conduction, electrical

Flow of charged particles through a material giving rise to electric current. Conduction in metals involves the flow of negatively charged free electrons. Conduction in gases and some liquids involves the flow of ions that carry positive charges in one direction and negative charges in the other. Conduction in a semiconductor such as silicon involves the flow of electrons and positive holes.

Conventionally, current is regarded as a movement of positive electricity from points at high potential to points at a lower potential. Metals contain many free electrons that move with little hindrance from the parent atoms, and they are therefore good conductors. In other solids the electrons are more tightly bound to the atoms and conduction is less. Increase of temperature in this case frees more electrons, so the conductivity of non-metals increases with rising temperature. Conduction in gases and in many liquids involves a flow not merely of electrons, but of ions or groups of ions as well. When a salt such as sodium chloride is dissolved in water, the chlorine atoms each gain an electron and become negatively charged, while the sodium atoms each lose one and become positively charged. These charged atoms, or ions, can move through the liquid and transport electricity (see electrolysis). Gases are, under normal circumstances, almost completely nonconducting. They may be ionized by irradiation with X-rays or by radioactive radiations. They are more readily maintained in a conducting state at high temperatures, as in the electric arc, or at low pressures, as in electric discharge lamps. Valves may be used to control the flow of electricity in a circuit. At very low temperatures certain metals such as lead become almost perfect conductors, and if a current is set up in a ring of a metal in this superconducting state, the current persists for a long time without any energy being supplied. In all other cases the flow of a current through a conductor is accompanied by a loss of energy as heat, and a continuous supply of energy is required to maintain the current. A magnetic field is always present in the space around a conductor in which a current is flowing.