Drift velocity

Introduction

When it comes to physics, drift velocity is an important topic that holds significant value in the total marks in the physics section. The other related topics include the mobility of an electron, the formula to calculate this velocity, and the net velocity of the electrons. 

Definition of drift velocity

Every material has suspended electrons in its sub-atomic configurations. These electrons are in random motion when no external field is applied to the material. However, in the electric field, the free electrons attain charge and slowly tend to align in a similar direction to the electric field. Thus, drift velocity is known as the net or average velocity with which the charged particles drift in the presence of such electric fields.

Understanding Net or Average Velocity

All conductive materials above zero Kelvin have free electrons in their sub-atomic configurations, which move at arbitrary velocity. In the presence of electric potential around this conductor, these free particles tend to align themselves in the positive direction. Additionally, as the electrons drift, they keep on colliding with other particles resulting in loss of their kinetic energy.

The constant electric field ensures acceleration of the charged particles resulting in frequent collisions. Since the acceleration happens in the direction of the applied electric field, the average velocity of these electrons occurs in the same direction of this applied field.

Thus, the net velocity of the electrons is always in a similar direction of the field irrespective of the small drifts by the electrons.

Understanding mobility of an electron

Let Vd is the drift velocity and E is the electric field applied to the conductor, then mobility ‘μ’ is represented as:

μ=VdE

Factors affecting the drift velocity:

The main factors on which the drift velocity is dependent are:

•     Temperature: With an increase in temperature, atoms of the material start vibrating at a fast speed. Hence, the associated movement of the electrons increases. Thus, drift velocity increases with an increase in the temperature of the conductor.
•     Potential difference: The potential difference applied across the cross-section of the conductors increases the current flowing through it. This increase in current causes the electrons to vibrate quickly. Thus, with an increase in potential difference, the drift velocity of the electrons increases.
•     Area: The more area required to be covered by the electrons decreases their energy. Electrons prefer to pass through the shortest distance, which significantly increases with an increase in the cross-sectional area of the conductor. Hence, with an increase in the cross-sectional area of the conductor, the drift velocity of the conductor decreases. Thus, the area of the conductor and its drift velocity are inversely proportional to each other.

The formula for calculating drift velocity:

Let ‘I’ is the charge passing through a conductive material in amperes,

‘n’ is the total number of charged particles or electrons,

‘A’ is the cross-sectional area of the material,

‘vd’ is the drift velocity of charged particles in the material, and

‘Q’ is the electron charge in Coulombs.

Then,

I=nAvdQ

Electric current and Drift velocity:

The drift velocity of a charged particle is infinitesimally small and is measured to be around 10-3 ms-1. Thus, if electrons are moving with such low velocity, they take around seventeen minutes to flow through a conductive material of length one meter. However, this speed does not impact the working of electrical appliances at home. It is because the current flows quite fast, usually at light speed and not with the speed at which the particles drift inside the material.

Hence, it is observed that electric current is not due to the drift velocity of the electrons. The current passing through a conductor is due to the applied electric field across its cross-sectional areas. There is no significant contribution of the drift velocity in electric current passing through a conductor.

Current density and Drift velocity:

The current density is termed as the quantity of charge per unit time flowing in a unit cross-section of a conductive material. The relation between current density and drift velocity is formalized as:

J = I/A.

Here, ‘J’ is the current density. We have seen in the earlier section that drift velocity is calculated with the formula:

I = nAvdQ 

where

‘I’ is the charge passing through a conductive material.

‘n’ is the total number of charged particles or electrons,

‘A’ is the cross-sectional area of the material,

‘vd’ is the drift velocity of the charged particles in the material, and

‘Q’ is the electron charge in Coulombs.

Thus, substituting the values for ‘I’ in current density, we get the following result:

J = nvdQ.

In other words, the drift velocity is directly proportional to the current density of the charged particles. Furthermore, on increasing the force of the electric field, the velocity increases resulting in a significant increase in the amount of current passing through the conductive material.

Conclusion:

We hope this brief article could help you understand the basics of drift velocity, a quick formula to calculate drift velocity, and the mobility of the electrons. It is the average velocity of the charged particles or electrons. The established relation between this velocity and electric current helps further relate it to the current density.