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 :

The magnetic field (B) at the center of a circular orbit due to an electron moving with speed v is given by B = μ0 I / (2 r), where I is the current due to the electron, and r is the radius of the orbit. The current I can be expressed as I = q / T, where q is the charge of the electron, and T is the period of the orbit. The period T is related to the speed v and radius r by T = 2 π r / v, so I = q v / (2 π r). Substituting this into the expression for B gives B = μ0 q v / (4 π r^2). Rearranging to solve for r, we get r = μ0 q v / (4 π * B). Thus, the radius r is proportional to v/B.