Question

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

Answer 1

Magnetic force on a charge is given from the Lorentz force formula as :

`F=qvBSin 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 = pi/2` or in a helical trajectory if other wise. The force vanishes when `theta = 0`.

Thus, it is the magnetic force which never changes the speed of a charged particle.