# Drift velocity

## Main Question or Discussion Point

for an electron, randomly moving inside a conductor , having applied an external electric field we have those electrons moving with a net speed called drift speed , against the direction of field.
so initially as electrons are moving randomly we consider their initial velocity o
and after time t =at
where a = acc. of electrons = e(field)/mass of electron
t = mean time between consecutive collisions of electrons
courtesy PHYSICS by halliday resnick krane vol 2
but i don't understand why don't we average the initial and final speed of electrons ie
drift speed = (0 + at)/2
??

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The acceleration of an electron in a real conductor is not constant. I think when subjected to an electric field, the speed of the electrons increases monotonically up to its "steady-state" drift velocity. If the applied field is not changing (DC), then after an initial transient time, the electrons are flowing at the drift velocity. If the applied field is a sinusoidal function (AC), then the current (and thus drift velocity) will also vary sinusoidally. An accelerating charge (i.e. a varying current) establishes an electromagnetic wave.

Drakkith
Staff Emeritus
I think drift speed is already an average since every electron will be moving at different speeds and constantly interacting with the material and the applied field.

For the acceleration happened just a moment (average), the drift velocity is the average speed electrons have in the conductor after the generation of the electric field.

vanhees71