Drift velocity is the net velocity with which a given particle
progresses in a given direction. The particle itself may in fact have
a much higher velocity; many orders of magnitude higher in some
cases. However, this high velocity is constantly changing (velocity is
a vector, and the direction is changing while the magnitude stays
approximately the same), so the progress of a particle in a given
direction will often be limited.
The concept of drift velocity is useful in many areas, including
fluid mechanics and electromagnetism.
In electromagnetism, electron-gas theory in particular, average drift velocity is a property of electrons in
conductors. It is useful in determining resistance and the various
electrical properties of the material. Mobility relates the drift
velocity of electrons to the applied electric field.
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