# Drift Velocity

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## 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 (m^{2})

**n** = charge density (m^{-3}) This is the number of charge carriers that can move per m^{3}

**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.

**If the cross-sectional area of a wire is A and the number of charge carriers per m ^{3} 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

Different materials will have different values of n, the number of charge carriers per m^{3}.

Good conductors such as metals have the most charge carriers. Semiconductors have about 1 x10^{10} times fewer charge carriers than metals. At low voltages insulators have no free electrons so that a current is unable to flow.