Drift Velocity
The average velocity acquired by free electrons in a conductor in the presence of an electric field, opposite to the direction of the field.
What is Drift Velocity?
The average velocity attained by charge carriers in a conductor when subjected to an external electric field.
Key formula / rule: Drift Velocity (general)
Key points
- Define drift velocity.
- Explain the origin of drift velocity in a conductor.
- Relate drift velocity to the applied electric field and relaxation time.
- Understand the relationship between drift velocity and electric current.
Common exam trap
Confusing drift velocity with the random thermal velocity of electrons.
Definitions
- Term
Drift Velocity
- Meaning
The average velocity attained by charge carriers in a conductor when subjected to an external electric field.
- Term
Relaxation Time (τ)
- Meaning
The average time interval between two successive collisions of a charge carrier with the ions of the conductor lattice.
- Term
Mobility (μ)
- Meaning
The magnitude of drift velocity per unit electric field.
Learning objectives
Define drift velocity.
Explain the origin of drift velocity in a conductor.
Relate drift velocity to the applied electric field and relaxation time.
Understand the relationship between drift velocity and electric current.
Formulae
- Name
Drift Velocity (general)
- Note
For electrons, where m is the mass of the electron. The negative sign indicates direction opposite to E.
- Expression
vd = -eEτ/m
- Name
Current due to Drift Velocity
- Note
Relates current to drift velocity, number density of charge carriers (n), area (A), and charge (e).
- Expression
I = nAevd
- Name
Mobility
- Note
Mobility (μ) is a measure of how easily charge carriers can move in a material under an electric field. For electrons, μ = eτ/m.
- Expression
μ = vd / E
Prerequisites
Basic understanding of electric fields and forces.
Concept of charge carriers (electrons in conductors).
Understanding of thermal motion.
Basic kinematics.
Common mistakes
Confusing drift velocity with the random thermal velocity of electrons.
Assuming drift velocity is large.
Incorrectly determining the direction of drift velocity for positive and negative charge carriers.
Forgetting the role of collisions in limiting the drift velocity.
Keywords
Drift Velocity
Electric Field
Charge Carriers
Conductor
Collisions
Relaxation Time
Current
Mobility
Practice preview
If the number density of free electrons in a conductor is doubled, while keeping the current and cross-sectional area constant, how does the drift velocity of electrons change?…
medium
What is drift velocity in the context of electric current?…
easy
A copper wire of cross-sectional area 1 x 10^-6 m^2 carries a current of 1.1 A. If the number density of free electrons in copper is 8.5 x 10^28 m^-3, what is the drift velocity of the electrons? (Charge of electron e = …
hard