Alpha particles $\left(\mathrm{m}=6.68 \times 10^{-27} \mathrm{~kg} \text { and } \mathrm{q}=+2 \mathrm{e}\right)$ accelerate to $2\ keV$ through a potential difference $V$. They enter a magnetic field $B = 0.2T$ perpendicular to their direction of motion. What is radius of their path?