Electron Drift Velocity Basic

What is the definition of Electron Drift Velocity?

Drift velocity is the average velocity with which electrons 'drift' in the presence of an electric field. It's the drift velocity (or drift speed) that contributes to the electric current. In contrast, thermal velocity causes random motion resulting in collisions with metal ions.

The definition of drift speed can be understood by imagining randomly moving electrons in a conductor. Free electrons move in a conductor with random speeds and random directions. When we apply an electric field through a conductor, randomly moving electrons experience an electric force in the direction of the field.

Electron Drift Velocity Calculation

The calculation of the electron drift velocity formula or equation is as below:

We start with the acceleration of the electrons, 

a = F/m = eE/m. 

The average velocity gained, i.e. the drift velocity, due to this acceleration = a*t = eEt/m.

The density of conduction electrons in copper. or 0.15 mm/s! Compare this to the random thermal speed, of the electrons which is about 106 m/s or about 1010 times faster!.

So we learned that when an external field is applied across the conductor the free electrons acquire a net velocity opposite to the direction of the electric field. The average velocity acquired by them is called drift velocity.

The density of charge carriers:

The density of charge carriers, n, is the number of free electrons per unit volume. It tells how good a conductor the material is. A lower value of n means that the electrons will have to move faster in order to produce the same current.

Conductors, like metals, have high values of n – around 10²⁸per m³

Insulators, such as plastic, will have very low values of n – these may be close to 0

Semiconductors, for example, silicon, have values of n in between the two – this means they conduct better than insulators but not as well as conductors.