Drift Velocity
Drift velocity is the average velocity with which free electrons in a conductor drift in the opposite direction of the applied electric field, overcoming frequent collisions with ions.
What is Drift Velocity?
The average velocity attained by free electrons in a conductor when subjected to an external electric field, moving in a direction opposite to the field.
Key formula / rule: Drift Velocity
Key points
- Define drift velocity and explain its origin.
- Relate drift velocity to the applied electric field and relaxation time.
- Understand the role of collisions in determining drift velocity.
- Differentiate drift velocity from random thermal motion.
Common exam trap
Confusing drift velocity with the random thermal velocity of electrons.
Definitions
- Term
Drift Velocity
- Meaning
The average velocity attained by free electrons in a conductor when subjected to an external electric field, moving in a direction opposite to the field.
- Term
Relaxation Time (τ)
- Meaning
The average time interval between two successive collisions of an electron with the ions in the crystal lattice of a conductor.
Learning objectives
Define drift velocity and explain its origin.
Relate drift velocity to the applied electric field and relaxation time.
Understand the role of collisions in determining drift velocity.
Differentiate drift velocity from random thermal motion.
Formulae
- Name
Drift Velocity
- Note
vd is drift velocity, e is the magnitude of electronic charge, E is the electric field strength, τ is the average relaxation time, and m is the mass of the electron. The negative sign indicates that the drift is opposite to the direction of the electric field.
- Expression
vd = - (eEτ)/m
- Name
Current in terms of Drift Velocity
- Note
I is the current, n is the number density of free electrons, A is the cross-sectional area of the conductor, e is the magnitude of electronic charge, and vd is the drift velocity.
- Expression
I = nAevd
Prerequisites
Basic understanding of electric fields and forces.
Concept of free electrons in conductors.
Kinematics (average velocity).
Common mistakes
Confusing drift velocity with the random thermal velocity of electrons.
Assuming electrons move in a straight line without collisions.
Forgetting the negative sign in the relationship between drift velocity and electric field (if considering magnitude).
Keywords
Drift Velocity
Electric Field
Electrons
Conductors
Collisions
Relaxation Time
Electric Current
Ohm's Law
Practice preview
A conductor of length L has a drift velocity v_d when a potential difference V is applied across it. If the length of the conductor is doubled while keeping the potential difference constant, what will be the new drift v…
hard
If the temperature of a conductor increases, what happens to the drift velocity of electrons, assuming the applied electric field remains constant?…
medium
The drift velocity (v_d) of free electrons in a conductor is related to the applied electric field (E) by which of the following relations?…
medium