because electrons are bound, total energy is zero... therefore, kinetic energy of electrons = potential energy of electrons. kinetic energy is 1/2mv^2 change in potential energy of the electron = charge of electron x change in electrical potential of the anode
electron travels in a circle, because there's a magnetic field toward us, and the velocity is tangent to the circle, and magnetic force on the electron is -evxB. toward center.
and centripetal force is always the same: mv^2/r
so equate those: evB=mv^2/r e=mv/rB e/m=v/rB