A conductor is an object or type of material which permits the flow of electric charges in one or more directions. For example, a wire is an electrical conductor that can carry electricity along its length.
All conductors contain electrical charges, which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the direct current is proportional to the voltage (as determined by Ohm'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.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. All non-superconducting materials offer some resistance and warm up during electric currents. Proper design of an electrical conductor takes into account the temperature of the conductor as well as the value of electric current. The motion of charges creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. The current carrying capacity of a conductor is limited by its ability to dissipate heat. This effect is especially critical in printed circuits, where conductors are relatively small and close together, 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 act both as electrical and thermal conductors. However, some non-metallic materials are practical electrical conductors without being good thermal conductors.One of the best conductors(it is also toxic) is lead. The best, is also expensive, is Silver.
Conductivity of some Materials
Wires are measured by their cross section. In many countries, the size is expressed in square millimeters.
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; 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
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 in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the 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 always true in practical situations. However, this formula still provides a good approximation for long thin conductors such as wires. Another situation for which this formula is not exact 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...
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