Drift Velocity Questions

Drift velocity is the average velocity of charged particles in a material due to an electric field. The SI unit of drift velocity is m/s. Drift velocity can be calculated using the following formula:

Here, I represents the current flowing through the conductor (in Amperes), n represents the number of electrons, A represents the area of the cross-section of the conductor (m²), v represents the drift velocity of the electrons, and Q represents the charge of an electron (in Coulomb).

Mobility of the Electron

The drift velocity of an electron for a unit electric field is known as the mobility of the electron. The mobility of an electron can be calculated by,

Depending on the nature of the charge carrier, mobility is always a positive quantity.

Current density is defined as the total amount of current passing per unit cross-sectional area of a conductor. If J is the current density (A/m²) and v is the drift velocity of the electrons then,

From this formula, electron drift velocity and current density are directly proportional. Furthermore, when the electric field intensity increases, the drift velocity increases and the current flowing through the conductor increases.

Read More: Drift Velocity

Important Questions with Answers

1. What is meant by drift velocity?

The average velocity attained by charged particles (e.g. electrons) in a material due to an electric field is called drift velocity.

2. The drift velocity does not depend upon _________.

1. the length of the wire
2. the cross-section of the wire
3. the number of free electrons
4. the magnitude of the electric field

Answer – A. the length of the wire

Explanation – The drift velocity does not depend on the length of the wire when dealing with a macroscopic wire. However, suppose the wire is too short compared to the average distance a charge carrier travels before undergoing a collision, it might begin to depend on the wire length, but a wire won’t be that short for all practical intents and purposes.

The formula of drift velocity is:

3. The drift velocity of electrons is _________.

1. \(\begin{array}{l}10^{-3}m/s\end{array} \)
2. \(\begin{array}{l}10^{-5}ms\end{array} \)
3. \(\begin{array}{l}10^{3}m/s\end{array} \)
4. \(\begin{array}{l}10^{5}ms\end{array} \)

Answer – A.

Explanation – An electron’s drift velocity is usually in terms of 10^-3m/s., which depends on the nature of the charge carrier.

4. What is the relation between drift velocity and electric current?

Drift velocity is directly in proportion with the electric current. And also, we can say that drift velocity is directly proportional to the magnitude of an electronic field. If V is drift velocity, is electron mobility, and E is the electric field, then we can represent this relation in mathematical form as,

5. How does the drift velocity of electrons in a conductor depend on the temperature?

Drift velocity decreases with an increase in temperature because the drift velocity is also slightly temperature-dependent: an increase in temperature causes atoms to vibrate more, which increases the number of collisions electrons have on their way through the wire and decreases the drift velocity.

6. Drift velocity is a scalar quantity.

1. True
2. False

Answer: B. False

Explanation: Drift velocity is a vector quantity because both magnitude and direction are needed to define it.

7. How does drift velocity work?

The drift velocity is directly proportional to the current. Electrons jump towards high potential terminals of a conductor when an electric field is applied. Electrons inside a conductor move at arbitrary velocities and directions without an electric field.

8. When drift velocity is so small, how is it that an electric bulb lights up as soon as we turn the switch on?

A particular electron does not have to reach the electric bulb to make current flow in the bulb. As soon as the switch is turned on, the potential difference is applied at the ends of the bulb’s terminals and an electric field is set up.

Changes occur at the initial state of the setting of the electric field, which gives rise to a changing magnetic field. The changing magnetic field gives rise to another changing electric field, and so on. This electromagnetic phenomenon takes place at the speed of electromagnetic waves, which is the same as the velocity of light,

The set electric field makes current flow through the bulb instantly.

9. The mobility of free electrons is greater than that of free holes because ___________.

1. they carry positive charges
2. they require low energy to continue their motion
3. they mutually collide more
4. they carry negative charges

Answer: B. they require low energy to continue their motion

Explanation: The mobility of free electrons is always greater than that of free holes because they require low energy to continue their motion.

10) Define the term: mobility of an electron in a conductor.​

The mobility of an electron in a conductor is defined as the ratio of its drift velocity in a conductor to the electric field through the conductor.

Practice Questions

1. What is the current density?
2. Explain the net velocity of the electrons.
3. How do you find drift velocity?
4. What is the relation between drift velocity and current density?
5. Why is drift velocity opposite to the electric field?

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