Electrical resistance is the ratio of the potential difference (i.e. voltage) across an electric component (such as a resistor) to the current passing through it:
(where V is the voltage and I the current)
It is thus a measure of the component's opposition to the flow of electric charge. Electrical resistance is usually denoted by symbol R. The SI unit for electrical resistance is ohm. Its reciprocal quantity is electrical conductance measured in siemens.
For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current flowing or the amount of applied voltage: the two are proportional and the proportionality constant is the electrical resistance. This is the content of Ohm's law.
Specific electrical resistance, a measure of a material's ability to oppose the flow of electric current, is also known as electrical resistivity.
The resistance R of a wire can be computed as
where L is the length of the wire, A is the crosssectional area and ρ is the electrical resistivity of the material.
See electrical conduction for the more information about the physical mechanisms for conduction in materials.
Some materials exhibit the property known as negative resistance.
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