An electron and a positron are moving in the +x-direction with the same velocity in a crossed electric and magnetic field (the fields are perpendicular). The question states it's impossible to separate them using this configuration. The electric field is pointing in the -y-direction and the magnetic field, out of the page (+z-direction). Taking the +y-direction as positive for the electric field and [itex]vB[/itex], I get this: [itex]F_e = -e(E-vB) = e(-E+vB) [/itex] (force on electron) [itex]F_p = e(-E-vB)[/itex] (force on positron) The cross product between the velocity (in the +x-direction) and the magnetic field (+z-direction) points the same for the electron and the positron (I think), -y-direction. As you can see [itex]F_e[/itex] and [itex]F_p[/itex] are not equal, so the particles would be separated, which is not supposed to be possible. But I just cannot find where I go wrong. Thank you!