Electrical resistivity limits current flow in conductors, semiconductors, and insulators. Ohm’s law relates resistance, voltage, and current. Electrical resistivity tomography creates 3D images. Different elements have varying electrical resistivity and uses. Contaminated contacts can cause plasma and shorten relay life, so electronic switches are recommended.
Electrical resistivity is the characteristic of a conductor, semiconductor, or insulator that limits the amount of current flow. It is determined by the atomic or molecular properties that can allow or prevent the flow of free electrons through the material. Electrical resistivity is nearly the same as electrical resistance with the slight difference in how electrical resistivity can relate to the resistance of a specific length of a material. For example, a base unit of resistivity might refer to the amount of resistance per unit length of copper wire.
Ohm’s law gives the relationship between electrical resistance (R), voltage (V), and current flow in amperes (A). Resistance is the ratio of voltage to current. At the same voltage, higher current is the result of lower resistance. An electrical fuse is thought to have a very low voltage drop when placed in series with an electrical load. If the load is 9.999 ohms and the fuse has a resistance of 0.001 ohms, a supply voltage of 10 volts (V) will produce a current of 1 A and the voltage across the fuse is negligible at 0.001 V.
Electrical resistivity tomography is an imaging tool that can present a three-dimensional profile of embedded materials. This is accomplished by using built-in electrodes and direct current (DC) to create a two-dimensional image. By using perpendicular picture planes, you can get an idea of the three-dimensional layout.
Various elements with significant electrical resistivity have different uses in electrical applications. Silver and gold are very low electrical resistivity elements that are used for special applications such as microbonding used in the semiconductor industry. Copper is the safe choice commercial conductor because of its acceptable electrical resistivity and relatively low price. Carbon is a low cost material of choice for medium to high strength resulting in huge varieties of carbon strength on the market. Tungsten’s high stability at relatively high temperatures makes it a common choice for incandescent and filament applications such as light bulbs, wirewound variable resistors, and electric heaters.
The electrical resistance of the contacts is usually very low when the conductive surfaces are not contaminated. In the case of relay contacts, the pressure between them temporarily determines how much resistance will decrease when the contact is closed. If the pressure is not sufficient and the current is high, it is possible for the contact to form plasma which can melt the contact. The spark generated by repeated closings shortens the life of the relay. In most cases, it is a good idea to use electronic DC switches such as the silicon controlled rectifier (SCR) or use electronic alternating current (AC) switches such as the three terminal AC circuit breaker (TRIAC).
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