Drift Velocity

What is Drift Velocity?

To understand the concept of drift velocity, we should first understand what drift is. Drift is the slow movement of an object toward something. The average velocity attained by the charged particles in a material due to the influence of the electric field is known as the drift velocity.

Drift velocity is directly proportional to current. It is also directly proportional to the magnitude of the external electric field in a resistive material. Drift velocity can be expressed in terms of Ohm’s law

u=µE

Where,

 u represents the drift velocity,

µ represents the electron mobility, and

E represents the electric field

The unit of these quantities is m/s, m2/ (V.s), and V/m respectively.

What is the Drift Velocity of an Electron?

When particles like electrons attain the average velocity under the influence of an electric field is known as the drift velocity. It is assumed that the particle's movement is along a plane, and hence the motion can be described as the axial drift velocity.

The concept of drift velocity can be understood by studying the random motion of free electrons moving around the conductor. These free electrons keep on moving in the conductor in a disorganized way with random velocities. But when the conductor is subjected to an electric field, some kind of electrical force is applied to the randomly moving electrons and in the direction of the field. The field forces the electrons to switch towards high potential while maintaining the randomness of the motion. Scientifically we can express that the electrons will drift towards higher potentials by maintaining the random motions.

Further, it has been observed that each electron has its velocity while they move towards the higher potential point of the conductor. This net velocity is known as the drift velocity of electrons. Since the movement of the electron is known as the drift velocity, the current that is generated due to the drift movement of the electrons in the electrically charged conductor is known as the drift current. Every current flowing through a conductor is known as drift current.

When the charged particles go around in a conductor, it is not in a straight line as they collide with other particles present in the conductor. Hence the average speed of the particles in the conductor is taken into observation. This is known as drift velocity.

Under the impact of the electric field, the average velocity gained by the free electrons due to which the electrons drift.

Formula to calculate drift velocity using current density

Formula to evaluate drift velocity of the electron in a conductor of constant cross-sectional area is given by:

Drift velocity formula

v = I/nAq

Where,

v = represents the drift velocity of the electrons

I = represents the current flowing through the conductor and measured in Amperes.

A = represents the area of the cross-section of the conductor measured in m2.

q = represent the charge of an electron and measure in Coulombs

n = represent the number of electrons.

Example of Drift Velocity

As expressed in the formula above for the drift velocity, if out of the four quantities any three quantities are known, then the missing quantity can be found easily. Hence, the drift velocity of electrons in a piece of metal displaying a current of 0.1 A will be around 1x10-5 m/s. Generally, the electrons move at very high speeds of around a million m/s along a wire, but the electron drift or move very slowly in the direction of the current. 

FAQ (Frequently Asked Questions)

1. Give Some Common Theory on how Drift Velocity and Electric Current are Related

Let’s take a conducting wire having length L and of uniform cross-section area A, which contains the electric field—assuming that the wire contains 'n' free electrons per unit volume, each possessing a drift velocity 'v.' Considering the small time interval t. The length covered by each electron during this time-interval is vt. Avt gives the volume of this specific portion of the area. There are several free electrons in this section amounting to nAvt, and they go through the cross-section A in time t.

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If we signify the charge with Q, then the charge crossing the area in time t is given by

Q = neAvt

Or it can be rewritten as I = Q /t = neAv

The relation between drift velocity and the electric current is expressed in the above equation.

If the charge carriers in motion are positive instead of negative, then the field force acting on the carriers would be following the electric field direction and movement, or the drift velocity of the particle will be from left to right. There is a relation between the current density and the drift velocity, expressed in the equation below.

J = I/A = nev                 

2. What is Mobility in Physics?

In a solid-state, mobility in physics is a measure of the ease with which a particular type of charged particle passes through a solid under the influence of an electric field. Such particles are drawn by electric fields and periodically interact with solid molecules. The combination of electric field and collision causes the particles to move at an average speed is known as drift speed. The charge carrier in most metals is a negatively charged electron.

How fast an electron is moving through a metal or semiconductor under the influence of the electric field is determined with the help of electron mobility.

Mathematically, electron mobility is given as:

Where vd​ is the drift velocity of the electron, and E is the external electric field.

SI unit of mobility is m^2V^−1s^−1.

3. Does Drift Velocity Depend on Diameter?

Drift velocity does not depend on the cross-sectional area or the diameter of any conductor. For the same p.d. Here, with V across the conductor, an increase in the cross-sectional area decreases the resistance

R= (rho)(length)/(cross-sectional area)

And so, increasing the current

I=V/R,