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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
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.