Drift velocity

The drift velocity is the average velocity of the moving charge carriers in the electrically conductive media, which is (as indicated by their field strength) on the action of electric fields due. Such media may be, for example, metallic conductors, semiconductors, or plasma solutions of electrolytes. Depending on the sign of the drifting particles or quasi-particles (eg electrons or holes ) is interpreted as the drift velocity mean velocity in or against the field direction.

In a metallic conductor to conduction electrons move without outside influence at speeds of about 106 m / s (see Fermi distribution ). This movement is a non-directional thermal movement which causes no current on average. This acts on the conduction electrons, however, an electric field, for example, caused by an externally applied voltage, the thermal movements are superimposed by the drift speed. This is usually in the range of and is therefore comparatively small.

The stronger the applied electric field, the greater the drift velocity. The average drift velocity is, however, limited. If this limit is reached, an increase of the current strength, only by increasing the cross-sectional area can be achieved. Is of the same cross -section, ie, the same number of available conduction electrons, the current density is increased, the greater part of the energy used will always be converted to thermal energy in the form of lattice vibrations caused by " collisions " at the atomic level, - to the conductor is liquefied or destroyed. This principle is used, for example, in fuses.

Along the crystal, there is interaction with phonons and disturbances in the grid, whereby part of the energy of the electrons in the form of Joule heat is transferred to the grating. To describe the electron motion through the crystal, you can use the average drift velocity v *:

The mobility depends on the purity of the crystal, but above all by the excitation of lattice modes by thermal energy (temperature) in metals.

The drift velocity of electrons in ionized gases ( plasmas ) can be, for example, with drift chambers measure (VDC, ie velocity drift chamber ).