Resistivity is a fundamental property of a material that measures its electrical resistance or how strongly it resists electric current. It is a characteristic property of each material and is useful in comparing various materials on the basis of their ability to conduct electric currents. Resistivity is commonly represented by the Greek letter ρ (rho) and is quantitatively equal to the resistance R of a specimen such as a wire, multiplied by its cross-sectional area A, and divided by its length l; ρ = RA/l. The SI unit of electrical resistivity is the ohm-metre (Ω⋅m) .
Resistivity and conductivity are intensive properties of materials, giving the opposition of a standard cube of material to current. Electrical resistance and conductance are corresponding extensive properties that give the opposition of a specific object to electric current. The resistivity of a material will also help categorize whether a material is a conductor, insulator, or a semiconductor.
The resistivity can be used to calculate the resistance of a wire of given geometry at a given temperature. The resistivity of an exceedingly good electrical conductor, such as hard-drawn copper, at 20° C (68° F) is 1.77 × 10-8 ohm-metre. If lengths are measured in centimeters, resistivity may be expressed in units of ohm-centimeter.
