# Electrical resistance

**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:

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

*L*is the length of the wire,

*A*is the cross-sectional 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.