An electron having kinetic energy 2.5 keV moves horizontally into the region of space in which there is a downward-directed uniform electric field of magnitude of 10 kV/m.
(i) Determine the magnitude and direction of the least uniform magnetic field which will cause the electron to continue to move horizontally? Avoid the gravitational force, which is small.
(ii) Is it possible for a proton to pass via the combination of fields undeflected? If so, under what conditions?