In physics a drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an average velocity of zero. Applying an electric field adds to this random motion a small net flow in one direction; this is the drift.
Drift velocity is proportional to current. In a resistive material it is also proportional to the magnitude of an external electric field. Thus Ohm's law can be explained in terms of drift velocity. The law's most elementary expression is:
When a potential difference is applied across a conductor, free electrons gain velocity in the direction opposite to the electric field between successive collisions (and lose velocity when traveling in the direction of the field), thus acquiring a velocity component in that direction in addition to its random thermal velocity. As a result, there is a definite small drift velocity of electrons, which is superimposed on the random motion of free electrons. Due to this drift velocity, there is a net flow of electrons opposite to the direction of the field.
This can also be written as:
But the current density and drift velocity, j and u, are in fact vectors, so this relationship is often written as:
Electricity is most commonly conducted through copper wires. Copper has a density of , and an atomic weight of , so there are . In one mole of any element there are atoms (the Avogadro number). Therefore, in of copper there are about atoms . Copper has one free electron per atom, so n is equal to electrons per cubic metre.
Assume a current , and a wire of diameter (radius = ). This wire has a cross sectional area A of ? × 2 = = . The charge of one electron is . The drift velocity therefore can be calculated:
Therefore, in this wire the electrons are flowing at the rate of . At 60 Hz alternating current, this means that within half a cycle the electrons drift less than 0.2 μm. In other words, electrons flowing across the contact point in a switch will never actually leave the switch.
By comparison, the Fermi flow velocity of these electrons (which, at room temperature, can be thought of as their approximate velocity in the absence of electric current) is around .