A dielectric (plural: dielectrics) is an electrically weakly or non-conductive substance in which the charge carriers present cannot move freely. A dielectric can be a gas, a liquid or a solid. The term dielectric is used in particular when there is an electric field in the area of space under consideration (from the Greek dia-: “through”, i.e. the field goes through the material).
The field sizes of the dielectric are the electric field strength and the electric flux density . You are in the electrostatic , i.e. H. time-constant case and linked in an isotropic medium by the permittivity via the following relationship:
Use of terms
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 decisive for their functioning.
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:
- In the case of displacement polarization , electrical dipoles are induced, i.e. dipoles are created by small charge shifts in the atoms or molecules or between differently charged ions. In an alternating field, the negative electron shell and the positive atomic nucleus “oscillate” back and forth in opposite directions. The movement of the atomic nucleus can be neglected due to its significantly larger mass (mass ratio of proton to electron ≈ 1836) compared to the movement of the electron shell. Therefore the atomic nucleus is considered to be stationary. The size of the induced dipole moment is therefore only dependent on the deflection of the electron shell. No thermal energy is generated with these vibrations . The effect can be described with the help of the Clausius-Mossotti equation .
- With orientation polarization , disordered, permanent dipoles of an insulator in the electric field are aligned against their thermal movement . In an alternating field, the molecules have to constantly reorient themselves, whereby energy from the field is converted into heat ( microwave oven ). The effect can be described with the Debye equation .
Dielectrics in capacitors
The following applies to a plate capacitor:
The higher the relative permittivity , the more energy can be stored in the electric field between the plates of a capacitor . The relative permittivity of the selected insulating material indicates how many times the capacity of a capacitor increases compared to vacuum (or air) as an insulating material.
An important parameter of a dielectric in capacitors and cables is also its breakdown strength , that is at what voltage the dielectric its insulation properties lost and there will be arcing between the capacitor plates or wires.
Depending on the application, the dielectric loss factor also plays a role in capacitor dielectrics. With alternating voltage it leads to the heating of the capacitor. The pronounced dielectric absorption of some materials can lead to a partial recharge of a capacitor after a complete discharge by short-circuiting.
Dielectrics in cables, high frequency and high voltage components
The insulating material between the conductors of a cable (especially high-frequency and coaxial cables ) is also referred to as the dielectric , which essentially determines its line impedance and the frequency-dependent attenuation per length (usually given in decibels [dB] or neper [Np] per km).
Typical materials for dielectrics in high-frequency applications are polyethylene , PTFE , ceramics (for example steatite , aluminum oxide ), mica or air. Dielectrics for high-frequency applications must generally have particularly low dielectric loss factors .
The same applies to high-voltage components such as cables or transformers . The dielectric consists primarily of the oil-soaked paper insulation between the cable conductor and shield or between the transformer windings . The dielectric properties of these components provide information about the quality of the insulation.
- Video: dielectric in the capacitor . Institute for Scientific Film (IWF) 2004, made available by the Technical Information Library (TIB), doi : 10.3203 / IWF / C-14819 .
- Arthur von Hippel, Editor: Dielectric Materials and Applications . Artech House, London, 1954, ISBN 0-89006-805-4 .