Drift Velocity — Definition
Definition
Imagine a bustling marketplace where people are moving in all directions, bumping into each other randomly. This is similar to how free electrons behave inside a conductor in the absence of any external influence. They are in constant, rapid, random motion due to their thermal energy, colliding frequently with the fixed positive ions of the conductor's lattice. The average velocity of these random motions is zero, meaning there's no net flow of charge in any particular direction.
Now, picture a gentle, invisible force pushing everyone in the marketplace slightly towards one end. Even though people are still moving randomly and colliding, there's now a very slight, overall shift in their position towards that one end.
This 'gentle push' in a conductor is analogous to an external electric field. When an electric field is applied across a conductor, it exerts a force on these free electrons, causing them to accelerate in a direction opposite to the field (since electrons are negatively charged).
However, this acceleration is short-lived because the electrons quickly collide with the lattice ions, losing the kinetic energy gained from the field and essentially 'resetting' their directed motion. After each collision, they again accelerate under the influence of the electric field until the next collision. This process repeats continuously.
So, instead of a continuous acceleration, the electrons acquire a very small, average velocity component in the direction opposite to the electric field, superimposed on their much larger, random thermal motion.
This tiny, average velocity, which is responsible for the net flow of charge and thus the electric current, is called the drift velocity (). It's important to understand that drift velocity is typically very small, on the order of millimeters per second, much, much slower than the speed of light or even the electrons' thermal speeds (which can be hundreds of kilometers per second).
Yet, it is this minuscule directed motion that defines electric current.