A beam of protons is accelerated from rest through a potential difference of 2000 V and then enters a uniform magnetic field which is perpendicular to the direction of the proton beam. If the flux density is 0.4 T, calculate the radius of the path which the beam describes. (Proton mass = 1.7 x 10^-27 kg, Electronic charge = 1.6 x 10^-19 C)