A charge $q = 5$ uC moves in a uniform magnetic field $B = 0.2$ T perpendicular to the induction lines with speed $v = 10^4$ m/s. The mass of the particle is $m = 2 \cdot 10^{-5}$ kg. Find the radius of the circle the particle moves along.
The Lorentz force is perpendicular to the velocity and acts as the centripetal force: $qvB = mv^2/r$, hence
$$r = \frac{mv}{qB} = \frac{2 \cdot 10^{-5} \cdot 10^4}{5 \cdot 10^{-6} \cdot 0.2} = \frac{0.2}{10^{-6}} = 2 \cdot 10^5 \text{ m}$$
$$r = 200\,000 \text{ m} = 200 \text{ km}$$
$r = 2 \cdot 10^5$ m (200 km)