Drift velocity

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

The drift velocity is the average velocity that a particle, such as an electron, attains in a material 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:

where u is the drift velocity, μ is the electron mobility (with units m2/(V⋅s)) of the material and E is the electric field (with units V/m).

Experimental measure

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, j is the current density flowing through the material, n is the charge-carrier number density, and q is the charge on the charge-carrier.

This can also be written as:

But the current density and drift velocity, j and u, are in fact vectors, so this relationship is often written as:


is the charge density (SI unit: coulombs per cubic metre).

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[citation needed]:


Numerical example

Electricity is most commonly conducted in a copper wire. Copper has a density of 8.94 g/cm3, and an atomic weight of 63.546 g/mol, so there are 140685.5 mol/m3. In one mole of any element there are 6.02×1023 atoms (Avogadro's constant). Therefore, in 1 m3 of copper there are about 8.5×1028 atoms (6.02×1023 × 140685.5 mol/m3). Copper has one free electron per atom, so n is equal to 8.5×1028 electrons per cubic metre.

Assume a current I = 1 ampere, and a wire of 2 mm diameter (radius = 0.001 m). This wire has a cross sectional area of 3.14×10−6 m2 (A = π × (0.001 m)2). The charge of one electron is q = −1.6×10−19 C. The drift velocity therefore can be calculated:

Dimensional analysis:

Therefore, in this wire the electrons are flowing at the rate of 23 μm/s. At 60 Hz alternating current, this means that within half a cycle the electrons drift less than 0.2 μm. In other words, electrons flowing across the contact point in a switch will never actually leave the switch.

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 1570 km/s.[2]

See also


  1. ^ Griffiths, David (1999). Introduction to Electrodynamics (3 ed.). Upper Saddle River, NJ: Prentice-Hall. p. 289.
  2. ^ http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html Ohm's Law, Microscopic View, retrieved 2015-11-16

External links

  • Ohm's Law: Microscopic View at Hyperphysics
Retrieved from "https://en.wikipedia.org/w/index.php?title=Drift_velocity&oldid=856575064"
This content was retrieved from Wikipedia : http://en.wikipedia.org/wiki/Drift_velocity
This page is based on the copyrighted Wikipedia article "Drift velocity"; it is used under the Creative Commons Attribution-ShareAlike 3.0 Unported License (CC-BY-SA). You may redistribute it, verbatim or modified, providing that you comply with the terms of the CC-BY-SA