 # Drift Velocity

## You are here

*Please note: you may not see animations, interactions or images that are potentially on this page because you have not allowed Flash to run on S-cool. To do this, click here.*

## Drift Velocity

You will now already know that current is a measure of the amount of charge moving per second.

This means that current is dependent on:

• the speed at which charged particles are moving.
• the charge they are carrying.
• the number of charged particles that are moving.

Charged particles do not travel in a straight line through a conductor, because they collide with other particles in the material. We therefore use the average speed the particle travels at along the conductor. This is called the drift velocity.

Current can be calculated using the equation:

I = vAnq

Where:

I = current (amps, A)

v = drift velocity (m/s)

A = cross-sectional area of the conductor (m2)

n = charge density (m-3) This is the number of charge carriers that can move per m3

q = charge on each charge carrier (coulombs, c)

The drift velocity of electrons in a piece of metal with a current of 0.1 A will be around 1x10-5 m/s, so imagine how long it takes one electron to travel along a 10 cm long wire! The electrons are actually travelling at speeds of up to a million m/s in the wire but only drift very slowly in the current direction.

#### Derivation for I = vAnq

If the cross-sectional area of a wire is A and the number of charge carriers per m3 is n, then:

Number of charge carriers per metre of wire = An

If each charge carrier is travelling at speed v (m/s) along the wire, then:

Number of charge carriers passing a point in the wire per second = vAn

If each charge carrier has a charge q, then:

The amount of charge passing along the wire per second (i.e. the current, I) = vAnq

#### Comparing Materials

Different materials will have different values of n, the number of charge carriers per m3.

Good conductors such as metals have the most charge carriers. Semiconductors have about 1 x1010 times fewer charge carriers than metals. At low voltages insulators have no free electrons so that a current is unable to flow.

### S-cool exclusive FREE TUTORIAL offer! 