Electrical conductor

The resistance of a given conductor depends on the material it is made of, and on its dimensions. For a given material, the resistance is inversely proportional to the cross-sectional area. For example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistance and conductance of a conductor of uniform cross section, therefore, can be computed as or large power cables carrying more than a few hundred amperes. Aside from the geometry of the wire, temperature also has a significant effect on the efficacy of conductors. Temperature affects conductors in two main ways, the first is that materials may expand under the application of heat. The amount that the material will expand is governed by the thermal expansion coefficient specific to the material. Such an expansion (or contraction) will change the geometry of the conductor and therefore its characteristic resistance. However, this effect is generally small, on the order of 10−6. An increase in temperature will also increase the number of phonons generated within the material. A phonon is essentially a lattice vibration, or rather a small, harmonic kinetic movement of the atoms of the material. Much like the shaking of a pinball machine, phonons serve to disrupt the path of electrons, causing them to scatter. This electron scattering will decrease the number of electron collisions and therefore will decrease the total amount of current transferred. ==Conductor materials==

Conduction materials include metals, electrolytes, superconductors, semiconductors, plasmas and some nonmetallic conductors such as graphite and conductive polymers. Copper has a high conductivity. Annealed copper is the international standard to which all other electrical conductors are compared; the International Annealed Copper Standard conductivity is , although ultra-pure copper can slightly exceed 101% IACS. The main grade of copper used for electrical applications, such as building wire, motor windings, cables and busbars, is electrolytic-tough pitch (ETP) copper (CW004A or ASTM designation C100140). If high conductivity copper must be welded or brazed or used in a reducing atmosphere, then oxygen-free high conductivity copper (CW008A or ASTM designation C10100) may be used. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires. Silver is 6% more conductive than copper, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies. Famously, of silver on loan from the United States Treasury were used in the making of the calutron magnets during World War II due to wartime shortages of copper. Aluminum wire is the most common metal in electric power transmission and distribution. Although only 61% of the conductivity of copper by cross-sectional area, its lower density makes it twice as conductive by mass. As aluminum is roughly one-third the cost of copper by weight, the economic advantages are considerable when large conductors are required. The disadvantages of aluminum wiring lie in its mechanical and chemical properties. It readily forms an insulating oxide, making connections heat up. Its larger coefficient of thermal expansion than the brass materials used for connectors causes connections to loosen. Aluminum can also "creep", slowly deforming under load, which also loosens connections. These effects can be mitigated with suitably designed connectors and extra care in installation, but they have made aluminum building wiring unpopular past the service drop. Very few organic compounds conduct electricity. For example, hydrocarbon oils, such as octane, are a common transformer insulator. Typical organic compounds do not conduct because they lack charge carriers. They are uncharged molecules, unable to perform ionic conduction; and their electrons are imperturbable at low energy, localized to relatively inert covalent bonds. Rare organic conductors arise from a failure of either condition: certain ionic liquids conduct through ion transport and conductive polymers delocalize a substantial portion of their electrons in a π system. While pure water is not an electrical conductor, even a small portion of ionic impurities, such as salt, can rapidly transform it into a conductor. ==Wire size==

Wires are measured by their cross sectional area. In many countries, the size is expressed in square millimetres. In North America, conductors are measured by American wire gauge for smaller ones, and circular mils for larger ones. ==Conductor ampacity==

The ampacity of a conductor, that is, the amount of current it can carry, is related to its electrical resistance: a lower-resistance conductor can carry a larger value of current. The resistance, in turn, is determined by the material the conductor is made from (as described above) and the conductor's size. For a given material, conductors with a larger cross-sectional area have less resistance than conductors with a smaller cross-sectional area. For bare conductors, the ultimate limit is the point at which power lost to resistance causes the conductor to melt. Aside from fuses, most conductors in the real world are operated far below this limit, however. For example, household wiring is usually insulated with PVC insulation that is only rated to operate to about 60 °C, therefore, the current in such wires must be limited so that it never heats the copper conductor above 60 °C, causing a risk of fire. Other, more expensive insulation such as Teflon or fiberglass may allow operation at much higher temperatures. ==Isotropy==

If an electric field is applied to a material, and the resulting induced electric current is in the same direction, the material is said to be an isotropic electrical conductor. If the resulting electric current is in a different direction from the applied electric field, the material is said to be an anisotropic electrical conductor. ==See also==