The drift velocity is the flow velocity that a particle, such as an electron, attains due to an electric field. It can also be referred to as axial drift velocity. In general, an electron will propagate randomly in a conductor at the Fermi velocity. An applied electric field will give this random motion a small net flow velocity in one direction.
In a semiconductor, the two main carrier scattering mechanisms are ionized impurity scattering and lattice scattering.
Because current is proportional to drift velocity, which in a resistive material is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.
The most elementary expression of Ohm's law is:

u= \mu E ,
where u is the drift velocity, μ is the electron mobility (with units m^{2}/(V⋅s)) of the material and E is the electric field (with units V/m).
Contents

Experimental measure 1

Numerical example 2

See also 3

References 4

External links 5
Experimental measure
The formula for evaluating the drift velocity of charge carriers in a material of constant crosssectional area is given by:^{[1]}

u = {j \over n q} ,
where u is the drift velocity of electrons, j is the current density flowing through the material, n is the chargecarrier number density, and q is the charge on the chargecarrier.
In terms of the basic properties of the rightcylindrical currentcarrying metallic ohmic conductor, where the chargecarriers are electrons, this expression can be rewritten as :

u = {m \; \sigma \Delta V \over \rho e f \ell} ,
where
Numerical example
Electricity is most commonly conducted in a copper wire. Copper has a density of 7000894000000000000♠8.94 g/cm^{3}, and an atomic weight of 6998635460000000000♠63.546 g/mol, so there are 7005140685500000000♠140685.5 mol/m^{3}. In one mole of any element there are 7023602000000000000♠6.02×10^{23} atoms (Avogadro's constant). Therefore in 7000100000000000000♠1 m^{3} of copper there are about 7028850000000000000♠8.5×10^{28} atoms (7023602000000000000♠6.02×10^{23} × 7005140685500000000♠140685.5 mol/m^{3}). Copper has one free electron per atom, so n is equal to 7028850000000000000♠8.5×10^{28} electrons per cubic metre.
Assume a current I = 3 amperes, and a wire of 6997100000000000000♠1 mm diameter (radius = 6996500000000000000♠0.0005 m). This wire has a cross sectional area of 6993784999999999999♠7.85×10^{−7} m^{2} (A = π × (6996500000000000000♠0.0005 m)^{2}). The charge of one electron is q = 3018840000000000000♠−1.6×10^{−19} C. The drift velocity therefore can be calculated:

\begin{align} u &= {I \over nAq}\\ u &= \frac{3}{\left(8.5 \times 10^{28}\right) \left(7.85 \times 10^{7}\right) \left(1.6 \times 10^{19}\right)}\\ u &= 0.00028 \end{align}
Dimensional analysis:
u = \dfrac{\text{A}}{\dfrac{\text{electron}}{\text{m}^3}{\cdot}\text{m}^2\cdot\dfrac{\text{C}}{\text{electron}}} = \dfrac{\dfrac{\text{C}}{s}}{\dfrac{1}{\text{m}}{\cdot}\text{C}} = \dfrac{\text{m}}{\text{s}}
Therefore in this wire the electrons are flowing at the rate of 3003720000000000000♠−0.00028 m/s.
By comparison, the Fermi flow velocity of these electrons (which, at room temperature, can be thought of as their approximate velocity in the absence of electric current) is around 7006157000000000000♠1570 km/s.^{[2]}
In the case of alternating current, the direction of electron drift switches with the frequency of the current. In the example above, if the current were to alternate with the frequency of F = 7001600000000000000♠60 Hz, drift velocity would likewise vary in a sinewave pattern, and electrons would fluctuate about their initial positions with the amplitude of:
A = \frac{1}{2F} \frac{2\sqrt{2}}{\pi} u = 2.1\times10^{6} \text{ m}
See also
References

^ Griffiths, David (1999). Introduction to Electrodynamics (3 ed.). Upper Saddle River, NJ: PrenticeHall. p. 289.

^ http://230nsc1.phyastr.gsu.edu/hbase/electric/ohmmic.html Ohm's Law, Microscopic View, retrieved Feb 14, 2009
External links

Ohm's Law: Microscopic View at Hyperphysics
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