The dielectric constant is the ratio between the absolute permittivity of a material and the absolute permittivity of a vacuum. The technical definition of "dielectric constant" or "relative permittivity" is complicated and still debated among electrical engineers. This is because the permittivity of a material depends on the frequency of the applied voltage. The term "static dielectric constant" is used to describe this ratio when a direct current or zero frequency voltage is applied.
Capacitors are devices that store electrical charges. They have a number of uses in electronics because of their unique characteristics. They are used as filters in power supplies to remove fluctuations in output voltage, as timing constants in oscillator circuits and to block direct current (DC) voltage while passing alternating current (AC) voltage in amplifier circuits.
A capacitor has two plates or ribbons of conductive material separated by an insulator. When a direct current is applied across the capacitor, a charge builds up between the plates. To prevent a loss of voltage across the gap between the plates, an insulator is placed between the plates. This insulator is known as the dielectric.
The term "constant" is misleading when referring to the dielectric or permittivity value of an insulator. As the applied frequency changes, so does the dielectric constant. The term generally used for the frequency-dependent dielectric value is "relative dielectric constant."
As the frequency increases, the relative dielectric constant decreases. As a result, capacitors are designed to operate within a given frequency range. One dielectric material might be suitable for use with lower frequencies, but it might not be able to function properly when exposed to high-frequency voltage. Capacitors in high-frequency circuits need insulators that have a much higher dielectric constant.
Some dielectric insulators actually contribute to the formation of the electric field in a capacitor. They do this by helping to concentrate and align the electric field. This characteristic, along with the material’s insulating ability, determine the dielectric constant or relative permittivity of a given material.