The relative permittivity or dielectric constant is a dimensionless physical constant that describes how an electric field affects a material. It is the ratio of the permittivity of a substance to the permittivity of free space. The relative permittivity of a material is a measure of its ability to store electric energy in an electrical field.
The relative permittivity of a material is typically denoted as ?r(?) and is defined as the ratio of the complex frequency-dependent permittivity of the material to the vacuum permittivity. It is a dimensionless number that is generally complex-valued; its real and imaginary parts are denoted as:
Re(?r(?)) and Im(?r(?)) respectively.
The relative permittivity of a medium is related to its electric susceptibility, ?e, as ?r(?) = 1 + ?e. In anisotropic media (such as non-cubic crystals), the relative permittivity is a second-rank tensor.
The relative permittivity of a material for a frequency of zero is known as its static relative permittivity.
The relative permittivity of a material is a fundamental property that affects the Coulomb force between two point charges in the material. Relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum. Likewise, relative permittivity is the ratio of the capacitance of a capacitor using that material as a dielectric, compared with a similar capacitor that has vacuum as its dielectric.
Relative permittivity is also commonly known as the dielectric constant, a term still used but deprecated by standards organizations in engineering as well as in chemistry.
The relative permittivity of a material depends on various factors such as temperature, pressure, frequency, and humidity. The relative permittivity of some materials at room temperature under 1 kHz is given in the following table:
| Material | Relative Permittivity |
|————–|————————–|
| Vacuum | 1 (by definition) |
| Air | 1.000 589 86 ± 0.000 000 50 (at STP, 900 kHz) |
| PTFE /Teflon | 2.1 |
| Polyethylene /XLPE | 2.25 |
| Polyimide | 3.4 |
| Polypropylene | 2.22.36 |
| Polystyrene | 2.42.7 |
| Carbon disulfide | 2.6 |
| BoPET | 3.1 |
| Paper, printing | 1.4 (200 kHz) |
| Electroactive polymers | 212 |
| Mica | 36 |
| Silicon dioxide | 3.9 |
| Sapphire | 8.911.1 (anisotropic) |
| Concrete | 4.5 |
| Pyrex (glass)| 4.7 (3.710) |
| Neoprene | 6.7 |
| Natural rubber | 7 |
| Diamond