A proton (m=1.67×10−27kg, q=1.6×10−19C) enters a uniform magnetic field of magnitude 0.20T with a speed of 4.0×106ms−1, the velocity being perpendicular to the field. Determine the radius of the circular path followed by the proton.
a) 2.1m
b) 0.21m
c) 0.104m
d) 0.021m