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The resitance of a wire or other objects is a measure of the potential difference V that must be impressed across the object to cause a current I of one ampere to flow through it:

R = V/I

The unit of resistance is the ohm, for which the symbol Ω is used. 1 Ω = 1 Volt / Ampere

The resistance R of a wire of length L and cross-sectional area A is:

R = ρL / A

where ρ is a constant called the resistivity. The resistivity is a characteristic of the material from which the wire is made. For L in mass m, A in m2, and R in Ω, the units of ρ are ρ · m

Resistance varies with temperature as well.

If a wire has a resistance Ro at a temperature T0, then its resistance R at temperature T is:

R = R0 + αR0( T - T0 )

where α is the temperature coefficient of resistance of the material of the wire. Usually α varies with temperature and so this relation is applicable only over a small temperature range. The units of α are K-1 or °C-1.

A similar relation applies to the variation of resistivity with temperature. If ρ0 and ρ are the resistivites at T0 and T, respectively, then:

ρ = ρ0 + αρ0( T - T0 )