Short Answer

Drift velocity is the average slow speed with which free electrons move through a conductor when an electric field is applied. Even though electrons move randomly inside a metal, the electric field pushes them slightly in one direction, creating a net flow of charge.

This small, directed motion of electrons is what we call drift velocity. It is usually very slow, even though electric current appears instantly in a circuit. Drift velocity depends on the electric field, charge of the electron, and the nature of the conductor.

Detailed Explanation

Drift velocity

Drift velocity is an important concept in understanding how electric current flows in a conductor. In any metal, free electrons move randomly in all directions even when no electric field is applied. Their motion is fast but completely disorderly, so there is no net movement of charge in any one direction. When an electric field is applied across the conductor, these electrons experience a force that pushes them slightly in the opposite direction of the field. Because of this, the random motion becomes slightly biased, creating a net movement of electrons. This slow, average motion in a specific direction is called drift velocity.

Drift velocity helps us understand how current flows at the microscopic level. Even though electric energy travels extremely fast through a circuit, the actual electrons move very slowly. Their directed movement forms the basis of electric current in metals.

Meaning of drift velocity

When a voltage is applied across a conductor, free electrons feel a force due to the electric field. They start drifting toward the positive terminal. Although they continue to move randomly due to collisions with atoms, their overall motion shifts in one direction. This average velocity with which electrons drift is known as drift velocity.

It is important to note:

Drift velocity is not the high-speed random motion of electrons.
It is the average slow movement in one direction.

In most practical circuits, the drift velocity is only a few millimeters per second.

Formula for drift velocity

The formula for drift velocity (vₑ) is:

vₑ = I / (n A q)

Where,

I is the electric current,
n is the number of free electrons per unit volume,
A is the cross-sectional area of the conductor,
q is the charge of an electron.

This formula shows that drift velocity increases when current increases and decreases when the number of free electrons in the metal is large.

Factors affecting drift velocity

Several factors influence drift velocity:

Electric field:
A stronger electric field pushes electrons more strongly, increasing drift velocity.
Current:
Higher current means electrons have a larger drift speed.
Nature of material:
Metals with more free electrons have smaller drift velocity for the same current.
Temperature:
At higher temperatures, atoms vibrate more, increasing collisions and reducing drift velocity.
Cross-sectional area:
A thicker wire allows more electrons to flow, resulting in smaller drift velocity for the same current.

Relation with electric current

Drift velocity is directly related to electric current. When electrons drift through a conductor, they carry charge with them. The current flowing in the conductor is due to this movement.

The relation is:

I = n A q vₑ

This shows:

If drift velocity increases, current increases.
If more electrons are available, the same current can flow with smaller drift velocity.

This equation is fundamental in understanding current at the microscopic level.

Drift velocity vs signal speed

A common misunderstanding is that electrons move very fast in a circuit. In reality:

Drift velocity is very slow (mm/s).
Electric energy or electrical signals move almost at the speed of light.

This happens because electrons push neighboring electrons, creating a chain reaction that transfers energy quickly. It is similar to how water pressure moves instantly through a pipe even though water molecules move slowly.

Examples of drift velocity

Household circuits:
When you switch on a light, electrons begin drifting slowly, but the light turns on immediately because the electric field sets up almost instantly.
Metal wires:
In copper wires, drift velocity is small, but current still flows because copper has many free electrons.
Low-temperature conductors:
When temperature drops, drift velocity increases because there are fewer collisions.
Thick wires:
A thick wire allows more current to flow with lower drift velocity.

These examples show how drift velocity affects electric current in real life.

Importance of drift velocity

Drift velocity helps explain:

How current flows through wires
How microscopic electron behavior results in macroscopic electrical effects
How resistance and temperature affect current
Why different materials conduct electricity differently

Drift velocity also forms the basis of the famous equation I = n A q vₑ, used widely in physics and electrical engineering.

Conclusion

Drift velocity is the average velocity with which electrons move through a conductor when an electric field is applied. Despite random motion, electrons drift slowly in a specific direction, creating electric current. It depends on current, electric field, material properties, and temperature. Drift velocity helps explain current flow at the microscopic level and is essential for understanding electrical behavior in conductors.