📘 Chapter 33: Drift Velocity (Class XII)
🔷 1. Introduction
In the previous chapter, we learned that free electrons inside a metallic conductor move randomly. Random motion alone cannot produce electric current because the average movement of electrons in every direction is zero.
When an external electric field is applied across the conductor, the free electrons acquire a small average velocity in one particular direction. This average velocity is known as Drift Velocity.
🔷 2. What is Drift Velocity?
Ethan: Professor, what exactly is drift velocity?
Professor: Drift velocity is not the actual random speed of electrons. Instead, it is the average velocity acquired by free electrons in a conductor due to an applied electric field.
Although electrons continue their random motion, the electric field gives them a slight preference to move in one direction. This average directed motion is called drift.
Academic Definition:
Drift velocity is defined as the average velocity with which free electrons drift through a metallic conductor under the influence of an applied electric field.
🔷 3. Physical Meaning of Drift Velocity
Ethan: Professor, what does "average velocity" mean here?
Professor: Imagine thousands of students walking randomly inside a large playground. Since they move in every direction, there is no overall movement.
Now suppose a teacher asks everyone to slowly walk toward the main gate. Although students still avoid each other and change directions occasionally, their overall movement is toward the gate.
Similarly, electrons continue colliding with atoms, but the electric field causes a tiny net motion opposite to the electric field. This net average motion is the drift velocity.
🖼️ Motion of Electrons under an Electric Field
The electric field produced by a battery exerts a force on free electrons. As a result, electrons slowly drift through the conductor while continuously colliding with metal ions. This directed motion produces electric current.
🔷 4. Symbol and Formula
Ethan: Professor, how is drift velocity represented?
Professor: Drift velocity is represented by the symbol vd.
vd = (−eEτ) / m
Where,
- vd = Drift velocity
- e = Charge of an electron
- E = Electric field intensity
- τ (Tau) = Relaxation time
- m = Mass of an electron
🔷 5. Why is the Negative Sign Used?
Ethan: Professor, why does the equation contain a negative sign?
Professor: Because electrons carry negative charge. Therefore, they move opposite to the direction of the electric field.
Electric Field → Positive to Negative
Electron Drift → Negative to Positive
🔷 6. Factors Affecting Drift Velocity
Ethan: Professor, upon what factors does drift velocity depend?
Professor: Drift velocity depends mainly on the following factors.
| Factor | Effect on Drift Velocity |
|---|---|
| Electric Field (E) | Higher electric field increases drift velocity. |
| Relaxation Time (τ) | Longer relaxation time increases drift velocity. |
| Mass of Electron (m) | Greater mass decreases drift velocity. |
🔷 7. Is Drift Velocity Large?
Ethan: Professor, do electrons travel very fast inside wires?
Professor: No. The drift velocity is surprisingly very small, usually only a few millimeters per second.
However, when a switch is turned on, the bulb glows almost immediately because the electric field propagates through the conductor nearly at the speed of light. The signal travels very fast, but the electrons drift slowly.
📦 8. Important Results (Must Remember)
- Drift velocity is an average velocity.
- It exists only when an electric field is applied.
- Random motion alone does not produce current.
- Electrons drift opposite to the electric field.
- Drift velocity is usually very small.
- Greater electric field produces greater drift velocity.
- Drift velocity is responsible for electric current.
🔷 9. Drift Velocity vs Random Motion
| Random Motion | Drift Motion |
|---|---|
| Occurs naturally. | Occurs due to electric field. |
| Direction changes continuously. | Has one average direction. |
| Produces no current. | Produces electric current. |
🧠 10. Conceptual Questions
🔹 Q1
Ethan: What is drift velocity?
Professor: It is the average directed velocity acquired by free electrons due to an applied electric field.
🔹 Q2
Ethan: Why is drift velocity called an average velocity?
Professor: Because electrons continuously collide with atoms and do not move uniformly.
🔹 Q3
Ethan: Does random motion create electric current?
Professor: No. Random motion has zero average displacement.
🔹 Q4
Ethan: In which direction do electrons drift?
Professor: Opposite to the electric field.
🔹 Q5
Ethan: Why is drift velocity very small?
Professor: Because electrons undergo frequent collisions with the positive ions of the metal lattice.
🔷 11. Applications of Drift Velocity
- Understanding electric current in metallic conductors.
- Design of electrical transmission systems.
- Semiconductor device analysis.
- Electronic circuit design.
- Electrical engineering and material science.
🔷 12. Summary
Drift velocity is the average velocity acquired by free electrons in a metallic conductor when an electric field is applied. Although electrons continue their random motion, the electric field gives them a preferred direction, producing a slow net movement called drift. This directed motion is responsible for electric current in conductors and forms the microscopic basis of electrical conduction.
✨ End of Chapter 33: Drift Velocity ✨
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