An electron moving in a circular path creates an equivalent current loop . The electric current $I$ is defined as the amount of charge passing a point per unit time, $I = q/t$ . Since the electron (charge $e$) completes $n$ rotations per second, its frequency is $n$, and the time for one rotation (period $T$) is $1/n$ . Therefore, the equivalent current is $I = e / (1/n) = ne$ . According to the sources, the magnitude of the magnetic field $B$ at the centre of a circular current loop of radius $r$ is given by $B = \frac{\mu_0 I}{2r}$ . Substituting $I = ne$ into this formula gives $B = \frac{\mu_0 n e}{2r}$.