Definition:Vacuum Permittivity

Physical Constant
The vacuum permittivity is the physical constant denoted $\varepsilon_0$ defined as:


 * $\varepsilon_0 := \dfrac {e^2} {2 \alpha h c}$

where:
 * $e$ is the elementary charge
 * $\alpha$ is the fine-structure constant
 * $h$ is Planck's constant
 * $c$ is the speed of light defined in $\mathrm m \, \mathrm s^{-1}$

Of the above, only the fine-structure constant $\alpha$ is a measured value; the others are defined.

It can be defined as the capability of an electric field to permeate a vacuum.

From Value of Vacuum Permittivity, it has the value:


 * $\varepsilon_0 = 8 \cdotp 85418 \, 78128 (13) \times 10^{-12} \, \mathrm F \, \mathrm m^{-1}$ (farads per metre)

It can equivalently be defined as:


 * $\varepsilon_0 := \dfrac 1 {\mu_0 c^2}$

where:
 * $\mu_0$ is the vacuum permeability defined in $\mathrm H \, \mathrm m^{-1}$ (henries per metre)
 * $c$ is the speed of light defined in $\mathrm m \, \mathrm s^{-1}$

Also known as
The vacuum permittivity is also known by the terms:


 * permittivity of free space
 * permittivity of empty space
 * permittivity in (or of) vacuum
 * distributed capacitance of the vacuum

The term electric constant has now apparently been accepted by many standards organisations worldwide.

However, as from $20$ May $2019$, the definition of $\varepsilon_0$ is no longer as a defined constant, but derived ultimately from the fine-structure constant, which is a measured value.

Hence many authorities (including ) prefer not to use electric constant.

The following terms are more or less obsolete:


 * dielectric constant
 * dielectric constant of vacuum

Note that the term dielectric constant is also still used sometimes to mean the absolute permittivity (of a material), and so suffers the additional problem of being ambiguous.

Some sources use $\epsilon_0$ instead of $\varepsilon_0$.

Either symbol is acceptable, but (having to choose one or the other) has settled on $\varepsilon_0$ as standard.

Also see

 * Value of Vacuum Permittivity
 * Definition:Vacuum Permeability