Electrical conductor.docx

1. Electrical conductor
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In physicsandelectrical engineering,aconductorisan objector type of material thatallowsthe flowof
charge (electriccurrent) inone ormore directions.Materialsmade of metal are commonelectrical
conductors.Electriccurrentis generatedbythe flow of negativelychargedelectrons,positivelycharged
holes,andpositive ornegativeionsinsome cases.
In orderfor currentto flowwithinaclosed electrical circuit,itisnotnecessaryforone chargedparticle
to travel fromthe componentproducingthe current(the currentsource) tothose consumingit(the
loads).Instead,the chargedparticle simplyneedstonudge itsneighborafinite amount,whowill nudge
itsneighbor,andonand on until a particle isnudgedintothe consumer,thuspoweringit.Essentially
whatis occurringis a longchainof momentumtransferbetweenmobile charge carriers;the Drude
model of conductiondescribesthisprocessmore rigorously.Thismomentumtransfermodel makes
metal an ideal choice fora conductor;metals,characteristically,possessadelocalizedseaof electrons
whichgivesthe electronsenoughmobilitytocollide andthusaffectamomentumtransfer.

2. As discussedabove,electronsare the primarymoverinmetals;however,otherdevicessuchasthe
cationicelectrolyte(s) of abattery,or the mobile protonsof the protonconductorof a fuel cell relyon
positive charge carriers.Insulatorsare non-conductingmaterialswithfew mobilechargesthatsupport
onlyinsignificantelectriccurrents.
Contents
1 Resistance andconductance
2 Conductormaterials
3 Wire size
4 Conductorampacity
5 Isotropy
6 See also
7 References
8 Furtherreading
8.1 Pioneeringand historical books
8.2 Reference books
9 External links
Resistance andconductance
A piece of resistivematerialwithelectrical contactsonbothends.
Main article:Electrical resistance andconductance
The resistance of a givenconductordependsonthe material itismade of,andon its dimensions.Fora
givenmaterial,the resistance isinverselyproportional tothe cross-sectionalarea.[1] Forexample,a
thickcopperwire has lowerresistance thananotherwise-identical thincopperwire.Also,foragiven
material,the resistance isproportional tothe length;forexample,alongcopperwire hashigher
resistance thananotherwise-identical shortcopperwire.The resistance Randconductance G of a
conductorof uniformcrosssection,therefore,canbe computedas[1]

3. {displaystyle{begin{aligned}R&=rho{frac{ell }{A}},[6pt]G&=sigma{frac{A}{ell
}}.end{aligned}}}{displaystyle{begin{aligned}R&=rho{frac{ell }{A}},[6pt]G&=sigma{frac{A}{ell
}}.end{aligned}}}
where {displaystyleell}ell isthe lengthof the conductor,measuredinmetres[m],A isthe cross-
sectionareaof the conductormeasuredinsquare metres[m2],σ(sigma) isthe electrical conductivity
measuredinsiemenspermeter(S·m−1),andρ (rho) isthe electrical resistivity(alsocalledspecific
electrical resistance) of the material,measuredinohm-metres(Ω·m).The resistivityandconductivity
are proportionalityconstants,andtherefore dependonlyonthe material the wire ismade of,notthe
geometryof the wire. Resistivityandconductivityare reciprocals:{displaystyle rho=1/sigma
}rho=1/sigma.Resistivityisameasure of the material'sabilitytooppose electriccurrent.
Thisformulaisnot exact:It assumesthe currentdensityistotallyuniforminthe conductor,whichisnot
alwaystrue inpractical situation.However,thisformulastillprovidesagoodapproximationforlongthin
conductorssuch as wires.
Anothersituationthisformulaisnotexactfor iswithalternatingcurrent(AC),becausethe skineffect
inhibitscurrentflownearthe centerof the conductor.Then,the geometrical cross-sectionisdifferent
fromthe effective cross-sectioninwhichcurrentactuallyflows,sothe resistance ishigherthan
expected.Similarly,if twoconductorsare neareachothercarrying ACcurrent,theirresistancesincrease
due to the proximityeffect.Atcommercial powerfrequency,theseeffectsare significantforlarge
conductorscarrying large currents,suchas busbarsin an electrical substation,[2] orlarge powercables
carryingmore thana fewhundredamperes.
Aside fromthe geometryof the wire,temperature alsohasasignificanteffectonthe efficacyof
conductors.Temperature affectsconductorsintwomainways,the firstisthatmaterialsmayexpand
underthe applicationof heat.The amountthat the material will expandisgovernedbythe thermal
expansioncoefficientspecifictothe material.Suchanexpansion(orcontraction) will change the
geometryof the conductorand therefore itscharacteristic resistance.However,thiseffectisgenerally
small,onthe order of 10−6. Anincrease intemperature will alsoincreasethe numberof phonons
generatedwithinthe material.A phononisessentiallyalattice vibration,orrathera small,harmonic
kineticmovementof the atomsof the material.Much like the shakingof apinball machine,phonons
serve todisruptthe path of electrons,causingthemtoscatter.Thiselectronscatteringwill decrease the
numberof electroncollisionsandtherefore will decrease the total amountof currenttransferred.
Conductormaterials
Main article:Electrical resistivityandconductivity
Furtherinformation:CopperconductorandAluminumbuildingwiring
Material ρ [Ω·m] at 20°C σ [

4. 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
Conductionmaterialsinclude metals,electrolytes,superconductors,semiconductors,plasmasandsome
nonmetallicconductorssuchas graphite andconductive polymers.
Copperhas a highconductivity.Annealedcopperisthe internationalstandardtowhichall other
electrical conductorsare compared;the InternationalAnnealedCopperStandardconductivityis58
MS/m, althoughultra-pure coppercanslightlyexceed101% IACS.The main grade of copper usedfor
electrical applications,suchasbuildingwire,motorwindings,cablesandbusbars,iselectrolytic-tough
pitch(ETP) copper(CW004A or ASTMdesignationC100140). If highconductivitycoppermustbe welded
or brazedor usedina reducingatmosphere,thenoxygen-free highconductivitycopper(CW008A or
ASTMdesignationC10100) may be used.[3] Because of itsease of connectionbysolderingorclamping,
copperis still the mostcommonchoice formostlight-gauge wires.
Silveris6% more conductive thancopper,butdue to cost itis notpractical in mostcases.However,itis
usedinspecializedequipment,suchassatellites,andasa thinplatingtomitigate skineffectlossesat
highfrequencies.Famously,14,700 shorttons (13,300 t) of silveronloanfromthe UnitedStates
Treasurywere usedinthe makingof the calutronmagnetsduringWorldWar II due to wartime
shortagesof copper.
Aluminumwire isthe mostcommonmetal inelectricpowertransmissionanddistribution.Although
only61% of the conductivityof copperbycross-sectional area,itslowerdensitymakesittwice as
conductive bymass.Asaluminumisroughlyone-thirdthe costof copperby weight,the economic
advantagesare considerable whenlarge conductorsare required.
The disadvantagesof aluminumwiringlieinitsmechanical andchemical properties.Itreadilyformsan
insulatingoxide,makingconnectionsheatup.Itslargercoefficientof thermal expansionthanthe brass
materialsusedforconnectorscausesconnectionstoloosen.Aluminumcanalso"creep",slowly
deformingunderload,whichalsoloosensconnections.These effectscanbe mitigatedwithsuitably
designedconnectorsandextracare ininstallation,buttheyhave made aluminumbuildingwiring
unpopularpastthe service drop.

5. Organiccompoundssuchas octane,whichhas 8 carbonatoms and 18 hydrogenatoms,cannotconduct
electricity.Oilsare hydrocarbons,since carbonhasthe propertyof tetracovalencyandformscovalent
bondswithotherelementssuchas hydrogen,since itdoesnotlose orgain electrons,thusdoesnotform
ions.Covalentbondsare simplythe sharingof electrons.Hence,thereisnoseparationof ionswhen
electricityispassedthroughit.Liquidsmade of compoundswithonlycovalentbonds cannotconduct
electricity.Certainorganicionicliquids,bycontrast,can conductan electriccurrent.
While pure waterisnot an electrical conductor,evenasmall portionof ionicimpurities,suchassalt,can
rapidlytransformitintoa conductor.
Wire size
Wiresare measuredbytheircrosssectional area.Inmanycountries,the size isexpressedinsquare
millimetres.InNorthAmerica,conductorsare measuredbyAmericanwire gauge forsmallerones,and
circularmilsfor largerones.
Conductorampacity
The ampacityof a conductor,that is,the amountof current itcan carry, isrelatedtoitselectrical
resistance:alower-resistance conductorcancarry a largervalue of current.The resistance,inturn,is
determinedbythe material the conductor ismade from(as describedabove) andthe conductor'ssize.
For a givenmaterial,conductorswithalargercross-sectionalareahave lessresistancethanconductors
witha smallercross-sectional area.
For bare conductors,the ultimate limitisthe point atwhichpowerlosttoresistance causesthe
conductorto melt.Aside fromfuses,mostconductorsinthe real worldare operatedfarbelow thislimit,
however.Forexample,householdwiringisusuallyinsulatedwithPVCinsulationthatisonlyratedto
operate toabout 60 °C, therefore,the currentinsuchwiresmustbe limitedsothatitneverheatsthe
copperconductorabove 60 °C, causinga risk of fire.Other,more expensiveinsulationsuchasTeflonor
fiberglassmayallowoperationatmuchhighertemperatures.
Isotropy
If an electricfieldisappliedtoamaterial,andthe resultinginducedelectriccurrentisinthe same
direction,the material issaidtobe an isotropicelectricalconductor.If the resultingelectriccurrentisin
a differentdirectionfromthe appliedelectricfield,the material issaidtobe an anisotropicelectrical
conductor.

6. See also
Classificationof materialsbasedonpermittivity
εr″
/
εr′
Currentconduction Fieldpropagation
0 perfectdielectric
losslessmedium
≪ 1 low-conductivitymaterial
poor conductorlow-lossmedium
gooddielectric
≈ 1 lossyconductingmaterial lossypropagationmedium
≫ 1 high-conductivitymaterial
goodconductorhigh-lossmedium
poor dielectric
∞ perfectconductor
Bundle conductor
Charge transfercomplex
Electrical resistivityandconductivity
Fourthrail
Overheadline
StephenGray,firstto identifyelectrical conductorsandinsulators
Superconductivity
Thirdrail
References
"Wire SizesandResistance"(PDF).Retrieved2018-01-14.
Finkand Beaty,StandardHandbookforElectrical Engineers11thEdition,pages17–19
"High conductivitycoppers(electrical)".CopperDevelopmentAssociation(U.K.).Archivedfromthe
original on2013-07-20. Retrieved2013-06-01.

7. Furtherreading
Pioneeringand historical books
WilliamHenryPreece.OnElectrical Conductors.1883.
OliverHeaviside.Electrical Papers.Macmillan,1894.
Reference books
Annual Bookof ASTM Standards:Electrical Conductors.AmericanSocietyforTestingandMaterials.
(everyyear)
IET WiringRegulations.InstitutionforEngineeringandTechnology.wiringregulations.net
External links
BBC: KeyStage 2 Bitesize:Electrical Conductors
The discoveryof conductorsand insulatorsbyGray,Dufayand Franklin.
WikimediaCommonshasmediarelatedtoElectrical conductors.
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