The values below are relative permittivity \(\epsilon_r \triangleq \epsilon/\epsilon_0\) for a few materials that are commonly encountered in electrical engineering applications, and for which permittivity emerges as a consideration. Note that “relative permittivity” is sometimes referred to as dielectric constant.
Here we consider only the physical (real-valued) permittivity, which is the real part of the complex permittivity (typically indicated as \(\epsilon'\) or \(\epsilon_r'\)) for materials exhibiting significant loss.
Permittivity varies significantly as a function of frequency. The values below are representative of frequencies from a few kHz to about 1 GHz. The values given are also representative of optical frequencies for materials such as silica that are used in optical applications. Permittivity also varies as a function of temperature. In applications where precision better than about 10% is required, primary references accounting for frequency and temperature should be consulted. The values presented here are gathered from a variety of references, including those indicated in “Additional References.”
Free Space (vacuum): \(\epsilon_r \triangleq 1\)
Solid Dielectrics:
| Material | \(\epsilon_r\) | Common uses |
| Styrofoam\(^1\) | 1.1 | |
| Teflon\(^2\) | 2.1 | |
| Polyethylene | 2.3 | coaxial cable |
| Polypropylene | 2.3 | |
| Silica | 2.4 | optical fiber\(^3\) |
| Polystyrene | 2.6 | |
| Polycarbonate | 2.8 | |
| Rogers RO3003 | 3.0 | PCB substrate |
| FR4 (glass epoxy laminate) | 4.5 | PCB substrate |
\(^1\) Properly known as extruded polystyrene foam (XPS).
\(^2\) Properly known as polytetrafluoroethylene (PTFE).
\(^3\) Typically doped with small amounts of other materials to slightly raise or lower the index of refraction
(\(=\sqrt{\epsilon_r}\)).
Non-conducting spacing materials used in discrete capacitors exhibit \(\epsilon_r\) ranging from about 5 to 50.
Semiconductors commonly appearing in electronics – including carbon, silicon, geranium, indium phosphide, and so on – typically exhibit \(\epsilon_r\) in the range 5–15.
Glass exhibits \(\epsilon_r\) in the range 4–10, depending on composition.
Gasses, including air, typically exhibit \(\epsilon_r\cong 1\) to within a tiny fraction of a percent.
Liquid water typically exhibits \(\epsilon_r\) in the range 72–81. Distilled water exhibits \(\epsilon_r \approx 81\) at room temperature, whereas sea water tends to be at the lower end of the range.
Other liquids typically exhibit \(\epsilon_r\) in the range 10–90, with considerable variation as a function of temperature and frequency. Animal flesh and blood consists primarily of liquid matter and so also exhibits permittivity in this range.
Soil typically exhibits \(\epsilon_r\) in the range 2.5–3.5 when dry and higher when wet. The permittivity of soil varies considerably depending on composition.
Additional Reading:
“Relative permittivity” on Wikipedia.