Electrical conductor

In science and engineering, conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons. See electrical conduction.

Conductors in context

For the purpose of electronics and electrical engineering, materials are classified according to their electrical resistance, which describes how readily they allow electric current to pass when a voltage is applied. Apart from conductors, materials are classed as insulators (very poor conductors), semi-conductors (materials whose ability to conduct electricity can be controlled), and superconductors which (below a critical temperature, usually cryogenic) offer no significant electrical resistance, allowing circular currents, once established, to flow indefinitely.

Details

Note: The following applies to direct current only. When the direction of voltage/current alternates, other effects (inductance and capacitance) come into play also.

All conductors contain movable electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on a wire (etc) made from the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the amount of current is proportional to the voltage (Ohm's law) provided the temperature remains constant and the material remains in the same shape and state. The ratio between the voltage and the current is called the resistance (measured in ohms) of the object between the points where the voltage was applied. The resistance across a standard mass (and shape) of a material at a given temperature is called the resistivity of the material. The inverse of resistance and resistivity is conductance and conductivity.

Most familiar conductors are metallic. Copper is the most common material for electrical wiring, and gold for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. See electrical conduction for more information on the physical mechanism for charge flow in materials.

Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. 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 close together, and inside an enclosure: the heat produced, if not properly removed, can cause fusing (melting) of the tracks.

Since all conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility.

Thermal and electrical conductivity often go together (for instance, most metals are both electrical and thermal conductors). However, some materials are practical electrical conductors without being a good thermal conductor.

Power engineering

In power engineering, a conductor is a piece of metal used to conduct electricity, known colloquially as an electrical wire.

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 materials

Of the metals commonly used for conductors, copper, has a high 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, aluminum has worse conductivity per unit volume, but better conductivity per unit weight. In many cases, weight is more important than volume making aluminum 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, aluminum is used over a steel core that provides much greater tensile strength than would the aluminum 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). Silver is a better conductor than gold, however, 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.

Conductor voltage

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.

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 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, causing a risk of fire. 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.

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

• Resistivity
• Charge transfer complex
• Bundle conductor
• Superconductivity

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