A circular coil of radius R carries an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a distance r from the centre of the coil, such that r >> R, varies as

The magnetic field due to a circular coil of radius Rat a point on the axis of the coil located at a distance r from the centre of the coil,B=μ04π  2πiR2 R2+r23/2Given, r >> R then we have, after neglecting R,B=μ04π2πiR2r3Also  area A= πR2B=μ02π  Air3  ⇒  B ∝ 1r3