Electrical resistivity (also known as specific electrical resistance) is a measure indicating how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrons. The SI unit for electrical resistivity is the ohm metre.

The electrical resistivity of a material is usually given by :

<math>\rho={{RS}\over l}<math>

where

ρ is the electrical resistivity (measured in ohm metres)

R is the resistance of a uniform specimen of the material (measured in ohms)

l is the length of the specimen (measured in metres)

S is the cross-sectional area of the specimen (measured in square metres)

Electrical resistivity can also be defined as:

<math>\rho={E \over J}<math>

where

E is the magnitude of the electric field (measured in volt metres)

J is the magnitude of the current density (measured in amperes per square metre)

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, due to an effect known as superconductivity.

The reciprocal quantity is electrical conductivity.

Contents

1 Typical values 2 Temperature dependence 3 SI electricity units 4 See also 5 External links

Typical values

Typical resistivities for various materials (at 20 �C; 10^-6 Ωm equals Ω·mm²/m) are shown in the table below:

Material	Resistivity (ohm metres)
Silver	0.0159 × 10-6
Copper	0.017 × 10-6
Gold	0.0244 × 10-6
Aluminium	0.0282 × 10-6
Tungsten	0.056 × 10-6
Iron	0.1 × 10-6
Steel, Stainless	0.72 × 10-6
Platinum	0.11 × 10-6
Lead	0.22 × 10-6
Nichrome (A nickel-chromium alloy commonly used in heating elements)	1.50 × 10-6
Carbon	35 × 10-6
Germanium	0.46
Silicon	640
Glass	1010 to 1014
Hard rubber	approximately 1013
Sulfur	1015
Quartz (fused)	75 × 1016

Temperature dependence

• The electric resistivity of a typical metal conductor increases linearly with the temperature.
• The electric resistivity of a typical semiconductor decreases exponentially with the temperature.

An even better approximation of the temperature dependence of the resistivity of a semiconductor is given by the Steinhardt-Hart equation:

<math>1/T = A + B \ln(R) + C (\ln(R))^3 \,<math>

where A, B and C are the so-called Steinhardt coefficients.

This equation is used to calibrate thermistors.

SI electricity units

SI electromagnetism units edit  (http://www.wordiq.com/definition/Template:SI_electromagnetism_units)
Name	Symbol	Dimensions	Quantity
ampere (SI base unit)	A	A	Current
coulomb	C	A�s	Electric charge, Quantity of electricity
volt	V	J/C = kg�m2�s−3�A−1	Potential difference
ohm	Ω	V/A = kg�m2�s−3�A−2	Resistance, Impedance, Reactance
ohm metre	Ω�m	kg�m3�s−3�A−2	Resistivity
farad	F	C/V = kg−1�m−2�A2�s4	Capacitance
farad per metre	F/m	kg−1�m−3�A2�s4	Permittivity
reciprocal farad	F−1	kg1�m2�A−2�s−4	Elastance
siemens	S	Ω−1 = kg−1�m−2�s3�A2	Conductance, Admittance, Susceptance
siemens per metre	S/m	kg−1�m−3�s3�A2	Conductivity
weber	Wb	V�s = kg�m2�s−2�A−1	Magnetic flux
tesla	T	Wb/m2 = kg�s−2�A−1	Magnetic flux density
ampere per metre	A/m	m−1�A	magnetic induction
ampere-turns per weber	A/Wb	kg−1�m−2�s2�A2	Reluctance
henry	H	V�s/A = kg�m2�s−2�A−2	Inductance
henry per metre	H/m	kg�m�s−2�A−2	Permeability
(dimensionless)	-	-	Magnetic susceptibility

See also

• electrical conductivity

External links

• http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Sensors/TempR.html

da:Elektrisk resistivitet de:Spezifischer_Widerstand fi:Ominaisvastus pl:Rezystywność sl:specifična upornost sv:Resistivitet