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PowerPedia:Conductor
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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 σ, defined as:
or its reciprocal resistivity ?:
In linear anisotropic materials, σ 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 printed circuits, 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 insulators. 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 electrical resistance; ranging from high to null resistance, there are semiconductors, ordinary metallic conductors (also called normal metals), and superconductors.
Conductor Size
In United States, conductors are measured by American wire gauge for smaller ones, and circular mils 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 band structure. Electrons, being fermions, follow the Pauli exclusion principle, 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 Fermi energy. 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 insulators. 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. Mott insulators and charge transfer insulators are two such classes of insulators.
Gases and plasmas
In air, and other ordinary gases below the breakdown field, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is extremely low, gases are dielectrics or insulators. However, once the applied electric field approaches the breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionizing, neutral gas atoms or molecules in a process called avalanche breakdown. The breakdown process forms a plasma 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 spark, arc or lightning.
Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules 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
Metals 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 Fermi energy). 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, copper has the highest conductivity. Silver is more conductive, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires.
Compared to copper, aluminium has worse conductivity per unit volume, but better conductivity per unit weight. In many cases, weight is more important than volume making aluminium the 'best' conductor material for certain applications. For example, it is commonly used for large-scale power distribution conductors such as overhead power lines. In many such cases, aluminium is used over a steel core that provides much greater tensile strength than would the aluminium alone [1][2].
Gold is occasionally used for very fine wires such as those used to wire bond integrated circuits to their lead frames. The contacts in electrical connectors are also commonly gold plated or gold flashed (over nickel). 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 copper, silver, or tin/lead alloys. This corrosion would have a very detrimental effect on connection quality over time; gold plating avoids that.
Resistance comes about in a metal because of the scattering of electrons from defects in the lattice or by phonons. A crude theory of conduction in simple metals is the Drude model, in which scattering is characterized by a relaxation time τ. The conductivity is then given by the formula
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 effective mass.
Semiconductors
A solid with filled bands is an insulator, but at finite temperature, electrons can be thermally excited from the valence band to the next highest, the conduction band. The fraction of electrons excited in this way depends on the temperature and the band gap, the energy difference between the two bands. Exciting these electrons into the conduction band leaves behind positively charged holes in the valence band, which can also conduct electricity. See semiconductor 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 Mott transition 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 superconducting state. By an interaction mediated by some other part of the system (in metals, phonons), the electrons pair up into Cooper pairs. The bosonic Cooper pairs form a superfluid which has zero resistance.
Electrolytes
Electric currents in electrolytes are flows of electrically charged atoms (ions). For example, if an electric field is placed across a solution of Na+ and Cl–, 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, redox 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 proton conductors 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 metals). 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 "perfect vacuum" 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 free electrons or ions through either field emission or thermionic emission. Thermionic emission occurs when the thermal energy exceeds the metal's work function, while field emission occurs when the electric field at the surface of the metal is high enough to cause tunneling, which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate an electron cloud as in the filament or indirectly heated cathode of vacuum tubes. Cold electrodes can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called cathode spots or anode spots) are formed. These are incandescent regions of the electrode surface that are created by a localized high current flow. These regions may be initiated by field emission, but are then sustained by localized thermionic emission once a vacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field. Vacuum tubes and sprytrons are some of the electronic switching and amplifying devices based on vacuum conductivity.
Conductor voltage and ampacity
The voltage on a conductor is determined by the connected circuitry and has nothing to do with the conductor itself. Conductors are usually surrounded by and/or supported by insulators and the insulation determines the maximum voltage that can be applied to any given conductor. 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 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 fuses, most conductors in the real world are operated far below this limit, however. For example, household wiring is usually insulated with PVC 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 Teflon or fiberglass may allow operation at much higher temperatures. The American wire gauge article contains a table showing allowable ampacities for a variety of copper wire sizes.
Power engineering
In power engineering, a conductor is a piece of metal used to conduct electricity, known colloquially as an electrical wire.
Patents
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References and external articles
| G Web | Sites on Conductors via Google Search |
| G groups | Newsgroups with Conductors via Google Groups |
| G Image | Images of Conductors via Google Image |
| Y Image | Images of Conductors via Yahoo! Images |
- 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]
- Wikipedia contributors, Wikipedia: The Free Encyclopedia. Wikimedia Foundation.
- H. Mann, Pure electrical conductor, DynLAB - Course on Modeling and Simulation : Electrical systems I, icosym-nt.cvut.cz.
