Electrical conductivity

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Not to be confused with electrical conductance, a measure of an object's or circuit's ability to conduct an electric current between two points, which is dependent on the electrical conductivity and the geometric dimensions of the conducting object.

Electrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current. The conductivity σ is defined as the ratio of the current density J to the electric field strength E:

\mathbf{J} = \sigma \mathbf{E}.

It is also possible to have materials in which the conductivity is anisotropic, in which case σ is a 3×3 matrix (or more technically a rank-2 tensor) which is generally symmetric.

Conductivity is the reciprocal (inverse) of electrical resistivity, ρ, and has the SI units of siemens per metre (S·m-1) and CGSE units of inverse second (s–1):

\sigma = {1\over\rho}.

Electrical conductivity is commonly represented by the Greek letter σ, but κ (esp. in electrical engineering science) or γ are also occasionally used.

An EC meter is normally used to measure conductivity in a solution.

Contents

[edit] Classification of materials by conductivity

  • A conductor such as a metal has high conductivity and a low resistivity.
  • An insulator like glass has low conductivity and a high resistivity.
  • The conductivity of a semiconductor is generally intermediate, but varies widely under different conditions, such as exposure of the material to electric fields or specific frequencies of light, and, most important, with temperature and composition of the semiconductor material.

The degree of doping in solid state semiconductors makes a large difference in conductivity. More doping leads to higher conductivity. The conductivity of a solution of water is highly dependent on its concentration of dissolved salts and sometimes other chemical species which tend to ionize in the solution. Electrical conductivity of water samples is used as an indicator of how salt-free, ion-free, or impurity-free the sample is; the purer the water, the lower the conductivity (the higher the resistivity). Conductivity measurements in water are often reported as "Specific Conductance", which is the conductivity of the water were it measured at 25 C.

[edit] Some electrical conductivities

Main article: electrical resistivities of the elements (data page)
Material Electrical Conductivity

(S·m-1)

 Notes
Silver 63.0 × 106 Best electrical (as well as thermal) conductor of any known metal
Copper 59.6 × 106
Annealed Copper 58.0 × 106 Referred to as 100% IACS or International Annealed Copper Standard. The unit for expressing the conductivity of nonmagnetic materials by testing using the eddy-current method. Generally used for temper and alloy verification of Aluminium.
Gold 45.2 × 106 Gold is commonly used in electrical contacts because it does not easily corrode.
Aluminium 37.8 × 106 Commonly used for High Voltage Mains electricity distribution cables[citation needed]
Sea water 4.8 Corresponds to an average salinity of 35 g/kg at 20 °C.[1]
Drinking water 0.0005 to 0.05 This value range is typical of high quality drinking water and not an indicator of water quality
Deionized water 5.5 × 10-6 Conductivity is lowest with monoatomic gases present; changes to 1.2 × 10-4 upon complete de-gassing, or to 7.5 × 10-5 upon equilibration to the atmosphere due to dissolved CO2 [2]
Jet A-1 Kerosene 50 to 450 × 10-12 [3]
n-hexane 100 × 10-12
Air 0.3 to 0.8 × 10-14 [4]

[edit] Complex conductivity

To analyse the conductivity of materials exposed to alternating electric fields, it is necessary to treat conductivity as a complex number (or as a matrix of complex numbers, in the case of anisotropic materials mentioned above) called the admittivity. This method is used in applications such as electrical impedance tomography, a type of industrial and medical imaging. Admittivity is the sum of a real component called the conductivity and an imaginary component called the susceptivity.

An alternative description of the response to alternating currents uses a real (but frequency-dependent) conductivity, along with a real permittivity. The larger the conductivity is, the more quickly the alternating-current signal is absorbed by the material (i.e., the more opaque the material is). For details, see Mathematical descriptions of opacity.

[edit] Temperature dependence

Electrical conductivity is strongly dependent on temperature. In metals, electrical conductivity decreases with increasing temperature, whereas in semiconductors, electrical conductivity increases with increasing temperature. Over a limited temperature range, the electrical conductivity can be approximated as being directly proportional to temperature. In order to compare electrical conductivity measurements at different temperatures, they need to be standardized to a common temperature. This dependence is often expressed as a slope in the conductivity-vs-temperature graph, which can be written as:

\sigma_{T'} = {\sigma_T \over 1 + \alpha (T - T')}

where

σT′ is the electrical conductivity at a common temperature, T′
σT is the electrical conductivity at a measured temperature, T
α is the temperature compensation slope of the material,
T is the measured absolute temperature,
T′ is the common temperature.

The temperature compensation slope for most naturally occurring waters is about 2 %/°C, however it can range between (1 to 3) %/°C. This slope is influenced by the geochemistry, and can be easily determined in a laboratory.

At extremely low temperatures (not far from absolute zero), a few materials have been found to exhibit very high electrical conductivity in a phenomenon called superconductivity.

[edit] See also

[edit] Notes and references

  1. ^ Physical properties of sea water
  2. ^ Pashley, R. M. (2005). "De-Gassed Water is a Better Cleaning Agent". The Journal of Physical Chemistry B 109: 1231. doi:10.1021/jp045975a. 
  3. ^ Jet A-1 Kerosene
  4. ^ Pawar, S. D. (2009). "Effect of relative humidity and sea level pressure on electrical conductivity of air over Indian Ocean". Journal of Geophysical Research 114: D02205. doi:10.1029/2007JD009716. 

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Electrical_conductivity"