(3 pag. 419) "A beam of Pb+ ions, containing two isotopes with atomic mass unit (u.m.a.) 207 and 208, enters a velocity selector where the magnitude of the electric field is E = 78 V/m and that of the magnetic field is B = 6.5 mT. The velocity of the ions is directed in the positive direction of the x-axis, while the magnetic field is directed in the negative direction of the z-axis.

Determine the direction of the electric field and the velocity of the ions selected by the device. Upon exiting the velocity selector, the ions enter a mass spectrometer where there is a uniform magnetic field Bs = 31 mT perpendicular to the velocity of the ions. Calculate the difference between the radii of the trajectories traveled by the two isotopes. [1,2 10⁴m/s; 4,0 mm]