Electrical conductivity is the ability of a material to carry the flow of an electric current (a flow of electrons). Imagine that you attach the two ends of a battery to a bar of iron and a galvanometer. (A galvanometer is an instrument for measuring the flow of electric current.) When this connection is made, the galvanometer shows that electric current is flowing through the iron bar. The iron bar can be said to be a conductor of electric current.
Replacing the iron bar in this system with other materials produces different galvanometer readings. Other metals also conduct an electric current, but to different extents. If a bar of silver or aluminum is used, the galvanometer shows a greater flow of electrical current than with the iron bar. Silver and aluminum are better conductors of electricity than is iron. If a lead bar is inserted, the galvanometer shows a lower reading than with iron. Lead is a poorer conductor of electricity than are silver, aluminum, or iron.
Many materials can be substituted for the original iron bar that will produce a zero reading on the galvanometer. These materials do not permit the flow of electric current at all. They are said to be nonconductors, or insulators. Wood, paper, and most plastics are common examples of insulators.
Another way of describing the conductivity of a material is through resistance. Resistance can be defined as the extent to which a material prevents the flow of electricity. Silver, aluminum, iron and other metals have a low resistance (and a high conductivity). Wood, paper, and most plastics have a high resistance (and a low conductivity).
The unit of measurement for electrical resistance is called the ohm (abbreviation: Ω). The ohm was named for German physicist Georg Simon Ohm (1789–1854), who first expressed the mathematical laws of electrical conductance and resistance in detail. Interestingly enough, the unit of electrical conductance is called the mho (ohm written backwards). This choice of units clearly illustrates the reciprocal (opposite) relationship between electrical resistance and conductivity.
How conductance takes place
Electrical conductivity occurs because of the ease with which electrons can be removed from atoms. All substances consist of atoms. In turn, all atoms consist of two main parts: a positively charged nucleus and one or more negatively charged electrons. An atom of iron, for example, consists of a nucleus with 26 positive charges and 26 negatively charged electrons.
The electrons in an atom are not all held with equal strength. Electrons close to the nucleus are strongly attracted by the positive charge of the nucleus and are removed from the atom only with great difficulty. Electrons farthest from the nucleus are held only loosely and are removed quite easily.
A block of iron can be thought of as a huge collection of iron atoms. Most of the electrons in these atoms are held tightly by the iron nuclei. But a few electrons are held loosely—so loosely that they act as if they don't even belong to atoms at all. Scientists sometimes refer to this condition as a cloud of electrons.
Normally these "free" electrons have no place to go. They just spin around randomly among the iron atoms. That situation changes, however, when a battery (or other source of electric current) is attached to the iron block. Electrons flow out of one end of the battery and into the other. At the electron-rich end of the battery, electrons flow into the piece of iron, pushing iron electrons ahead of them. Since all electrons have the same negative charge, they repel each other. Iron electrons are pushed away from the electron-rich end of the battery towards the electron-poor end. In other words, an electric current flows through the iron.
Insulators have a very different structure. They too consist of atoms (nuclei and electrons), but very few free electrons can be found in insulators. Those electrons tend to be bound tightly to nuclei in chemical bonds. Attaching a battery to an insulator has no effect since there are no free electrons to be pushed through the material.
Electrons are not the only particles capable of carrying an electric current. Ions can do it, too. An ion is an atom or group of atoms with an electric charge. Suppose you dissolve a crystal of table salt (sodium chloride) in water. Salt crystals consist of positive sodium ions and negative chloride ions. In the solid state, these ions are not free to move around. Once they are dissolved in water, however, they become completely mobile. They are free to "swim" about in the water and to respond to an electric current from a battery. That current supplies electrons that cause positive sodium ions to flow in one direction and negative chloride ions to flow in the opposite direction.
A good example of this effect can be seen in the conductivity of water. Pure water consists only of water molecules. The electrons in water molecules are held tightly by hydrogen and oxygen atoms and are not free to move. Attaching a battery to a container of water produces no electric current because pure water is an insulator. But a few grains of table salt added to the water changes things completely. Sodium ions and chloride ions are released from the salt, and the salt water solution becomes conductive.
Semiconductivity and superconductivity
Some materials cannot be classified as either conductors or insulators. Semiconductors, for example, are materials that conduct an electric current but do so very poorly. Semiconductors were not well understood until the mid-twentieth century, when a series of remarkable discoveries revolutionized the field of electrical conductivity. These discoveries have made possible a virtually limitless variety of electronic devices, ranging from miniature radios and handheld calculators to massive solar power arrays and orbiting telescopes.
Superconductivity is a property that appears only at very low temperatures, usually close to absolute zero (−273°C). At such temperatures, certain materials lose all resistance to electric current; they become perfect conductors. Once an electric current is initiated in such materials, it continues to flow without diminishing and can go on essentially forever.
The discovery of superconductivity holds enormous potential for the development of electric appliances. In such appliances, a large fraction of the electrical energy supplied to the device is lost in overcoming electrical resistance within the device. That lost energy shows up as waste heat. If the same appliance were made of a superconducting material, no energy would be lost because there would be no resistance to overcome. The appliance would become, at least in principle, 100 percent efficient.
[ See also Superconductor ]