Circles in a Field

At right angles to a uniform field, a charge moves on a circle: the magnetic force is the centripetal force.

$$qvB = \frac{mv^2}{r}\quad\Rightarrow\quad r = \frac{mv}{qB}$$

The magnetic force always points to the centre: the charge goes round.

One turn takes $T = \frac{2\pi m}{qB}$, whatever the speed.