Drift velocity

In physics, drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field.

Applying an electric field adds to this random motion a small net flow in one direction; this is the drift.

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: where u is drift velocity, μ is the material's electron mobility, and E is the electric field.

In the MKS system, drift velocity has units of m/s, electron mobility, m2/(V·s), and electric field, V/m.

Due to this drift velocity, there is a net flow of electrons opposite to the direction of the field.

The formula for evaluating the drift velocity of charge carriers in a material of constant cross-sectional area is given by:[1] where u is the drift velocity of electrons, j is the current density flowing through the material, n is the charge-carrier number density, and q is the charge on the charge-carrier.

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: where is the charge density (SI unit: coulombs per cubic metre).

In terms of the basic properties of the right-cylindrical current-carrying metallic ohmic conductor, where the charge-carriers are electrons, this expression can be rewritten as:[citation needed] where Electricity is most commonly conducted through copper wires.

Assume a current I = 1 ampere, and a wire of 2 mm diameter (radius = 0.001 m).

The elementary charge of an electron is e = −1.6×10−19 C. The drift velocity therefore can be calculated:

At 60 Hz alternating current, this means that, within half a cycle (1/120th sec.

In context, at one ampere around 3×1016 electrons will flow across the contact point twice per cycle.

But out of around 1×1022 movable electrons per meter of wire, this is an insignificant fraction.

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 1570 km/s.