Electrical resistivity

Electrical resistivity (also known as specific electrical resistance) is a measure indicating how strongly a material opposes the flow of electric current; if the resistivity of the material is small, that means that material is effective to carry electrons.

The resistivity of a material is usually denoted by the lower-case Greek letter rho (ρ) and is given by /em>, where Rresistance and a cross-section area S. The units of ρ are ohm meters. Its reciprocal quantity is electrical conductivity.

Also the resistivity is the magnitude of the electric field divided by the magnitude of the current density.

In general, electrical resistivity of metals increases with temperature, while the resistivity of semiconductors decreases with temperature.

As the temperature of a metal is reduced, the resistance usually reduces until it reaches a constant value, known as the residual resistivity. This value depends not only on the type of metal, but on its purity and thermal history.

Some materials lose all electrical resistivity at sufficiently low temperatures; this effect is known as superconductivity.

The table of resistivity for various materials (at 20 degree):

Material	Resistivity (Ohm-meters)
Silver	1.59 x 10-8
Copper	1.7 x 10-8
Gold	2.44 x 10-8
Aluminum	2.82 x 10-8
Tungsten	5.6 x 10-8
Iron	10 x 10-8
Platinum	11 x 10-8
Lead	22 x 10-8
Nichrome (A nickel-chromium alloy commonly used in heating elements)	1.50 x 10-6
Carbon	3.5 x 10-5
Germanium	0.46
Silicon	640
Glass	1010 to 1014
Hard rubber	approximately 1013
Sulfur	1015
Quartz (fused)	75 x 16