Electrical resistivity and conductivity
The electrical conductivity is a physical quantity that describes to what extent a substance conducts electrical current. It decides whether a substance is suitable as an insulator or as an electrical conductor. It is also used to identify substances.
The electric current consists of moving charge carriers. It is caused by a voltage difference between two poles. The material between the poles depends on the magnitude of the electrical current flowing at a given voltage. The quotient of current (I) and voltage (U) is referred to as electrical conductance (G).
G = I / U
The electrical conductance depends on both the material properties and the dimensions of the material. The larger the cross-sectional area and the shorter the distance between the poles, the more current flows. For general statements about the material the dependence on the dimensions has to be excluded. This is done by referring the conductance (G) to the cross-sectional area (A) and the distance (l). This results in the electrical conductivity (sigma) of a substance.
Sigma = G * L / A
The unit of measure of electrical conductivity is Siemens per meter [S / m].
The term “electrical resistance” refers to the reciprocal of the conductance. The specific electrical resistance is the reciprocal of the conductivity.
Conductivity measurement is done indirectly by measuring the current that sets at a given voltage under defined conditions. Modern measuring instruments deliver the value directly by converting the determined current with the help of the device constants. It must be remembered that the electrical conductivity depends on the temperature. The relationship between electrical quantities and temperature is further investigated in thermoelectrics.