The figure shows a region of length ‘l’ with a
uniform magnetic field of 0.3 T in it and a
proton entering the region with velocity 4 $ \times 105
ms–1 making an angle 60o with the field. If the
proton completes 10 revolution by the time it
cross the region shown, ‘l’ is close to (mass of
proton = 1.67 \times 10–27 kg, charge of the proton =
1.6 \times $ 10–19 C)