Electron Drift Velocity: Overview, Questions, Preparation

Drift is the act of moving slowly towards something. When an electric field flows through a conductor, the charged particles tend to drift in the electric field's direction. It is known as drift velocity.

Electrons in the atom tend to move in all directions. They carry their velocity and do not require any outer force. 

However, when an electric field is applied, there is a higher potential generated in the conductor. It influences the electrons to move towards the higher potential in a single direction.

The net average velocity thus attained by these electrons, due to the electric field is known as electron drift velocity. The current produced by the movement of electrons due to drift velocity is called drift current.

Electron Drift Velocity is stated as follows:

The average velocity attained by electrons in a material due to an electric field is called drift velocity.

The unit of measurement for electron drift velocity is m/s or m2/V.s.

The formula for calculating the drift velocity of an electron is as follows:

I = nAvQ

Therefore, v = I/nAQ

Where,

I is the current flowing in amperes.

n is the total no. of electrons present.

A is the area of the conductor.

v is the drift velocity.

Q is the total charge present on an electron.

What is the mobility of an electron?

The time an electron takes to move through a conductor when an electric field is applied is known as its mobility.

The mobility of an electron is calculated as follows:

μ = E/Vd

What is the relationship between electron drift velocity and electric current?

The drift velocity of an electron is small. It is 10-3/Ms-1. Hence, for an electron to flow through a conductor, it should take about 17 minutes. However, an electronic appliance starts as soon as the switch turns on without a delay of 17 minutes. It occurs because a current flows at the speed of light. The current does not move at the speed of the drifting of electrons. Hence, a light bulb turns on the moment a switch is pressed, without any delay.

What is the relationship between drift velocity and current density?

The total current passing through a cross-sectional area of a conductor is called current density at a given point of time.

From the formula of electron drift velocity, we know,

v = I/nAQ

However, current density (J) = I/A

Therefore, 

J = nvQ

Where,

J represents current density in A/ms. 

v represents the drift velocity of the electrons.

Thus we can say that current density and the drift velocity of electrons are directly proportional.

Electron drift velocity in class 12

In class 12, the chapter, current electricity defines the terms velocity, electric current, current density, etc. It also provides formulas to calculate these quantities. Current electricity has a weightage of 7-8 marks in the exam. Hence, two or three questions from this chapter may be asked.

1. A current of 3A  is flowing in a copper conductor with a cross-section of 1mm^2 (1×10^-6m^2). n = 8.5 x 1028 per m3

Solution:

As per the formula,

3 = 8.5×1028×1×10−6×v×1.6×10−19

Where, Q=1.6×10−19 C

Therefore,

v = 2.205882×10^−4  ms−1 

Q: Why is drift velocity small?

Q: Name one factor on which drift velocity depends.

Q: What does the negative sign in drift velocity mean?

Q: What is the relationship between drift velocity and temperature?

Q: Does resistance affect drift velocity?