Let’s Get Physics-l: Electron Drift Velocity

Let’s begin by discussing metal and its crystalline structure. In metals, the atoms are arranged in long, neat lines which we refer to as a crystal lattice. In between these atoms we have the delocalised or conduction electrons which move in between the atoms randomly and freely. You can consider it as a very organised town with the atoms being buildings and vehicles being the electrons. When the metal conducts electricity these electrons drift slowly along the metal.

Without potential difference, the electrons move randomly but with no real overall net movement of electrons around the metal. However, if potential difference acts across the ends of the wire current begins to flow through the wire. The electrons maintain their random movement but they now have a net movement towards the positive potential, drifting slowly along the wire.

With so many electrons in a metal, the slow nature of the drift could be seen as a touch peculiar. However, as is often the case, we do have an explanation for this. The movement of electrons is rather limited thus their movement is small. This is further affected by collisions with the tightly packed metal atoms getting in the way of electron movement.

We can calculate the mean drift velocity given that we know the density of metal atoms, such as copper  (Which we can obtain using the density of the metal and its atomic mass (Though it is often given to you)). This can then be used in the equation:

I = nAev

  • I – Current (A)
  • n – number of free electrons per unit volume (m^-3)
  • A – cross-sectional area of the wire (m^2)
  • e – elementary charge (1.6×10^-19 C)
  • v – mean drift velocity of electrons (ms^-1)

We can rearrange this equation to give us v on its own by dividing I by the product of nAe.


Summary

  • Metal Structure – Crystal lattice where atoms are arranged in long, neat rows and delocalised electrons can move randomly between them.
    • Without p.d – random movement of electrons with no net movement
    • With p.d – random movement of electrons with slow net movement towards positive potential
  • Drift velocity – the average velocity of an electron as it moves through a wire.
  • I = nAev
    • I – Current (A)
    • n – number of free electrons per unit volume (m^-3)
    • A – cross-sectional area of the wire (m^2)
    • e – elementary charge (1.6×10^-19 C)
    • v – mean drift velocity of electrons (ms^-1)

There you have it, electron drift velocity (Not the most interesting of topics, I know, but no less important overall). As ever, if you have any feedback, thoughts, questions or requests please leave them in the comments.

 

 

 

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