Electrical properties of materials
© copyright M.J.Morris 2002
The atomic structure of some materials produce free electrons. As their name implies free electrons can move freely through the material. The motion of the electrons is complicated, however we can break it down into two types, random motion and uniform drift. These types of motion will be described below but it is important to point out that it is only the uniform drift of electrons that produces electrical current.
Thermal energy cause the free electrons to move with random motion. This motion is characterised by fast short movements with random changes in direction.
This movement does not cause any electrical effects on a macroscopic scale because
This motion is similar to the motion of water molecules in a glass of still water at room temperature. The individual water molecules will be moving randomly due to the thermal energy. However there is no net movement of water in the glass.
When an electrical field is set up within the material the free electrons drift towards the positive potential. They still retain the random motion due to thermal energy but now they drift away from the "fixed" position referred to in the previous example. This is similar to the movement of water molecules in a stream. The water molecules may move with some random motion due to thermal energy but they all drift gradually downstream. This produces visible movement of the water on a macroscopic scale.
Electron drift velocity is measure of the net movement of electrons through the material. The drift velocity is much lower than the velocity of the random motion (which is around 1000 km/s). The actual value of drift velocity varies but a typical figure for metals is 1mm/s (1 millimetre per second).