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Capacitor dielectric constant & permittivity
- description of capacitor permittivity and dielectric constant with equations and a table of values for common materials and substances
Capacitor, capacitance theory , etc includes:
• Capacitor / capacitance basics • Capacitor equations • Dielectric constant and permittivity • ESR, loss tangent and Q • Capacitor conversion chart / table • Capacitor markingsPermittivity and dielectric constant are two terms that are at the very heart of capacitor technology. The dielectric is the material that provides the insulation between the capacitor plates, and many of the characteristics of the capacitor will be dependent upon the properties of the dielectric used.
Capacitor permittivity and dielectric constant
The terms permittivity and dielectric constant are essentially the same for most purposes, although care must be taken when interesting some of the terms are relative permittivity and other terms have some specific meanings.
It is that property of a dielectric material that determines how much electrostatic energy can be stored per unit of volume when unit voltage is applied, and as a result it is of great importance for capacitors and capacitance calculations and the like.
In general permittivity uses the Greek letter epsilon as its symbol: ε.
Definitions of some specific terms related to dielectric constant and permittivity are given below:
- Absolute permittivity: is the measure of permittivity in a vacuum and it is how much resistance is encountered when forming an electric field in a vacuum. The absolute permittivity is normally symbolised by ε0. The permittivity of free space - a vacuum - is equal to approximately 8.85 x 10-12 Farads / metre (F/m)
- Relative permittivity: is permittivity of a given material relative to that of the permittivity of a vacuum. It is normally symbolised by: εr.
- Static permittivity: of a material is its permittivity when exposed to a static electric field. Often a low frequency limit is placed on the material for this measurement. A static permittivity is often required because the response of a material is a complex relationship related to the frequency of the applied voltage.
- Dielectric constant: This is the relative permittivity for a substance or material.
It can be seen from the definitions or permittivity that constants are related according toth e following equation:
Where:
εr = relative permittivity
εs = permittivity of the substance in Farads per metre
ε0 = permittivity of a vacuum in Farads per metre
Choice of capacitor dielectric
Capacitors use a variety of different substances as their dielectric material. The material is chosen for the properties it provides. One of the major reasons for the choice of a particular dielectric material is its dielectric constant. Those with a high dielectric constant enable high values of capacitance to be achieved - each one having a different permittivity or dielectric constant. This changes the amount of capacitance that the capacitor will have for a given area and spacing.
The dielectric will also need to be chosen to meet requirements such as insulation strength - it must be able to withstand the voltages placed across it with the thickness levels used. It must also be sufficiently stable with variations in temperature, humidity, and voltage, etc.
Relative permittivity of common substances
The table below gives the relative permittivity of a number of common substances.
| Substance | Relative Permittivity |
|---|---|
| Ebonite | 2.7 - 2.9 |
| Glass | 5 - 10 |
| Marble | 8.3 |
| Mica | 5.6 - 8.0 |
| Paraffin wax | 2 - 2.4 |
| Porcelain | 4.5 - 6.7 |
| Rubber | 2.0 - 2.3 |
| Calcium titanate | 150 |
| Strontium titanate | 200 |
| Air 0C | 1.000594 |
| Air 20C | 1.000528 |
| Carbon monoxide 25C | 1.000634 |
| Carbon dioxide 25C | 1.000904 |
| Hydrogen 0C | 1.000265 |
| Helium 25C | 1.000067 |
| Nitrogen 25C | 1.000538 |
| Sulphur dioxide 22C | 1.00818 |
The values given above are what may be termed the "static" values of permittivity. They are true for steady state or low frequencies. It is found that the permittivity of a material usually decreases with increasing frequency. It also falls with increasing temperature. These factors are normally taken into account when designing a capacitor for electronics applications. Some materials have a more stable level of permittivity and hence they are used in the higher tolerance capacitors. However this often has to be balanced against other factors. Some materials have very high levels of permittivity, and hence they enable capacitors to be made much smaller. This factor may be particularly useful when the size of the capacitor is particularly important.
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