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.

Why is the electron drift velocity zero in the absence of an applied field?

However the electrons are continually scattering off the lattice of metal atoms so the direction of the electron motion is randomised and the average velocity relative to the lattice of metal atoms is zero. The drift velocity is this average velocity, so in the absence of any applied field the drift velocity is zero.

What is drift velocity of free electrons?

It is defined as the average velocity with which free electrons get drifted towards the positive end of a conductor (opposite to the electric field) under the influence of an external electric field.