Which force field can accelerate an electron, but never change its speed?
Magnetic force on a charge is given from the Lorentz force formula as :
As we see, the force always acts perpendicular to the initial velocity vector and as a result doesn't modify it's magnitude (speed) but, only changes it's direction. (The particle tends to revolve along a circle if
Thus, it is the magnetic force which never changes the speed of a charged particle.