# Drift velocity

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 'rattle around' 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.

Drift velocity is expressed in the following equations:

where *J* is the current density, *ρ* is free charge density (with units C/m^{3}), and u is the drift velocity, and where *μ* is the electron mobility (with units m^{2}/(V⋅s)) and *E* is the electric field (with units V/m).

## Mathematical formula

The formula for evaluating the drift velocity of charge carriers in a material of constant cross-sectional area is given by:^{[1]}

where *u* is the drift velocity of electrons, *I* is the current flowing through the material, *n* is the charge-carrier density, *A* is the area of cross-section of the material and *q* is the charge on the charge-carrier.

In terms of the basic properties of the right-cylindrical current-carrying metallic ohmic conductor, where the charge-carriers are electrons, this expression can be rewritten as {{ safesubst:#invoke:Unsubst||date=__DATE__ |$B=
{{#invoke:Category handler|main}}{{#invoke:Category handler|main}}^{[citation needed]}
}}:

where,

*u*is again the drift velocity of the electrons, in m⋅s^{−1};*m*is the molecular mass of the metal, in kg;*ΔV*is the voltage applied across the conductor, in V;*ρ*is the density (mass per unit volume) of the conductor, in kg⋅m^{−3};*e*is the elementary charge, in C;*f*is the number of free electrons per atom.*ℓ*is the length of the conductor, in m; and*σ*is the electric conductivity of the medium at the temperature considered, in S/m;

## Numerical example

Electricity is most commonly conducted in a copper wire. Copper has a density of Template:Val, and an atomic weight of Template:Val, so there are Template:Val. In one mole of any element there are Template:Val atoms (Avogadro's constant). Therefore in Template:Val of copper there are about Template:Val atoms (Template:Val × Template:Val). Copper has one free electron per atom, so *n* is equal to Template:Val electrons per cubic metre.

Assume a current *I* = 3 amperes, and a wire of Template:Val diameter (radius = Template:Val). This wire has a cross sectional area of Template:Val (*A* = π × Template:Val^{2}). The charge of one electron is *q* = Template:Val. The drift velocity therefore can be calculated:

Dimensional analysis:

Therefore in this wire the electrons are flowing at the rate of Template:Val.

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 Template:Val.^{[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* = Template:Val, drift velocity would likewise vary in a sine-wave pattern, and electrons would fluctuate about their initial positions with the amplitude of:

## See also

## References

- ↑ {{#invoke:citation/CS1|citation |CitationClass=book }}
- ↑ http://230nsc1.phy-astr.gsu.edu/hbase/electric/ohmmic.html Ohm's Law, Microscopic View, retrieved Feb 14, 2009

## External links

- Ohm's Law: Microscopic View at Hyperphysics