View source for Physics:Electrical conductor

{{short description|Object or material which allows the flow of electric charge with little energy loss}}
[[File:Electrical wires near Putim.jpg|thumb|right|Overhead conductors carry electric power from generating stations to customers.]] 
{{Electromagnetism|cTopic=Electrostatics}}

In [[Physics:Physics|physics]] and [[Electrical engineering|electrical engineering]], a '''conductor''' is an object or type of [[Physics:Material|material]] that allows the flow of [[Physics:Electric charge|charge]] ([[Physics:Electric current|electric current]]) in one or more directions. Materials made of [[Chemistry:Metal|metal]] are common electrical conductors. The flow of negatively charged [[Physics:Electron|electron]]s generates electric current, positively charged [[Physics:Electron hole|holes]], and positive or negative [[Physics:Ion|ion]]s in some cases. 

In order for current to flow within a closed [[Physics:Electrical network|electrical circuit]], one charged particle does not need to travel from the component producing the current (the [[Physics:Current source|current source]]) to those consuming it (the [[Engineering:Electrical load|load]]s). Instead, the charged particle simply needs to nudge its neighbor a finite amount, who will nudge ''its'' neighbor, and on and on until a particle is nudged into the consumer, thus powering it. Essentially what is occurring is a long chain of [[Physics:Momentum|momentum]] transfer between mobile charge carriers; the [[Physics:Drude model|Drude model]] of conduction describes this process more rigorously. This momentum transfer model makes metal an ideal choice for a conductor; metals, characteristically, possess a delocalized sea 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 cationic [[Physics:Electrolyte|electrolyte]](s) of a [[Physics:Battery (electricity)|battery]], or the mobile protons of the [[Chemistry:Proton conductor|proton conductor]] of a fuel cell rely on positive charge carriers. [[Physics:Insulator (electricity)|Insulator]]s are non-conducting materials with few mobile charges that support only insignificant electric currents.

==Resistance and conductance==
[[File:Resistivity geometry.png|thumb|A piece of resistive material with electrical contacts on both ends.]]
{{main|Physics:Electrical resistance and conductance}}
The [[Physics:Electrical resistance and conductance|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.<ref name=":0">{{Cite web|url=https://web.stanford.edu/class/ee281/materials/references/WireResistances.pdf|title=Wire Sizes and Resistance|access-date=2018-01-14}}</ref> For example, a thick copper [[Engineering:Wire|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 {{math|''R''}} and conductance {{math|''G''}} of a conductor of uniform cross section, therefore, can be computed as<ref name=":0" />

: <math>
\begin{align}
R & = \rho \frac \ell A, \\[6pt]
G & = \sigma \frac A \ell.
\end{align}
</math>

where <math>\ell</math> is the length of the conductor, measured in [[Metre|metre]]s [m], ''A'' is the cross-section area of the conductor measured in [[Square metre|square metre]]s [m<sup>2</sup>], σ (sigma) is the [[Physics:Electrical conductivity|electrical conductivity]] measured in [[Siemens (unit)|siemens]] per meter (S·m<sup>−1</sup>), and ρ (rho) is the [[Physics:Electrical resistivity|electrical resistivity]] (also called ''specific 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 are [[Multiplicative inverse|reciprocals]]: <math>\rho=1/\sigma</math>. Resistivity is a measure of the material's ability to oppose electric current.

This formula is not exact: It assumes the [[Physics:Current density|current 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 with [[Physics:Alternating current|alternating current]] (AC), because the [[Physics:Skin effect|skin effect]] inhibits current flow near the center of the conductor. Then, the ''geometrical'' cross-section is different from the ''effective'' 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 the [[Physics:Proximity effect (electromagnetism)|proximity effect]]. At [[Physics:Utility frequency|commercial power frequency]], these  effects are significant for large conductors carrying large currents, such as [[Engineering:Busbar|busbar]]s in an [[Physics:Electrical substation|electrical substation]],<ref>Fink and Beaty, ''Standard Handbook for Electrical Engineers 11th Edition'', pages 17–19</ref> 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<sup>−6</sup>. An increase in temperature will also increase the number of phonons generated within the material. A [[Physics:Phonon|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==
{{main|Physics:Electrical resistivity and conductivity}}
{{further|Chemistry:Copper conductor|Chemistry:Aluminum building wiring}}

{| class="wikitable floatright"
|-
! Material !! ρ [Ω·m] at 20°C !! σ [{{sfrac|S|m}}] at 20°C
|-
| Silver, Ag || 1.59 × 10<sup>−8</sup> || 6.30 × 10<sup>7</sup>
|-
| Copper, Cu || 1.68 × 10<sup>−8</sup> || 5.96 × 10<sup>7</sup>
|-
| Aluminum, Al || 2.82 × 10<sup>−8</sup>|| 3.50 × 10<sup>7</sup>
|}

Conduction materials include [[Chemistry:Metal|metal]]s, [[Physics:Electrolyte|electrolyte]]s, [[Physics:Superconductor|superconductor]]s, [[Physics:Semiconductor|semiconductor]]s, [[Physics:Plasma|plasma]]s and some nonmetallic conductors such as [[Chemistry:Graphite|graphite]] and [[Chemistry:Conductive polymer|conductive polymer]]s.

[[Chemistry:Copper|Copper]] has a high [[Physics:Electrical conductivity|conductivity]]. [[Engineering:Annealing (metallurgy)|Annealed]] copper is the international standard to which all other electrical conductors are compared; the [[International Annealed Copper Standard]] conductivity is {{val|58|u=MS|up=m}}, although ultra-pure copper can slightly exceed 101% IACS.  The main grade of copper used for electrical applications, such as building wire, [[Physics:Electric motor|motor]] windings, cables and [[Engineering:Busbar|busbar]]s, is [[Chemistry:Oxygen-free copper#Specification|electrolytic-tough pitch (ETP) copper]] (CW004A or ASTM designation C100140).  If high conductivity copper must be [[Chemistry:Welding|welded]] or [[Engineering:Brazing|brazed]] or used in a reducing atmosphere, then [[Chemistry:Oxygen-free copper|oxygen-free high conductivity copper]] (CW008A or ASTM designation C10100) may be used.<ref>{{cite web |url=http://www.copperinfo.co.uk/alloys/copper/ |title=High conductivity coppers (electrical) |publisher=Copper Development Association (U.K.) |access-date=2013-06-01 |url-status=dead |archive-url=https://web.archive.org/web/20130720000331/http://www.copperinfo.co.uk/alloys/copper/ |archive-date=2013-07-20 }}</ref> Because of its ease of connection by [[Physics:Soldering|soldering]] or clamping, copper is still the most common choice for most light-gauge wires.

[[Chemistry:Silver|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 [[Astronomy:Satellite|satellite]]s, and as a thin plating to mitigate [[Physics:Skin effect|skin effect]] losses at high frequencies.  Famously, {{convert|14700|ST|t}} of silver on loan from the United States Treasury were used in the making of the [[Physics:Calutron|calutron]] magnets during World War II due to wartime shortages of copper. <ref>{{cite web |url=https://www.pugetsound.edu/sites/default/files/file/7579_AmericanScientist-Article%20Treasury%20Silver_0.pdf |title=From Treasury Vault to the Manhattan Project |publisher=American Scientist |access-date=2022-10-27}}</ref>

[[Chemistry:Aluminum|Aluminum]] wire is the most common metal in [[Finance:Electric power transmission|electric power transmission]] and [[Engineering:Electric power distribution|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 [[Chemistry:Aluminum building wiring|aluminum building wiring]] unpopular past the [[Engineering:Service drop|service 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 organic [[Physics:Ionic liquid|ionic liquid]]s, by contrast, can conduct an electric current. 

While pure [[Chemistry:Water|water]] is not an electrical conductor, even a small portion of ionic impurities, such as [[Chemistry:Salt|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 [[Engineering:Ampacity|ampacity]] of a conductor, that is, the amount of [[Physics:Electric current|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 [[Engineering:Fuse (electrical)|fuses]], most conductors in the real world are operated far below this limit, however. For example, household wiring is usually insulated with [[Chemistry:Polyvinyl chloride|PVC]] insulation that is only rated to operate to about 60&nbsp;°C, therefore, the current in such wires must be limited so that it never heats the copper conductor above 60&nbsp;°C, causing a risk of [[Astronomy:Fire|fire]]. Other, more expensive insulation such as [[Engineering:Teflon|Teflon]] or [[Chemistry:Fiberglass|fiberglass]] may allow operation at much higher temperatures.

==Isotropy==
If an [[Physics:Electric field|electric field]] is applied to a material, and the resulting induced [[Physics:Electric current|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==
{{div col|colwidth=15em}}
*Bundle conductor
*Charge transfer complex
*[[Engineering:Electrical cable|Electrical cable]]
*[[Physics:Electrical resistivity and conductivity|Electrical resistivity and conductivity]]
* Fourth rail
* [[Engineering:Overhead line|Overhead line]]
* Stephen Gray, first to identify electrical conductors and insulators
*[[Physics:Superconductivity|Superconductivity]]
* [[Engineering:Third rail|Third rail]]
{{div col end}}

==References==
{{reflist}}

==Further reading==
===Pioneering and historical books===
* William Henry Preece. ''On Electrical Conductors''.  1883.
* Oliver Heaviside. ''Electrical Papers''. Macmillan, 1894.

===Reference books===
* ''Annual Book of ASTM Standards: Electrical Conductors.'' American Society for Testing and Materials. (every year)
* ''IET Wiring Regulations.'' Institution for Engineering and Technology. [http://www.wiringregulations.net/index.asp?search=bic&bic=WR0010&t=IET+WIRING+REGULATIONS wiringregulations.net]

==External links==
* [http://www.bbc.co.uk/schools/ks2bitesize/science/physical_processes/circuits_conductors/read1.shtml BBC: Key Stage 2 Bitesize: Electrical Conductors]
* [http://histoires-de-sciences.over-blog.fr/2018/04/history-of-electricity.the-discovery-of-conductors-and-insulators-by-gray-dufay-and-franklin.html The discovery of conductors and insulators by Gray, Dufay and Franklin.]



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{{Sourceattribution|Electrical conductor}}