What works from dielectric material

As dielectric is a volume in which there is an electric field (v. Greek. slide-: "Through", d. H. the field passes through the material) without any significant electrical conductivity being present. Dielectrics can be empty space (vacuum) or all volumes that are filled with electrically non-conductive materials (insulators or insulating materials) [1].

The field sizes of the dielectric are the electric field strengthE. and the electrical flux densityD., which in the electrostatic case, i.e. in the case that is constant over time, via the dielectric constant are linked via the following relationship:

The dielectric constant is made up of the electric field constant and the material-specific permittivity or dielectric constant (Values ​​greater than 1; the relative permittivity of air is approximately 1 as in a vacuum) together:

Use of the term

Insulators such as the insulating material between capacitor plates, coaxial cables and the like are called dielectric in the narrower sense. Antennas can also have function-determining dielectric components.

Furthermore, the liquid in an electrical discharge machine that prevents the electrode sparks from being too long is called a dielectric.

Insulating materials that only serve to electrically isolate conductive parts from one another are generally not referred to as dielectrics, although their dielectric properties can be important for their function.

Polarization of a dielectric

Since the charge carriers cannot move freely in a dielectric, they are polarized by an external electric field. A distinction is made between two types of polarization:

  1. Displacement polarization: Electric dipoles are induced, i.e. H. Dipoles are created by a slight shift in charge in the atoms or molecules or between differently charged ions. The effect can be described with the help of the Clausius-Mossotti equation.
  2. Orientation polarization: Alignment of disordered, permanent dipoles of an insulator in the electric field against their thermal movement. The effect can be described with the Debye equation.

Dielectric in capacitors

The capacityC. of a capacitor depends essentially on the dielectric used and its permittivity or dielectric constant, the electrode area A. and the distance d of the electrodes from each other.

The following applies to a plate capacitor:

The higher the dielectric constant the more energy can be stored in the electric field between the plates of a capacitor.

Another important parameter of a dielectric in capacitors and cables is its dielectric strength, i.e. at which voltage the dielectric loses its insulating properties and flashovers occur between the capacitor layers.

Depending on the application, the dielectric loss factor also plays a role in capacitor dielectrics. In the case of alternating voltage, it leads to heating of the capacitor.

Dielectrics in cables and high frequency components

As dielectric This is also the term used to describe the insulating material between the conductors of a cable (especially high-frequency and coaxial cables), which essentially determines its characteristic impedance and the frequency-dependent attenuation per length (usually given in decibels [dB] or neper [Np] per km).

Dielectric antenna elements and dielectric waveguides are used in high-frequency technology and here obey the same laws of refraction as in optics or fiber optic cables.

Typical dielectrics in high-frequency applications are polyethylene, PTFE, ceramics (e.g. steatite, aluminum oxide), mica or air.

Dielectrics for high-frequency applications generally have to have particularly low dielectric loss factors.

Literature sources

  1. Ralf Kories, Heinz Schmidt-Walter: Paperback of electrical engineering. Publisher Harri Deutsch, 2004 - ISBN 3817117345

Category: Electrostatics