Problem 3.390

Non-relativistic protons move rectilinearly in the region of space where there are uniform mutually perpendicular electric and magnetic fields with $E=4.0 \mathrm{kV} / \mathrm{m}$ and $B=50 \mathrm{mT} .$ The trajectory of the protons lies in the plane $x z$ (Fig. 3.102 ) and forms an angle $\varphi=30^{\circ}$ with the $x$ axis. Find the pitch of the helical trajectory along which the protons will move after the electric field is switched off.