Drift velocity is a key concept in the study of electrical current and is defined as the average velocity at which free electrons move through a conductor under the influence of an electric field. When an electric field is applied to a conductor, it creates a force on the free electrons, which results in their movement and the flow of current. The drift velocity is the measure of this movement of free electrons, and it is crucial in determining the overall electrical conductivity of a material.

The drift velocity is given by the equation $v_d = \frac {I}{neA}$, where I is the current, n is the number of free electrons per unit volume, e is the charge of each electron, and A is the cross-sectional area of the conductor.

It is important to note that the drift velocity is not the same as the speed at which individual free electrons move. Rather, it is an average velocity that takes into account the many different speeds of the free electrons as they move through the wire. The drift velocity is also much slower than the speed of light, and it is usually measured in millimeters per second.

In conclusion, drift velocity is a crucial aspect of electrical current and is an essential concept for understanding how current flows through conductors. It provides insight into the movement of free electrons in a wire and how it relates to the overall conductivity of the material.