Electrical conductor
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In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow of negatively charged electrons generates electric current, positively charged holes, and positive or negative ions in some cases.
In order for current to flow within a closed
As discussed above, electrons are the primary mover in metals; however, other devices such as the cationic
Resistance and conductance
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.[1] 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 R and conductance G of a conductor of uniform cross section, therefore, can be computed as[1]
where is the length of the conductor, measured in
This formula is not exact: It assumes the 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 alternating current (AC), because the 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 proximity effect. At commercial power frequency, these effects are significant for large conductors carrying large currents, such as busbars in an electrical 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 the
Conductor materials
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 include
The disadvantages of aluminum wiring lie in its mechanical and chemical properties. It readily forms an insulating oxide, making connections heat up. Its larger
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 ionic liquids, by contrast, can conduct an electric current.
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
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
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
εr′ |
conduction |
propagation
|
---|---|---|
0 | perfect dielectric lossless medium | |
≪ 1 | low-conductivity material poor conductor |
low-loss medium good dielectric |
≈ 1 | lossy conducting material | lossy propagation medium |
≫ 1 | high-conductivity material good conductor |
high-loss medium poor dielectric |
∞ | perfect conductor |
- Bundle conductor
- Charge transfer complex
- Electrical cable
- Electrical resistivity and conductivity
- Fourth rail
- Overhead line
- Stephen Gray, first to identify electrical conductors and insulators
- Superconductivity
- Third rail
References
- ^ a b "Wire Sizes and Resistance" (PDF). Retrieved 2018-01-14.
- ^ Fink and Beaty, Standard Handbook for Electrical Engineers 11th Edition, pages 17–19
- ^ "High conductivity coppers (electrical)". Copper Development Association (U.K.). Archived from the original on 2013-07-20. Retrieved 2013-06-01.
- ^ "From Treasury Vault to the Manhattan Project" (PDF). American Scientist. Retrieved 2022-10-27.
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. wiringregulations.net Archived 2021-04-02 at the Wayback Machine
External links
- BBC: Key Stage 2 Bitesize: Electrical Conductors
- The discovery of conductors and insulators by Gray, Dufay and Franklin.