Drift velocity: Definition from Answers.com

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will rattle around in a conductor at the Fermi velocity randomly. An applied electric field will give this random motion a small net 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 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: $J_{\it drift} = \sigma \cdot v_{\it avg}$ , where $J d r i f t$ is the current density , $σ$ is charge density in units C/m³, and v_avg is the average velocity of the carriers(drift velocity);

$v_{\it avg} = \mu \cdot E$ , where μ is the electron mobility in (m/s)/(V/m) and E is the electric field in V/m.

1 Derivation
2 See also
3 References
4 External links

Derivation

To find an equation for drift velocity, one can begin with the definition of current:

$I = \frac{\Delta Q}{\Delta t}$

where

ΔQ is the small amount of charge that passes through an area in a small unit of time, Δt.

One can relate ΔQ to the motion of charged particles in a wire expecting a dependence on the number density of the charge carriers and using dimensional analysis:

$Δ Q$	$= \left( \mathrm{number \ of \ charged \ particles} \right) \times \left( \mathrm{charge \ per \ particle} \right)$
	$= \left( n A \Delta x \right) q$

where

n is the number of charge carriers per unit volume

A is the cross sectional area

Δx is a small length along the wire

q is the charge of the charge carriers

Now, normally particles move randomly, but under the influence of an electric field in the wire, the charge carriers gain an average velocity in a specific direction. This is what's called drift velocity, v_d. And since Δx = v_d Δt, we can plug it into the above equation.

$\Delta Q = \left( n A v_d \Delta t \right) q$

Putting that back into the original equation and re-arranging to solve for the drift velocity:

$v_d = \frac{I}{n q A}$

Alternative derivation

Using the definition of current density:

$J_{drift} = \sigma \cdot \nu_{drift}$

where $σ$ is the density of charge per volume and the fact that

$J= \frac{I}{A}$

We can simply express:

$\sigma = n \cdot q$

to get

$\frac{I_{drift}}{A} = n \cdot q \cdot \nu_{drift}$

and the same result as above:

$v_d = \frac{I}{n q A}$

As a numerical example,for a copper wire of 1 square mm area, carrying a current of 3 amperes, the drift velocity of electrons would be about 0.00028 metres per second (or just about an hour to travel one metre). ^[1]

References

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

External links

Ohm's Law: Microscopic View at Hyperphysics

This condensed matter physics-related article is a stub. You can help Wikipedia by expanding it.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Donate to Wikimedia

	ionic mobility (physics)
	drift mobility (solid-state physics)
	hole mobility (electronics)

	In a copper wire of cross section 1 square centimetre in which current of 200A flows in copper there are 8.510to the power of 28 freeelectron per cubic metre .calculate the drift velocity? Read answer...
	When an object moves with constant velocity does its average velocity during any time interval differ from its instantaneous velocity at any instant? Read answer...
	When an object moves with inconstant velocity does its average velocity differ from its instantaneous velocity? Read answer...

	Why drift velocity saturates to thermal velocity at higher Electric fields in semiconductors?
	Drift velocity is always between 0.1 to 1 cm per second then how come the high current is produced?
	How does the drift velocity of electron is dependent on temperature?

		Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more
		Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Drift velocity". Read more

Drift velocity

Contents

Derivation

See also

References

External links