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|}In There was an error working with the wiki: Code[105] and There was an error working with the wiki: Code[106], conductors are materials that contain movable charges of Electricity. Electrical conduction is the movement of electrically charged particles through a transmission medium. The movement can form an electric current in response to an electric field. The underlying mechanism for this movement depends on the material. Conduction in metals and resistors is well described by Ohm's Law, which states that the current is proportional to the applied electric field. When an electric potential difference is impressed across separate points on a conductor, the mobile charges within the conductor are forced to move, and an electric current between those points appears in accordance with There was an error working with the wiki: Code[107]. While many conductors are There was an error working with the wiki: Code[108]lic, there are many non-metallic conductors as well, including all Plasmas.

Introduction

Under normal conditions, all materials offer some resistance to flowing charges, which generates heat. Thus, proper design of an electrical conductor includes an estimate of the temperature that the conductor is expected to endure without damage, as well as the quantity of electrical current. Electrical conduction movement can form an electric current in response to an electric field and the underlying mechanism for this movement depends on the material. Conduction in metals and resistors is well described by Ohm's Law, which states that the current is proportional to the applied electric field. The ease with which current density (current per area) j appears in a material is measured by the conductivity ?, defined as:

:Current (electricity) = ? Electric field

or its reciprocal There was an error working with the wiki: Code[82] ?:

:Current (electricity) = Electric field / ?

In linear anisotropic materials, ? and ? are tensors.

The motion of charges also creates an Electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and volume (length x cross-sectional area) has no real limit to the current it can carry without being destroyed as long as the heat generated by the resistive loss is removed and the conductor can withstand the radial forces. This effect is especially critical in There was an error working with the wiki: Code[109]s, where conductors are relatively small and the heat produced, if not properly removed, can cause fusing (melting) of the tracks.

Non-conducting materials lack mobile charges and are called There was an error working with the wiki: Code[83]s. A material can be an electrical conductor without being a thermal conductor, although a metal can be both an electrical conductor and a thermal conductor. Electrically conductive materials are usually classified according to their There was an error working with the wiki: Code[84]s), and There was an error working with the wiki: Code[110]s.

Conductor Size

In United States, conductors are measured by There was an error working with the wiki: Code[111] for smaller ones, and There was an error working with the wiki: Code[112] for larger ones. For example, a '4/0' conductor is about a half inch in diameter, while a '795 000' conductor is about an inch in diameter. In other places, conductors are often measured by their cross section in square millimeters.

Conductor Mediums

Solids

In crystalline solids, atoms interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into bands. Whether a material conducts or not is determined by its There was an error working with the wiki: Code[113]. Electrons, being There was an error working with the wiki: Code[114]s, follow the There was an error working with the wiki: Code[115], meaning that two electrons cannot occupy the same state. Thus electrons in a solid fill up the energy bands up to a certain level, called the There was an error working with the wiki: Code[116]. Bands which are completely full of electrons cannot conduct electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full (i.e. the Fermi energy is between two bands) are There was an error working with the wiki: Code[117]s. In some cases, however, the band theory breaks down and materials that are predicted to be conductors by band theory turn out to be insulators. There was an error working with the wiki: Code[118] and There was an error working with the wiki: Code[119] are two such classes of insulators.

Gases and plasmas

In air, and other ordinary There was an error working with the wiki: Code[85] value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and There was an error working with the wiki: Code[86] that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light emitting conductive path, such as a There was an error working with the wiki: Code[87] or There was an error working with the wiki: Code[120].

Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their There was an error working with the wiki: Code[121]s or atoms. A plasma can be formed by high Temperature, or by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current.

Metals

There was an error working with the wiki: Code[122] are good conductors because they have unfilled space in the valence energy band. In the absence of an electric field, there exist electrons travelling in all directions and many different velocities up to the Fermi velocity (the velocity of electrons at the There was an error working with the wiki: Code[123]). When an electric field is applied, a slight imbalance develops and mobile electrons flow. Electrons in this band can be accelerated by the field because there are plenty of nearby unfilled states in the band.

Of the metals commonly used for conductors, There was an error working with the wiki: Code[88]. There was an error working with the wiki: Code[124] is more conductive, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as There was an error working with the wiki: Code[125]s, and as a thin plating to mitigate There was an error working with the wiki: Code[126] losses at high frequencies. Because of its ease of connection by There was an error working with the wiki: Code[127] or clamping, copper is still the most common choice for most light-gauge wires.

Compared to copper, There was an error working with the wiki: Code[89]. In many such cases, aluminium is used over a There was an error working with the wiki: Code[128] core that provides much greater There was an error working with the wiki: Code[129] than would the aluminium alone http://slate.msn.com/id/2123556/http://www.eurekalert.org/features/doe/2005-03/drnl-mpt030905.php.

There was an error working with the wiki: Code[90] There was an error working with the wiki: Code[91] frames. The contacts in electrical connectors are also commonly gold There was an error working with the wiki: Code[92] or gold flashed (over There was an error working with the wiki: Code[130]). Contrary to popular belief, this is not done because gold is a better conductor it is not. Instead, it is done because gold is very resistant to the surface corrosion that is commonly suffered by There was an error working with the wiki: Code[131], There was an error working with the wiki: Code[132], or There was an error working with the wiki: Code[133]/There was an error working with the wiki: Code[134] alloys. This corrosion would have a very detrimental effect on connection quality over time There was an error working with the wiki: Code[135] avoids that.

Resistance comes about in a metal because of the There was an error working with the wiki: Code[136] of electrons from defects in the lattice or by There was an error working with the wiki: Code[137]. A crude theory of conduction in simple metals is the There was an error working with the wiki: Code[138], in which scattering is characterized by a relaxation time &tau. The conductivity is then given by the formula

:\sigma = \frac{ne^2 \tau}{m}

where n is the density of conduction electrons, e is the electron charge, and m is the electron mass. A better model is the so-called semiclassical theory, in which the effect of the periodic potential of the lattice on the electrons gives them an There was an error working with the wiki: Code[139].

Semiconductors

A solid with filled bands is an insulator, but at finite temperature, electrons can be thermally excited from the There was an error working with the wiki: Code[93]s in the valence band, which can also conduct electricity. See There was an error working with the wiki: Code[140] for more details.

In semiconductors, impurities greatly affect the concentration and type of charge carriers. Donor (n-type) impurities have extra valence electrons with energies very close to the conduction band which can be easily thermally excited to the conduction band. Acceptor (p-type) impurities capture electrons from the valence band, allowing the easy formation of holes. If an insulator is doped with enough impurities, a There was an error working with the wiki: Code[141] can occur, and the insulator turns into a conductor.

Superconductors

In metals and certain other materials, a transition occurs at low temperature (sub-cryogenic) to the There was an error working with the wiki: Code[94] state. By an interaction mediated by some other part of the system (in metals, phonons), the electrons pair up into There was an error working with the wiki: Code[95] Cooper pairs form a There was an error working with the wiki: Code[142] which has zero resistance.

Electrolytes

Electric currents in There was an error working with the wiki: Code[96]+ and There was an error working with the wiki: Code[97]&ndash, the sodium ions will move constantly towards the negative electrode (anode), while the chlorine ions will move towards the positive electrode (cathode). If the conditions are right, There was an error working with the wiki: Code[143] reactions will take place at the electrode surfaces, releasing electrons from the chlorine, and allow electrons to be absorbed into the sodium. Water-ice and certain solid electrolytes called There was an error working with the wiki: Code[144]s contain positive hydrogen ions which are free to move. In these materials, currents of electricity are composed of moving protons (as opposed to the moving electrons found in There was an error working with the wiki: Code[145]s). In certain electrolyte mixtures, populations of brightly-colored ions form the moving electric charges. The slow migration of these ions during an electric current is one example of a situation where a current is directly visible to human eyes.

Vacuum

Since a "There was an error working with the wiki: Code[98]" contains no charged particles, vacuums normally behave as very good insulators. However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting There was an error working with the wiki: Code[99], which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate an There was an error working with the wiki: Code[100] or indirectly heated There was an error working with the wiki: Code[101]s are some of the electronic switching and amplifying devices based on vacuum conductivity.

Conductor voltage and ampacity

The There was an error working with the wiki: Code[102] of a conductor, that is, the amount of Current (electricity) it can carry, is related to its electrical resistance: a lower-resistance conductor can carry more 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 There was an error working with the wiki: Code[103], most conductors in the real world are operated far below this limit, however. For example, household wiring is usually insulated with There was an error working with the wiki: Code[104] insulation that is only rated to operate to about 60 C, therefore, the current flowing in such wires must be limited so that it never heats the copper conductor above 60 C. Other, more expensive insulations such as There was an error working with the wiki: Code[146] or There was an error working with the wiki: Code[147] may allow operation at much higher temperatures. The There was an error working with the wiki: Code[148] article contains a table showing allowable ampacities for a variety of copper wire sizes.

Power engineering

In There was an error working with the wiki: Code[149], a conductor is a piece of metal used to conduct electricity, known colloquially as an There was an error working with the wiki: Code[150].

Patents

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References and external articles

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Hamid V. Ansari, What the electric conductor is, Ed 01.12.31. sci.materials, Nov 11 2002.

William L. Bahn, Best electrical conductor? alt.engineering.electrical, Apr 24 1999.

Electrical Conductor, Eric Weisstein's World of Physics

Conductors and Insulators, hyperphysics.phy-astr.gsu.edu

Electrical Conductivity Protocol, globe.gov [PDF]

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

H. Mann, Pure electrical conductor, DynLAB - Course on Modeling and Simulation : Electrical systems I, icosym-nt.cvut.cz.

See also

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Resistivity

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