Drift Velocity

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.

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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.

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

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.

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