Drift speed

The drift speed is the average speed of moving charge carriers due to an external field. In electrically conductive media, the drift speed is the speed that is due to the effect of electrical fields (characterized by their field strength ). Such media can e.g. B. metallic conductors , semiconductors , solutions of electrolytes or plasmas . The mobility is the proportionality factor between the applied electric field and the drift speed: ${\ displaystyle {\ vec {v}} ^ {*}}$ ${\ displaystyle E}$ ${\ displaystyle \ mu}$ ${\ displaystyle E}$

{\ displaystyle {\ vec {v}} ^ {*} = \ mu \ cdot {\ vec {E}}}

In suspensions , for example , the drift speed is the (average) speed that a particle reaches when an external force is applied. Then the drift speed is given by

{\ displaystyle {\ vec {v}} ^ {*} = \ mu \ cdot {\ vec {F}}}

.

Depending on the sign of the drifting particles (e.g. electrons ) or quasiparticles (e.g. holes ), the drift speed is interpreted as the mean speed in or against the field direction.

Electrons in metallic conductors

In a metallic conductor , conduction electrons move at speeds of approx. 10 ⁶ m / s without any external influence (see Fermi distribution ). This movement is an undirected thermal movement that does not produce any current on average . However, if an electric field acts on these conduction electrons, for example caused by an externally applied voltage , the thermal movements are superimposed by the drift speed. This is mostly in the range of 10 ⁻⁴ m / s = 0.1 mm / s and is therefore comparatively small. Along the crystal , the electrons interact with phonons and there is interference in the lattice , as a result of which part of the energy of the electrons is transferred to the lattice in the form of Joule heat .

The stronger the applied electric field, the higher the drift speed. However, the mean drift speed is limited. If this limit is reached, an increase in the current intensity can only be achieved by increasing the cross-sectional area . If the current density is increased with the same cross-section, i.e. the same number of available conduction electrons, larger and larger parts of the energy used are converted into thermal energy in the form of lattice vibrations through "collisions" at the atomic level - until the conductor is liquefied or destroyed. This principle is z. B. used in fuses .

The mean drift speed can be used to describe the electron movement through the crystal . The mobility in metals depends on the purity of the crystal, but above all on the excitation of lattice vibrations by thermal energy ( temperature ). ${\ displaystyle v ^ {*} = \ mu \ cdot E}$ ${\ displaystyle \ mu}$

Alternating current (which has an electric field strength that oscillates over time ) does not generate any charge transport in the mean over time, since the electrons, following the electric field, oscillate back and forth in place.

Electrons in gases

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

Electrophoresis

Charged colloidal particles such as proteins or nucleic acids move with the drift speed during electrophoresis.

literature

David Halliday, Robert Resnick, Jearl Walker: Physics. Wiley-VCH, 2005, ISBN 3-527-40366-3 .