PRINCIPLES OF ELECTRICAL CONDUCTION

1. Electrical conductors and insulators

Electrical conductors are materials that readily support the passage of an electric current under an applied voltage. Metals are typical conductors, and are characterised by having a high ELECTRICAL CONDUCTIVITY and low RESISTIVITIES .

On the other hand, insulators (such as glass, quartz, rubber, wood and most plastics) do not normally conduct electricity, and have low conductivities and high resistivities.

Semiconductors have resistivities that are indermediate between those of conductors and insulators. The table on the right gives an idea of the values that are involved.

Material Conductivity S·m-1 Resistivity Ω·m
CONDUCTORS
Copper 
Silver 
Gold 
Steel 
5.81x107
6.8x107
4.1x107
5x106
1.72x10-8
1.47x10-8
2.44x10-8
2x10-7
INSULATORS
Glass 
Wood 
Quartz (fused) 
Typical plastics 
10-10 - 10-14
10-8 - 10-11
1.3x10-18
~10-13
1010 - 1014
108 - 1011
7.5x1017
~1013
SEMICONDUCTORS
Pure graphite 
Pure silicon 
Pure germanium 
2.9x104
4.35x10-4
1.7
3.5x10-5
2.3x103
0.60

Electrical conductivity can be explained by the so-called ENERGY BAND THEORY (), which is outlined below in a (very!) simplified form.

Solids can be considered as having "bands" which can accommodate very large but finite numbers of electrons. A band of low energy, called the VALENCE BAND, contains the valence electrons, that is, the electrons in the outer electron shell of the atom. This band is shown in green in the diagram above. In metals, this band is not filled, and so the electrons in this band can move freely under an applied voltage.

In insulators, the valence band is completely filled with electrons, and they are "jammed" so they cannot move. If enough energy is supplied, a few electrons can gain sufficient energy to "jump" into a CONDUCTION BAND of higher energy (shown in yellow in the above diagram), where, due to their small number, they are free to move under the applied voltage, thus giving rise to the very low electrical conductivity observed with insulators. The important point here is that the energy gap between the valence and conduction bands in insulators is large compared to the corresponding gap in conductors ().

At high values of the applied electric field (that is, at high voltages), large numbers of electrons can jump into the conduction band. This results in the insulator being able to conduct electricity with resulting INSULATOR BREAKDOWN.

With semiconductors, the energy gap between the valence band (which is normally filled with electrons) and the conduction band is relatively small, so that electrons can "jump" between the valence and conduction bands with relative ease. At room temperature, small numbers of electrons are thermally excited and cross the gap from the valence to the conduction band. Increased temperatures result in a larger number of electrons gaining access to the conduction band, thus increasing the electrical conductivity of the material.

Why does the conductivity of a conductor decrease with increasing temperature?

(Click here for a discussion)

2. Conduction in ionic solutions

Solutions of ionic substances conduct electricity, and these solutions (and the solutes) are known as ELECTROLYTES. Molten salts also conduct electricity. Consider the diagram below:

A battery is connected to a conductor, which makes contact with a sodium chloride solution. The contact is made through two platinum ELECTRODES (). The electrode connected to the positive terminal of the battery is called the ANODE, while the one connected to the negative terminal is called the CATHODE. The ammeter will register a current, indicating that the solution is a conductor. This arises because the positively charged cations, Na+, move to the negatively charged cathode, while the negatively charged anions, Cl- move towards the positively charged anode. Click here to see an animation - we are not concerned here with what happens at the electrodes, beyond noting that cations are dischareged at the cathode, and anions discharged at the anode.) The important thing to remember is that the current is carried by the excess electrons of the anions.

3. Additional questions


Resistivity and temperature

In the absence of an applied electric field, there is no current in the conductor, because the electrons move randomly in all directions and hence there is no net flow of charge. If an electric field is applied, electrons move against the field colliding here and there with the metallic ions. This means that the path travelled by an individual electron is not straight. The actual displacement of the electron in a direction against the field in a given small time interval is shown as s in the diagram on the left. The more the number of collisions, the slower the net speed of the electron in the direction of motion.

When a conductor is heated, there is an increase in the number of collisions that take place within the conductor. Consequently, the net speed of the electron in the direction of motion is slowed down, and the flow of charge in unit time, which is what an electric current is, is reduced.