Question

Magnetic field B on the axis of a circular coil and far away distance x from the centre of the coil are related as :

• A. $B\propto x^{-3}$
• B. $B\propto x^{-2}$
• C. $B\propto x^{-1}$
• D. None of these

Answer 1

Answer: (A)

Magnetic field on the axis of circular coil is $4\pi (R^2+x^2{)}^{3/2}$
where R is the radius of the coil and x is the distance of the observation point. At the far away point $x$ > > $R$, so $R^2$ may be neglected as compared to $x^2$. Hence magnetic field is related to x as $B\propto x^{-3}$