Drift Velocity & Electron Mobility: Definitions & Formula

Drift Velocity & Electron Mobility: Definitions & Formula
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  • 0:03 Drift Velocity
  • 1:19 Electron Mobility &…
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Lesson Transcript
Instructor: Matthew Bergstresser
Electric charge moving through a conductor is electric current. Mobile charge carriers don't have a straight path through a conductor. This lesson describes the drift velocity of charge carriers and how to calculate it.

Drift Velocity

Have you ever been in a large crowd heading in a certain direction? Think about a mass of students heading into the cafeteria at lunch time or a massive crowd of people moving towards their seats at a concert or sporting event. Now that you have that image in your head, pick one person in the crowd, and imagine he has white paint on the bottom of his shoes.

After the crowd has dissipated, you can see the white shoe prints marking his path. It's not a straight line from his starting point to his ending point, is it? He zigged and zagged, jockeying for position as he drifted towards his destination. This is an analogy for what happens to electrons or charge carriers when they move through a wire.

Anything that carries charge through a conductor is a charge carrier. For the purposes of this lesson, let's assume our charge carriers are electrons moving through a simple battery-powered circuit on their way to illuminate a light bulb.

It is a common misconception to think electrons just flow through a wire at the speed of light. The misconception could come from many diagrams and animations that show this exact thing in an attempt to explain electric current, or it could be because the instant you turn on a light switch, it lights up. Let's look at what really happens.

Electron Mobility & Drift Velocity

Quantum mechanics tells us that electrons exhibit random motion. When the switch is flipped turning on our light bulb, an electric field is immediately created along the wire inducing the electrons to move to higher potential, which is the positive side of the battery. When the electrons move towards the positive terminal, they bounce into the atoms in the wire. The combination of their intrinsic random motion and their collisions with atoms cause the electrons to zig and zag along their journey. In this diagram, the black dots are copper atoms, and the little red dots represent an electron as it bounces its way through the atoms. The path is jagged, but the net displacement is to the right.


drift

Electron mobility is how quickly an electron can move through a conductor. It's determined by the drift velocity and the strength of the electric field as shown in this equation:


electron_mobility

Where:

  • μ is electron mobility
  • vd is the drift velocity in meters-per-second (m/s)
  • E is the strength of the electric field in newtons-per-coulomb (N/C)

The formula for drift velocity is:


drift_vel

Where:

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