Conductivity

Conductivity is the measure of a material's ability to conduct electric current or heat. In physics, it most commonly refers to electrical conductivity of a material, defined as the reciprocal of resistivity. It also describes thermal conductivity in the context of heat transport. Conductivity can apply to solids, liquids, and gases, and often depends on temperature, structure, and composition.

Electrical conductivity sigma is measured in siemens per meter (S/m). Metals typically have high sigma due to free electrons; insulators have very low sigma. In metals, conductivity decreases with temperature because electron scattering from lattice vibrations increases. In semiconductors, conductivity tends to increase with temperature as more charge carriers are thermally excited. Ionic conductors such as electrolytes conduct via mobile ions; their conductivity depends on ion concentration and mobility, and is usually reported in S/m or S/cm.

Thermal conductivity, denoted k, measures how readily a material transfers heat. Units are W/m·K. Metals generally have high thermal conductivity, while polymers and aerogels are low. Some materials exhibit anisotropy, with different conductivities along different directions. The Wiedemann–Franz law relates electronic contributions to thermal and electrical conductivities in metals at a given temperature.

Measurement and related concepts: For solids, conductivity can be measured with a four-point probe or impedance spectroscopy, yielding sigma. For liquids, conductivity meters rely on electrochemical cells and a cell constant to convert conductance to conductivity. Conductivity is sensitive to impurities, temperature, and moisture, and is widely used in quality control, electrochemistry, and geophysical investigations.