Dielektrisk

A dielectric is an insulating material that can be polarized by an applied electric field. When exposed to the field, electric dipoles within the material tend to align with the field, and the material reduces the effective internal field compared with the external field. Dielectrics are characterized by properties such as permittivity, polarization behavior, and dielectric strength, and they are widely used to control electric fields in devices including capacitors and electrical insulators. In Danish, dielektrisk is the term used for dielectric or dielectric-related properties.

The relation between the electric displacement D, the electric field E, and the material is D = ε0 εr E, where ε0 is the vacuum permittivity and εr is the relative permittivity (dielectric constant) of the material. Polarization can arise from electronic, ionic, and orientational mechanisms, depending on frequency and temperature. The relative permittivity εr is generally frequency-dependent and can differ for alternating current signals and direct current fields.

Practical dielectric performance is characterized by dielectric loss and dielectric strength. Dielectric loss describes energy dissipated as heat, often expressed by the loss tangent tanδ = ε''/ε'. The complex permittivity ε = ε' - jε'' captures this behavior. Dielectrics also have a dielectric strength, the maximum electric field the material can withstand before electrical breakdown, which limits operating voltages.

Applications include capacitors, insulating films and coatings, transformer and power-system insulation, and insulating liquids such as transformer oil. Dielectrics come in several families—polymers, ceramics, and liquids—each with different permittivity, losses, and breakdown strength. In crystalline dielectrics, anisotropy can lead to a dielectric tensor, and some materials such as ferroelectrics show large, field-dependent permittivity.