- Drift Velocity Definition: Drift velocity is defined as the net velocity of free electrons moving randomly in a conductor due to an electric field.
- Random Electron Motion: Despite an electric field, electrons move randomly but drift towards the positive terminal, creating drift current.
- Drift Current: The consistent flow of electrons caused by drift velocity is known as drift current.
- Electron Mobility: Electron mobility (μe) is the ratio of drift velocity (ν) to the applied electric field (E), showing how easily electrons move through a conductor.
- Impact of Electric Field: A stronger electric field increases electron drift velocity, resulting in a higher drift current.
What is Drift Velocity?
Drift velocity is the net velocity of free electrons in a conductor when an electric field is applied. The electrons still move at many speeds and in many directions. The field adds a force that pulls them toward the positive terminal, opposite the field direction.
The field does not stop that random motion. It only adds a slow net movement toward higher potential. The electrons keep colliding and changing direction as they make that net progress.
The average of that net movement toward the high-potential end is the drift velocity of the electrons.
The electrical current produced by that electron drift in a stressed conductor is the drift current. In a uniform metal this is the conduction current. In a semiconductor a concentration gradient can also produce a separate diffusion current.
The Relationship between Drift Velocity and Electron Mobility
A metal contains free electrons at room temperature. Those free electrons remain available at any temperature above absolute zero.
Free electrons in the conductor move randomly, often colliding with atoms and changing direction.
When a steady electric field is applied, the electrons gain a net motion toward the positive terminal of the applied voltage. That path is not a straight line.
As the electrons move toward the positive potential, they keep colliding with atoms and scattering. Each collision removes some kinetic energy. The electric field then accelerates them again toward the positive potential.
The next collision repeats that loss and regain of kinetic energy. The field cannot stop the random motion, but it does produce a net drift toward the positive terminal.
The electrons therefore have an average drift velocity toward the positive terminal, opposite the applied electric field. A stronger field accelerates them more after each collision, so the average drift velocity rises.
If ν is the drift velocity and E is the applied electric field, electron mobility μe is the ratio of ν to E.
Where μe is referred to as electron mobility.
Drift Velocity, Drift Current, and Electron Mobility: An Animation
The steady movement of electrons at that drift velocity is the drift current.
Drift velocity, drift current and electron mobility therefore describe one chain: field, net electron speed and the current that speed produces.
The current caused by the steady flow of electrons due to drift velocity is called drift current.
