi×10^{-7}×250×20×10^{-3}}{2×
40×10^{-3}}$
$(\because $ n = no. of turns, I = current through
coil, r = radius of coil)
$\Rightarrow B=\frac{4\pi ×250 ×20×10^{-7-3+3}}{2×40}$
=250×3.14×$10^{-7}$
$=785×10^{-7}=0.785×10^{-4}$tesla
=0.785 gauss