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

1 Answer
May 30, 2015

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.