25/1/2014 Electrical conductor - Wikipedia, the free encyclopedia

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Overhead conductors carry electric

power from generating stations to

customers.

Electrical conductorFrom Wikipedia, the free encyclopedia

In physics and electrical engineering, a conductor is an object or typeof material which permits the flow of electric charges in one or moredirections. For example, a wire is an electrical conductor that cancarry electricity along its length.

In metals such as copper or aluminum, the movable charged particlesare electrons. Positive charges may also be mobile, such as thecationic electrolyte(s) of a battery, or the mobile protons of the protonconductor of a fuel cell. Insulators are non-conducting materials withfew mobile charges and which support only insignificant electriccurrents.

Contents

1 Physics

2 Wire size

3 Conductance

4 Conductor materials5 Conductor ampacity

6 Isotropy

7 Bibliography

7.1 Pioneering and historical books

7.2 Reference books8 References

9 External links

10 See also

Physics

All conductors contain electrical charges, which will move when an electric potential difference (measured involts) is applied across separate points on the material. This flow of charge (measured in amperes) is what ismeant by electric current. In most materials, the direct current is proportional to the voltage (as determined byOhm's law), provided the temperature remains constant and the material remains in the same shape and state.

Copper is the most common material used for electrical wiring (see main article: Copper wire and cable). Silver

is the best conductor, but it is expensive. It has a resistivity of 1.6 × 10−8 Ω⋅m. Because gold does not corrode,it is used for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors,including graphite, solutions of salts, and all plasmas. There are even conductive polymers. (See electricalconduction for more information on the physical mechanism for charge flow in materials).

All non-superconducting materials offer some resistance and warm up during electric currents. Proper design ofan electrical conductor takes into account the temperature of the conductor as well as the value of electriccurrent. The motion of charges creates an electromagnetic field around the conductor that exerts a mechanical

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A piece of resistive material with

electrical contacts on both ends.

radial squeezing force on the conductor. The current carrying capacity of a conductor is limited by its ability todissipate heat. This effect is especially critical in printed circuits, where conductors are relatively small and closetogether, and inside an enclosure: the heat produced can melt the tracks.

Thermal and electrical conductivity often go together. For instance the sea of electrons causes most metals to actboth as electrical and thermal conductors. However, some non-metallic materials are practical electrical

conductors without being good thermal conductors.[citation needed]One of the best conductors(it is also toxic) islead. The best, is also expensive, is Silver.

Wire size

Wires are measured by their cross section. In many countries, the size is expressed in square millimeters. InNorth America conductors are measured by American wire gauge for smaller ones, and circular mils for largerones.

Conductance

Main article: Electrical resistance and conductance

The resistance of a given conductor depends on the material it ismade of, and on its dimensions. For a given material, the resistance isinversely proportional to the cross-sectional area; for example, a thickcopper wire has lower resistance than an otherwise-identical thincopper wire. Also, for a given material, the resistance is proportionalto the length; for example, a long copper wire has higher resistancethan an otherwise-identical short copper wire. The resistance R andconductance G of a conductor of uniform cross section, therefore,can be computed as

where is the length of the conductor, measured in metres [m], A is the cross-section area of the conductor

measured in square metres [m²], σ (sigma) is the electrical conductivity measured in siemens per meter (S·m−1),and ρ (rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured inohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only onthe material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals:

. Resistivity is a measure of the material's ability to oppose electric current.

This formula is not exact: It assumes the current density is totally uniform in the conductor, which is not alwaystrue in practical situations. However, this formula still provides a good approximation for long thin conductorssuch as wires.

Another situation for which this formula is not exact is with alternating current (AC), because the skin effectinhibits current flow near the center of the conductor. Then, the geometrical cross-section is different from theeffective cross-section in which current is actually flowing, so the resistance is higher than expected. Similarly, if

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two conductors are near each other carrying AC current, their resistances will increase due to the proximityeffect. At commercial power frequency, these effects are significant for large conductors carrying large currents,

such as busbars in an electrical substation,[1] or large power cables carrying more than a few hundred amperes.

Conductor materials

Main article: Electrical_resistivity_and_conductivity#Resistivity_of_various_materials

Further information: Copper wire and cable and Aluminum wire

Conduction materials include metals, electrolytes, superconductors, semiconductors, plasmas and somenonmetallic conductors such as graphite and Conductive polymers.

Copper has a high conductivity. Annealed copper is the international standard to which all other electricalconductors are compared. 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 designationC100140). This copper has an electrical conductivity of at least 101% IACS (International Annealed CopperStandard). If high conductivity copper needs to be welded or brazed or used in a reducing atmosphere, then

oxygen-free high conductivity copper (CW008A or ASTM designation C10100) may be used.[2] Because ofits ease of connection by soldering or clamping, copper is still the most common choice for most light-gaugewires.

Silver is more conductive than copper, but due to cost it is not practical in most cases. However, it is used inspecialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies.

Aluminum wire, which has 61% of the conductivity of copper, has been used in building wiring for its lower cost.By weight, aluminum has higher conductivity than copper, but it has properties that cause problems when usedfor building wiring. It forms a resistive oxide within connections, causing terminals of wiring devices to heat.Aluminum can "creep", slowly deforming under load, eventually causing device connections to loosen, and alsohas a different coefficient of thermal expansion compared to the materials used for connections. This acceleratesthe loosening of connections. These effects can be avoided by using wiring devices approved for use withaluminum.

Aluminum wires used for low voltage distribution, such as buried cables and service drops, require use ofcompatible connectors and installation methods to prevent heating at joints. Aluminum is also the most commonmetal used in high-voltage transmission lines, in combination with steel as structural reinforcement.

Anodized aluminum surfaces are not conductive. This affects the design of electrical enclosures that require theenclosure to be electrically connected. Organic compounds such as octane, which has 8 carbon atoms and 18hydrogen atoms, cannot conduct electricity. Oils are hydrocarbons, since carbon has the property oftetracovalency and forms covalent bonds with other elements such as hydrogen, since it does not lose or gainelectrons, thus does not form ions. Covalent bonds are simply, the sharing of electrons. Hence, there is noseparation of ions when electricity is passed through it. So the liquid (oil or any organic compound) cannotconduct electricity.

Conductor ampacity

The ampacity of a conductor, that is, the amount of current it can carry, is related to its electrical resistance: alower-resistance conductor can carry a larger value of current. The resistance, in turn, is determined by thematerial the conductor is made from (as described above) and the conductor's size. For a given material,

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conductors with a larger cross-sectional area have less resistance than conductors with a smaller cross-sectionalarea.

For bare conductors, the ultimate limit is the point at which power lost to resistance causes the conductor tomelt. Aside from fuses, most conductors in the real world are operated far below this limit, however. Forexample, 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 muchhigher temperatures.

The American wire gauge article contains a table showing allowable ampacities for a variety of copper wiresizes.

Isotropy

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

Bibliography

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. wiringregulations.net

(http://www.wiringregulations.net/index.asp?

search=bic&bic=WR0010&t=IET+WIRING+REGULATIONS)

References

1. ^ Fink and Beaty, Standard Handbook for Electrical Engineers 11th Edition, pages 17–19

2. ^ High conductivity coppers (electrical), Copper Development Association (U.K.),http://www.copperinfo.co.uk/alloys/copper/

External links

BBC: Key Stage 2 Bitesize: Electrical Conductors

(http://www.bbc.co.uk/schools/ks2bitesize/science/physical_processes/circuits_conductors/read1.shtml)

GSU: Hyperphysics: Conductors and Insulators (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html)

See also

25/1/2014 Electrical conductor - Wikipedia, the free encyclopedia

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ResistivityCharge transfer complex

Bundle conductor

Superconductivity

Semiconductor

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