Electrical conductor

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Inphysics andelectrical engineering, aconductor is an object or type ofmaterial that allows the flow ofcharge (electric current) in one or more directions. Materials made ofmetal are common electrical conductors. The flow of negatively chargedelectrons generates electric current, positively chargedholes, and positive or negativeions in some cases.

Overhead conductors carry electric power from generating stations to customers.

In order for current to flow within a closedelectrical circuit, one charged particle does not need to travel from the component producing the current (thecurrent source) to those consuming it (theloads). Instead, the charged particle simply needs to nudge its neighbor a finite amount, who will nudgeits neighbor, and on and on until a particle is nudged into the consumer, thus powering it. Essentially what is occurring is a long chain ofmomentum transfer between mobilecharge carriers; theDrude model of conduction describes this process more rigorously. This momentum transfer model makes metal an ideal choice for a conductor; metals, characteristically, possess a delocalizedsea of electrons which gives the electrons enough mobility to collide and thus affect a momentum transfer.

As discussed above, electrons are the primary mover in metals; however, other devices such as the cationicelectrolyte(s) of abattery, or the mobile protons of theproton conductor of a fuel cell rely on positive charge carriers.Insulators are non-conducting materials with few mobile charges that support only insignificant electric currents.

Resistance and conductance

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A piece of resistive material with electrical contacts on both ends

Main article:Electrical resistance and conductance

Theresistance 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.^[1] For example, a thick copperwire 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 resistanceR and conductanceG of a conductor of uniform cross section, therefore, can be computed as^[1]

{\begin{aligned}R&=\rho {\frac {\ell }{A}},\\[6pt]G&=\sigma {\frac {A}{\ell }}.\end{aligned}}

where $\ell$ is the length of the conductor, measured inmetres [m],A is the cross-section area of the conductor measured insquare metres [m²], σ (sigma) is theelectrical conductivity measured insiemens per meter (S·m⁻¹), and ρ (rho) is theelectrical resistivity (also calledspecific electrical resistance) of the material, measured in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity arereciprocals: $\rho =1/\sigma$ . Resistivity is a measure of the material's ability to oppose electric current.

This formula is not exact: It assumes thecurrent density is totally uniform in the conductor, which is not always true in practical situation. However, this formula still provides a good approximation for long thin conductors such as wires.

Another situation this formula is not exact for is withalternating current (AC), because theskin effect inhibits current flow near the center of the conductor. Then, thegeometrical cross-section is different from theeffective cross-section in which current actually flows, so the resistance is higher than expected. Similarly, if two conductors are near each other carrying AC current, their resistances increase due to theproximity effect. Atcommercial power frequency, these effects are significant for large conductors carrying large currents, such asbusbars in anelectrical substation,^[2] 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 thethermal 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. Aphonon 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

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Main article:Electrical resistivity and conductivity

Further information:Copper conductor andAluminum building wiring

Material	ρ [Ω·m] at 20 °C	σ [⁠S/m⁠] at 20 °C
Silver, Ag	1.59 × 10⁻⁸	6.30 × 10⁷
Copper, Cu	1.68 × 10⁻⁸	5.96 × 10⁷
Aluminum, Al	2.82 × 10⁻⁸	3.50 × 10⁷

Conduction materials includemetals,electrolytes,superconductors,semiconductors,plasmas and some nonmetallic conductors such asgraphite andconductive polymers.

Copper has a highconductivity.Annealed copper is the international standard to which all other electrical conductors are compared; theInternational Annealed Copper Standard conductivity is58 MS/m, 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 andbusbars, iselectrolytic-tough pitch (ETP) copper (CW004A orASTM designation C100140). If high conductivity copper must bewelded orbrazed or used in a reducing atmosphere, thenoxygen-free high conductivity copper (CW008A or ASTM designation C10100) may be used.^[3] Because of its ease of connection bysoldering 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 assatellites, and as a thin plating to mitigateskin effect losses at high frequencies. Famously, 14,700 short tons (13,300 t) of silver on loan from theUnited States Treasury were used in the making of thecalutron magnets during World War II due to wartime shortages of copper.^[4]

Aluminum wire is the most common metal inelectric power transmission anddistribution. 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 largercoefficient 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 madealuminum building wiring unpopular past theservice drop.

Organic compounds such as octane, which has 8 carbon atoms and 18 hydrogen atoms, cannot conduct electricity. Oils are hydrocarbons, since carbon has the property of tetracovalency and forms covalent bonds with other elements such as hydrogen, since it does not lose or gain electrons, thus does not form ions. Covalent bonds are simply the sharing of electrons. Hence, there is no separation of ions when electricity is passed through it. Liquids made of compounds with only covalent bonds cannot conduct electricity. Certain organicionic liquids, by contrast, can conduct an electric current.

While purewater is not an electrical conductor, even a small portion of ionic impurities, such assalt, can rapidly transform it into a conductor.

Wire size

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Wires are measured by their cross sectional area. In many countries, the size is expressed in square millimetres. In North America, conductors are measured byAmerican wire gauge for smaller ones, andcircular mils for larger ones.

Conductor ampacity

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Theampacity of a conductor, that is, the amount ofcurrent 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 fromfuses, most conductors in the real world are operated far below this limit, however. For example, household wiring is usually insulated withPVC 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 offire. Other, more expensive insulation such asTeflon orfiberglass may allow operation at much higher temperatures.

Isotropy

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If anelectric field is applied to a material, and the resulting inducedelectric current is in the same direction, the material is said to be anisotropic electrical conductor. If the resulting electric current is in a different direction from the applied electric field, the material is said to be ananisotropic electrical conductor.

Classification of materials based on permittivity
⁠ε_r″/ε_r′⁠	Current conduction	Field propagation
0		perfect dielectric lossless medium
$\ll 1$	low-conductivity material poor conductor	low-loss medium good dielectric
$\approx 1$	lossy conducting material	lossy propagation medium
$\gg 1$	high-conductivity material good conductor	high-loss medium poor dielectric
$\infty$	perfect conductor

References

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^^a ^b"Wire Sizes and Resistance"(PDF). Retrieved2018-01-14.
^Fink and Beaty,Standard Handbook for Electrical Engineers 11th Edition, pages 17–19
^"High conductivity coppers (electrical)". Copper Development Association (U.K.). Archived fromthe original on 2013-07-20. Retrieved2013-06-01.
^"From Treasury Vault to the Manhattan Project"(PDF). American Scientist. Retrieved2022-10-27.