Table 1 gives the relative permittivity ε (often called the dielectric constant) of some common gases at a temperature of 20 °C and pressure of one atmosphere (101.325 kPa). Values of the permanent dipole moment µ are also included. The density dependence of the permittivity is given by the equation
where ρm is the molar density, N is Avogadro’s number, k is the Boltzmann constant, T is the temperature, and α is the molecular polarizability. Therefore, in regions where the gas can be considered ideal, ε – 1 is approximately proportional to the pressure at constant temperature. For nonpolar gases (µ = 0), ε –1 is inversely proportional to temperature at constant pressure.
The number of significant figures indicates the accuracy of the values given. The values of ε for air, Ar, H2, He, N2, O2, and CO2 are recommended as reference values; these are accurate to 1 ppm or better. Column definitions for Table 1 are as follows.
Column heading | Definition |
Name | Name of gas; listed alphabetically by name |
Mol. form. | Molecular formula for gas |
ε | Relative permittivity (dielectric constant) at 20 °C and 1 atmosphere pressure (101.325 kPa) |
µ | Permanent dipole moment, in Debye units (1 D = 3.33564 × 10–30 C m) |
Table 2 gives the permittivity of water vapor in equilibrium with liquid water as a function of temperature (derived from Ref. 4).
Name | Mol. form. | ε | μ/D |
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Acetylene | HC≡CH | 1.00124 | 0 |
Aira | 1.0005360 | 0 | |
Ammonia | NH3 | 1.00622 | 1.4718 |
Argon | Ar | 1.0005169 | 0 |
Boron trifluoride | BF3 | 1.0011 | 0 |
Bromomethane | CH3Br | 1.01028 | 1.8203 |
Butane | C4H10 | 1.00266 | 0.05 |
Carbon dioxide | CO2 | 1.0009217 | 0 |
Carbon monoxide | CO | 1.00065 | 0.10980 |
Chloroethane | C2H5Cl | 1.01325 | 2.05 |
Chloroethene | CH2=CHCl | 1.0075 | 1.45 |
Chloromethane | CH3Cl | 1.01080 | 1.8963 |
Cyclopropane | C3H6 | 1.00178 | 0 |
Dimethyl ether | CH3OCH3 | 1.0062 | 1.30 |
Ethane | C2H6 | 1.001403 | 0 |
Ethylene | CH2=CH2 | 1.00135 | 0 |
Fluoromethane | CH3F | 1.00973 | 1.858 |
Helium | He | 1.0000645 | 0 |
Hydrogen | H2 | 1.0002532 | 0 |
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