Electrical conductivity
The electrical conductivity, also referred to as conductivity, is a physical quantity that indicates the ability of a substance to conduct electric current.
The symbol of the electrical conductivity is σ (Greek sigma) or κ (Greek kappa ) and γ (Greek gamma). The derived SI unit of electrical conductivity is S / m (Siemens per meter). The reciprocal of the electric conductivity is called resistivity.
The electrical conductivity is defined as the constant of proportionality between the current density and the electric field strength:
In the special case of constant electrical conductivity equation, this definition corresponds to Ohm's law.
Conductivity tensor
In the special case of an isotropic ( non-directional ) and linear (field- size independent ) medium, the electrical conductivity is a scalar ( one-dimensional value ). Only in this simple to use but common, case, therefore, takes place the power line in the conductor proportional and in the same direction as the electric field causing the power line.
In an anisotropic material, and the linear electrical conductivity tensor is a second level ( dyad ), so a multi-dimensional value. Examples of materials having such properties are materials with layered structures such as graphite and high temperature superconductors.
Origin and reciprocal
It should be noted that the above equation - it is one of the three fundamental constitutive equations - can not be derived from the Maxwell's equations. Maxwell's equations with the continuity laws and the constitutive equations are the foundation of the non-relativistic electrodynamic field theory dar.
The conductance is the reciprocal of resistance. The conductance of a normalized sized piece of conductive material is therefore the reciprocal of the resistivity ρ (Greek rho) and is called the specific conductance ( conductivity = ) σ (Greek sigma) respectively. Both are on the formula
Linked.
Symbols and Units
The symbol for the electrical conductivity is the Greek letter σ ( sigma). Other commonly used symbols for the electrical conductivity are κ ( kappa) and γ ( gamma).
The derived SI unit of electrical conductivity is S / m (Siemens per meter), so A / (V · m). Very common is also S / cm, m / ( Ω · mm ² ) and S · m / mm ², wherein the connections 1 S / cm = 100 S / m, 1 m / ( Ω · mm ²) = S · m / mm ² = 106 S / m applies.
Another particularly common in the U.S. unit is IACS, for English International Annealed Copper Standard. Here, the conductivity is expressed as a percentage of the conductivity of pure annealed copper. 100 % IACS corresponds to a conductivity of 58 x 106 S / m or 58 MS / m.
Electrical conductivity of various substances
According to the electric conductivity are divided substances in
- Superconductor (many metals, various alloys, ceramics and some few fullerenes )
- Conductor ( especially all metals)
- Semiconductors (e.g. silicon, germanium )
- Non-conductors or insulators (most non-metals, and hydrocarbons, and many organic compounds)
- In electrolyte solutions finally one speaks of an electrolytic conductivity. This refers to the specific conductivity of the resistance of a single electrolyte solution between two electrodes with a distance L of 1 cm and a cross-section q of 1 cm ², before at 18 ° C, now in accordance with DIN / e standard at 25 ° C. Related to its concentration conductivity of an electrolyte see Molar conductivity.
Why is a substance electrically conductive?
The conductivity of a substance or mixture of substances is dependent on the availability of mobile charge carriers. This can be loosely bound electrons in organic molecules as they are often described by resonance structures such as in metals, but also ions or delocalized electrons.
Examples
Ion
Pure (ie, distilled or demineralised ) water has a very low conductivity (about one billionth compared to metals, but still about 1000 times more conductive than an insulator ). Be added to that release free moving ions in aqueous solution water salts, acids or bases, the conductivity (4% salt, the conductivity even in distilled water increased to 1000- fold) increases.
Therefore fires in high-voltage equipment (eg switchgear ) should not be extinguished with water, to avoid exposing the personnel extinguishing the risk of electric shock. Wet extinguisher ( extinguishing media water) from at least 1 m distance ( spray ) or 3 m distance ( jet ) can be used in accordance with DIN VDE 0132 in low voltage systems up to 1000 V.
Dopant (electrons, electron holes )
In semiconductors, one uses targeted impurities of the base material, so-called. Dopants in order to influence its conductivity. If the base material treated with electron donors ( elements with more outer electrons than the base material ), it is called n ( egativ ) doping, with the addition of electron acceptors ( elements with fewer electrons than the base material ), however, p ( ositiv ) doping. The p-type doping the electron created voids, also known as holes or " electron -hole ", which as for the management of the electric current and thus increasing the conductivity contribute as the excess electrons in the case of n-doped semiconductor.