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| {{msg:electromagnetism}} | |||
| '''Electrical resistance''' is the ratio of the ] (i.e. voltage) across an electric component (such as a ]) to the ] passing through it: | |||
| :<math>R=V/I</math> | |||
| where ''R'' is the resistance, ''V'' the voltage and ''I'' the current. | |||
| Resistance is thus a measure of the component's opposition to the flow of ]. The ] unit of electrical resistance is the ]. Its ] quantity is '''electrical conductance''' measured in ]. | |||
| '''Differential resistance''' or '''incremental resistance''' is defined as the slope of the ''V-I'' graph at a particular point, thus: | |||
| :<math>R=dV/dI</math> | |||
| This quantity is sometimes called simply ''resistance'', although the two definitions are equivalent only for an ohmic component such as an ideal resistor. If the ''V-I'' graph is not monotonic (i.e. it has a peak or a trough), the differential resistance will be negative for some values of voltage and current. This property is often known as '']'', although it is more correctly called ''negative differential resistance'', since the absolute resistance ''V''/''I'' is still positive. | |||
| 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 ]: the two are ] and the proportionality constant is the electrical resistance. This case is described by ]. | |||
| '''Specific electrical resistance''', a measure of a material's ability to oppose the flow of electric current, is also known as ]. | |||
| The resistance ''R'' of a wire can be computed as | |||
| :<math> R = {L \rho \over A} \; , </math> | |||
| where ''L'' is the length of the wire, ''A'' is the cross-sectional area and ρ is the electrical resistivity of the material. | |||
| See ] for the more information about the physical mechanisms for conduction in materials. | |||
| ==See also== | |||
| *] | |||