A particle of charge q and mass m moves with speed v perpendicular to a uniform field B, describing a circle. The radius of the circular path is:

1. The magnetic force provides the centripetal force: $\dfrac{mv^2}{r} = qvB$. 2. Cancel one factor of $v$: $\dfrac{mv}{r} = qB$. 3. Solve for $r$: $r = \dfrac{mv}{qB}$. 4. So the larger the momentum $mv$, the larger the circle. _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", p.138_