When a charge enters a magnetic field perpendicularly ($v\perp B$), it moves in a circle. The magnetic force provides the centripetal force: $qvB=\frac{m{v}^2}{r}⟹v/r=\frac{qB}{m}$

• The angular frequency $\omega =v/r=\frac{qB}{m}$.
• The frequency $f=\frac{\omega }{2\pi }=\frac{qB}{2\pi m}$.
• Note: The frequency is independent of the speed $v$ and radius $r$ of the particle (within non-relativistic limits).