Mean drift velocity (4.1.2)
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When a potential difference is applied across the ends of a wire, by the terminals of a cell or battery, electrons move through the wire.
Electrons move around a circuit from the negative terminal of a cell or battery to the positive terminal.
It is important to note that conventional current is the flow of positive charge, which is in the opposite direction to the flow of negatively charged electrons. This is called electron current.
As electrons move through a wire, they collide with static metal ions, causing them to be scattered in random directions. Electrons can also collide with defects in the wire.
Despite the collisions, the electric force due to the potential difference at the ends of the wires still causes a net movement of electrons through the wire. This is called electron drift.
- The velocity of the electrons is referred to as their mean drift velocity.
- Electrons move through wires much more slowly than through free space.
An example of this is the standard copper wire used in the laboratory carrying a current, electrons move as slowly as
The current in flowing through a wire with cross-sectional area in is given by the formula below:
Where:
- is the number density: the number of electrons per cubic metre of material, in ,
- is the electron charge and is equal to , and
- is the mean drift velocity of the charge carriers and is measured in .
The current equation shows that the following properties will result in a greater current through a wire:
- Larger area : There is more space for electrons to flow through the wire.
- Greater electron density : There are more electrons to flow through the wire.
- Higher mean drift velocity : Due to the increased rate at which electrons flow through any point in the wire.
Question walkthrough
Mean Drift Velocity
Use I = nqvA to describe how the mean drift velocity must change when a wire is replaced with a narrower one carrying the same current.
The number density is the number of electrons per cubic metre of material is measured in SI units of
A greater number of electrons in a given volume means more electrons are available to carry current. Therefore, a higher value of results in a better electrical conductivity.
Materials are classified in terms of their number density and hence their electrical conductivity:
- Metals like copper have a high number density: generally in the range to Metals are good conductors of electricity.
- Semiconductors such as silicon have an intermediate number density: generally in the range Semiconductors do not conduct electricity as well as metals, but their number density can be increased by raising their temperature.
- Insulators have a low number density – practically zero – and do not conduct electricity.
Some examples of the value for different common materials are listed in the table below.
