Electrical resistivity is the characteristic of a conductor, a semiconductor, or an insulator that limits the amount of current flow. It is determined by the atomic or molecular properties that may either allow or impede the flow of free electrons through the material. Electrical resistivity is almost the same as electrical resistance with the slight difference in the way electrical resistivity may refer to resistance of a specific length of a material. For instance, a basic unit of resistivity could refer to the amount of resistance per unit length of a copper cable.
Ohm’s law provides the relationship between the electrical resistance (R), the voltage (V), and the current flow in amperes (A). Resistance is the ratio of the voltage to the current. For the same voltage, a higher current is a result of a lower resistance. An electrical fuse is meant 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 10-volt (V) supply voltage 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 is able to present a three-dimensional profile of embedded materials. This is accomplished by using embedded electrodes and direct current (DC) to create a two-dimensional image. By using perpendicular image planes, it is possible to have an idea of the three-dimensional layout.
Various elements with notable electric 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 chosen commercial conductor sure to its acceptable electrical resistivity and relatively low price. Carbon is a low-cost material of choice for medium to high resistance resulting in huge varieties of carbon resistance in the market. The high stability of tungsten in relatively high temperatures makes it a common choice for incandescent and filament applications such as light bulbs, wire-wound variable resistors, and electric heaters.
Contact electrical resistance is usually very low when the conductive surfaces are not contaminated. In the case of relay contacts, the pressure that temporarily joins them determines how low the resistance will drop when the contact is closed. If the pressure is not enough and the current is high, it is possible for the contact to form plasma that can melt the contact. The spark generated due to repeated closures shortens the relay lifespan. 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 like the three-terminal AC (TRIAC) switch.
Source / Courtesy : WiseGeek