An electric conductor (or simply “conductor”) is any material that can efficiently conduct electricity. Good conductors include some metal, ionic solutions, and ionized gasses.
Conduction, which is the movement of charges through a material medium, is caused done by the presence of an electric field in the conducting medium. Good conductors are materials that have available negative or positive charges, like electrons or ions, which can move and so constitute a current. Semiconductors are less effective conductors of electricity, while many other materials, such glass and air, are insulators.
In metals, the atomic nuclei form crystalline structures, where electrons from outer orbits are mobile, or “free.” The current (the net transfer of electric charge per unit time) is carried by the free electrons. Yet the transfer of energy is done much faster than the actual movements of the electrons. Among metals at room temperature, silver is the best conductor, followed by copper. Iron is a relatively poor conductor.
In electrolytic solutions, the positive and negative ions of the dissolved salts can carry current. Pure water is a good insulator, and various salts are fair conductors; together, as sea water, they make a good conductor.
Gases are usually good insulators. Yet when they become ionized under the influence of strong electrical fields, they may conduct electricity. Some of the energy is emitted as light photons, with most spectacular effects are seen in lightning.
In semiconductors like germanium and silicon, a limited number of free electrons or holes (positive charges) are available to carry current. Unlike metals, the conductivity of semiconductors increases with temperature, as more electrons are becoming free.
Electricity is transmitted by metal conductors. Wires are usually soft and flexible. They may be bare or, more commonly, coated with flexible insulating material. In most cases, they have a circular crosssection. Cables have larger cross-sections than wires and are usually stranded, built up as an assembly of smaller solid conductors. Cords are small-diameter flexible cables that are usually insulated. Multiconductor cable is an assembly of several insulated wires in a common jacket. Bus-bars are rigid and solid, made in shapes like rectangular, rods or tubes, and are used in switchboards.
Most conductors are made from copper or aluminum, which are both flexible materials. While copper is a better conductor, aluminum is cheaper and lighter. For overhead lines the conductors are made with a steel or aluminum-alloy core, surrounded by aluminum. The conductors are supported on insulators, which are usually ceramic or porcelain. They may be coated with rubber, polyethylene, asbestos, thermoplastic, and varnished cambric. The specific type of the insulating material depends on the voltage of the circuit, the temperature, and whether the circuit is exposed to water or chemicals.
A perfect conductor is a material through which charges can move with no resistance, while in a perfect insulator it is impossible for charges to move at all. However, all conducting materials have some resistance to the electrical energy, with several major effects. One is the loss of electrical energy that converts to heat; the other is that the heating of the conductors causes them to age. In addition, the energy loss within the conductors causes a reduction in the voltage at the load. The voltage drop needs to be taken into consideration in the design and operation of the circuit, since most utility devices are operating within a narrow range of voltage, and lower than desired voltage may not be sufficient for their operation.
Superconductors carry electric current without any resistance, therefore without resistive energy loss. Superconductors exhibit several characteristics that are unknown in common conductors. For instance, they may repel external magnetic fields; magnets placed over superconducting materials will remain suspended in the air. While there is a great potential in using superconductors as carriers of electrical energy, and for frictionless means of transportation, currently their use is limited. One of the reasons is their relatively low operating temperature; mostly close to the absolute zero, some higher, up to 138K (–135°C).
See also Electronics.