The dielectric constant and its effects on the properties of a capacitor

Written by Anthony Kenny

November 15, 2017

A typical capacitor comprises two conductive plates and a non-conductive dielectric material. The dielectric material separates the two conductive metal electrode plates. Applying voltage to the electrode plates of a capacitor causes an electric field in the non-conductive dielectric material. This electric field stores energy. The dielectric constant, also commonly known as relative permittivity, is the measure of the ability of a material to store electrical energy, and is one of the key properties of a dielectric material.

The capacitance of a parallel plate capacitor is a function of distance between plates, plate area, and dielectric material constant. An increase in plate area and dielectric constant results in an increase in capacitance while an increase in the separation distance between the plates results in a decrease in capacitance. Different dielectric materials have different dielectric constants.

Effects of dielectric constant on the characteristics of a capacitor

The dielectric material of a capacitor polarizes when voltage is applied. This process reduces the electric field and causes negatively charged electrons to shift slightly towards the positive terminal. Although the electrons do not shift far enough to cause a flow of current, the process creates an effect that is critical to the operation of capacitors. Removing the source of voltage causes the dielectric material to lose polarization. However, if the material has weak molecular bonds, it can remain in polarized state even when the source of voltage is removed.

A capacitor stores energy in the electric field when a voltage is applied. The capacity to store electrical energy varies from one dielectric material to another. The amount of electrical energy that a capacitor can store is influenced by the amount of polarization that occurs when voltage is applied. Materials with high dielectric constants can store more energy compared to those with low dielectric constants. The electric susceptibility of a material is a measure of the ease with which it polarizes in response to an electric field. Good dielectric materials have high electric susceptibility.

The dielectric constant is one of the key parameters to consider when selecting a dielectric material for a capacitor. This constant is measured in farads per meter and determines the amount of capacitance that a capacitor can achieve. Dielectric materials with high dielectric constants are used when high capacitance values are required, although, as mentioned above, other parameters that determine the capacitance of a capacitor include the spacing between the electrodes and the effective plate area.

Dielectric constants of common dielectric materials

All materials are capable of storing electrical energy when they are exposed to an electric field. The storage capacity varies from one material to another. The permittivity of materials is usually given relative to the permittivity of free space, usually symbolized by ϵ0. The permittivity of vacuum is commonly known as absolute permittivity and refers to the amount of resistance required to form an electric field in a vacuum. The absolute permittivity of free space is approximately 8.85418782 × 10-12 m-3 kg-1 s4 A2.

The permittivity of a dielectric material relative to that of free space is referred to as relative permittivity, usually symbolized by ϵr, or dielectric constant.

The table below shows the dielectric constants of commonly used dielectric materials.

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Variations in temperature cause discontinuities in the permittivity of a dielectric material, and have a significant effect on the dielectric constant of a material. For instance, an increase in temperature causes a decrease in permittivity, and the dielectric constant of a material drops sharply when the temperature falls below the freezing point.

When selecting a dielectric material for a capacitor, it is also important to consider the effect of frequency on the material’s properties. The permittivity that a material exhibits when it is exposed to an electric field is dependent on the frequency of the voltage source. When a material is placed in a static electric field, the permittivity that it exhibits is referred to as static permittivity. The permittivity of a material decreases with an increase in frequency of the voltage source.

A primary drive today is towards circuit miniaturisation. To produce miniature circuits components with a smaller footprint are required. The dielectric constant of a capacitor determines the capacitance that can be achieved. Dielectric materials with high dielectric constants are used when capacitors with smaller physical sizes are required.

Apart from dielectric constant, it is also important to consider dielectric loss and dielectric strength when selecting a dielectric material for a capacitor. The dielectric strength is a measure of the voltage that an insulator will withstand before it allows current to flow through it. The dielectric loss refers to the energy that a dielectric material dissipates when a variable voltage is applied.

Conclusion

A dielectric material is used to separate the conductive plates of a capacitor.  This insulating material significantly determines the properties of a component. The dielectric constant of a material determines the amount of energy that a capacitor can store when voltage is applied. A dielectric material becomes polarized when it is exposed to an electric field. When polarization occurs, the effective electric field is reduced. Since the permittivity of a material is dependent on frequency and temperature, the dielectric constant is usually given at specific conditions, usually at low frequencies. Moreover, the dielectric constant of a material is usually given relative to the permittivity of free space.

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