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Electrical Conductivity: Materials
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The dc electrical conductivity of a complicated composite material, like cement paste, depends on microstructure in several ways. In general, for an insulating material containing pores that are filled with a conductor, the bulk conductivity σ, averaged over the sample dimensions, can be written [3] as σ = β φ σo, where φ is the porosity, σo is the conductivity of the material filling the pores, and β is a dimensionless factor summarizing parameters like pore connectivity and tortuosity. The factor of porosity corrects for the cross-sectional area available for flow. In porous sandstone rocks, for example, where much work has been carried out in relating microstructure and electrical conductivity, the dimensionless quantity F = σo / σ, called the formation factor, has been defined [4]. For the relatively low porosities encountered in sandstones, a power law in φ is often found to be a reasonable description of experimental values of F [4]. This general relationship is usually called Archie's law [5].
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Copper is the standard by which electrical materials are rated and conductivity ratings are expressed as a relative measurement to copper. These ratings will frequently be expressed as "28 IACS". IACS is the abbreviation for International Annealed Copper Standard and the number preceding "IACS" is the percentage of conductivity a material has relative to copper, which is considered to be 100% conductive. This does not mean that copper has no resistance (is 100% conductive in an absolute sense), but rather that it is the standard by which other materials are measured. The higher the % IACS, the more conductive the material is. This standard refers to a pure, "standard" copper having a resistivity of 1.7241 microhm-cm at 20°C (68°F).
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Subsurface materials exhibit a very large range of electrical conductivity values. Fresh rock is generally a poor conductor of electricity, but a select group of metallic minerals containing iron, copper or nickel are very good conductors. Layers of graphite are ... very good conductors.
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