# Which force field can accelerate an electron, but never change its speed?

$F = q v B S \in \theta$
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 $\theta = \frac{\pi}{2}$ or in a helical trajectory if other wise. The force vanishes when $\theta = 0$.