Drift Velocity of Electrons

My book says; the electrons move with an average velocity which is independent of time, although electrons are accelerated. This phenomenon of drift and the velocity is called Drift velocity. My question us that why at all the electrons' motion is independent of time and what would have happened if the motion of electrons were dependent of time?
- Shambhavi Singh (age 16)
Varanasi, UP, India

The average drift velocity only becomes independent of time after a brief period of change right after the electric field is switched on. The electrons at first accelerate, on average, giving an increasing drift velocity. The scattering of electrons by defects and sound waves causes an average force that opposes the drift motion. After a very short time the drift becaomes large enough so that that force balances the force from the applied field, and the drift velocity becomes independent of time.

If the voltage changes over time, for example because the material is connected to an ac power source, then the drift velocity will change too. That happens all the time in ordinary electrical wires.

Mike W.

 

(published on 05/24/2018)