Absolute and Relative Permittivity of a Medium
While discussing electrostatic phenomenon, a certain property of the medium called its permittivity plays an important role. Every medium is supposed to possess two permittivities :
(i) absolute permittivity (ε)
and
(ii) relative permittivity (εr).
For measuring relative permittivity, vacuum or free space is chosen as the reference medium. It has an absolute permittivity of 8.854 × 10−12 F/m
Absolute permittivity ε0 = 8.854 × 10
−12 F/m
Relative permittivity, εr = 1
Being a ratio of two similar quantities, εr has no units.
Now, take any other medium. If its relative permittivity, as compared to vacuum is εr, then its
absolute permittivity is ε = ε0 εr F/m
If, for example, relative permittivity of mica is 5, then, its absolute permittivity is
ε = ε0 εr = 8.854 × 10−12 × 5 = 44.27 × 10−12 F/m
Definitions :
Definitions of some specific terms related to dielectric constant and permittivity are given below:
- Absolute permittivity: Absolute permittivity is defined as 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 / meter (F/m)
- Relative permittivity: Relative permittivity is defined as the permittivity of a given material relative to that of the permittivity of a vacuum. It is normally symbolised by: εr.
- Static permittivity: The static permittivity of a material is defined as 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: The dielectric constant is defined as the relative permittivity for a substance or material.
Relative permittivity of common substances
The table below gives the relative permittivity of a number of common substances.
| RELATIVE PERMITTIVITY OF COMMON SUBSTANCES | |
| SUBSTANCE | RELATIVE PERMITTIVITY |
| Calcium titanate | 150 |
| FR4 PCB material | 4.8 typically |
| Glass | 5 – 10 |
| Mica | 5.6 – 8.0 |
| Paper | 3.85 |
| Polyethylene) | 2.25 |
| Polyimide | 2.25 |
| Polypropylene | 2.2 – 2.36 |
| Porcelain (ceramic) | 4.5 – 6.7 |
| PTFE (Teflon) | 2.1 |
| Rubber | 2.0 – 2.3 |
| Silicon | 11.68 |
| Silicon dioxide | 3.9 |
| Strontium titanate | 200 |
| Air 0°C | 1.000594 |
| Air 20°C | 1.000528 |
| Carbon monoxide 25°C | 1.000634 |
| Carbon dioxide 25°C | 1.000904 |
| Hydrogen 0°C | 1.000265 |
| Helium 25°C | 1.000067 |
| Nitrogen 25°C | 1.000538 |
| Sulphur dioxide 22°C | 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.
When the design of a capacitor is undertaken the characteristics of the dielectric form one of the main decisions about the capacitor.
Some materials have a very stable dielectric constant and can be used in high stability capacitors, whereas other dielectric materials enable very high levels of volumetric capacitance to be achieved, i.e. high levels of capacitance in a small volume. Normally there is a balance as no single dielectric has ideal characteristics for everything.
Although ceramic capacitors are very popular there are many different ceramics that can be used. These give rise to ceramic capacitors being denoted by the various names for the ceramic performance levels: C0G, Y5V, X7R, NP0, etc.
Read article – Units of Resistivity
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