An electron moves in a circular orbit with a uniform speed v. It produces a magnetic field B at the centre of the circle. The radius of the circle is proportional to

Electric current corresponding to the revolution of electron is i=ev2πr. Magnetic field due to circular current at the centre is B  =  μ04π . 2πir  =  μ04π . evr2 ⇒  r= μ04π .evB  ⇒  r ∝  vB