In "electromagnetism, absolute permittivity, often simply called permittivity, usually denoted by the Greek letter ε (epsilon), is the measure of resistance that is encountered when forming an "electric field in a particular "medium. More specifically, permittivity describes the amount of charge needed to generate one unit of "electric flux in a particular medium. Accordingly, a charge will yield more electric flux in a medium with low permittivity than in a medium with high permittivity. Thus, permittivity is the measure of a material's ability to resist an electric field, not its ability to ‘permit’ it (as the name ‘permittivity’ might seem to suggest).
The "SI unit for permittivity is "farad per "meter (F/m or F·m^{−1}).
The lowest possible permittivity is that of a vacuum. "Vacuum permittivity, sometimes called the electric constant, is represented by ε_{0} and has a value of approximately 8.85×10^{−12} F/m.
The permittivity of a "dielectric medium is often represented by the ratio of its absolute permittivity to the electric constant. This dimensionless quantity is called the medium’s "relative permittivity, sometimes also called "permittivity". Relative permittivity is also commonly referred to as the dielectric constant, a term which has been deprecated in physics and engineering^{[1]} as well as in chemistry.^{[2]}
By definition, a perfect vacuum has a relative permittivity of exactly 1. The difference in permittivity between a vacuum and air can often be considered negligible, as κ_{air} = 1.0006.
Relative permittivity is directly related to "electric susceptibility (χ), which is a measure of how easily a dielectric "polarizes in response to an "electric field, given by
otherwise written as
The standard SI unit for permittivity is Farad per meter (F/m or F·m^{−1}).
In "electromagnetism, the "electric displacement field D represents how an electric field E influences the organization of electric charges in a given medium, including charge migration and electric "dipole reorientation. Its relation to permittivity in the very simple case of linear, homogeneous, "isotropic materials with "instantaneous" response to changes in electric field is
where the permittivity ε is a "scalar. If the medium is "anisotropic, the permittivity is a second rank "tensor.
In general, permittivity is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a "nonlinear medium, the permittivity can depend on the strength of the electric field. Permittivity as a function of frequency can take on real or complex values.
In SI units, permittivity is measured in "farads per meter (F/m or A^{2}·s^{4}·kg^{−1}·m^{−3}). The displacement field D is measured in units of "coulombs per "square meter (C/m^{2}), while the electric field E is measured in "volts per meter (V/m). D and E describe the interaction between charged objects. D is related to the charge densities associated with this interaction, while E is related to the forces and potential differences.
The vacuum permittivity ε_{0} (also called permittivity of free space or the electric constant) is the ratio D/E in "free space. It also appears in the "Coulomb force constant,
Its value is^{[3]}
where
The constants c_{0} and μ_{0} are defined in SI units to have exact numerical values, shifting responsibility of experiment to the determination of the meter and the "ampere.^{[5]} (The approximation in the second value of ε_{0} above stems from π being an "irrational number.)
The linear permittivity of a homogeneous material is usually given relative to that of free space, as a relative permittivity ε_{r} (also called "dielectric constant, although this term is deprecated and sometimes only refers to the static, zerofrequency relative permittivity). In an anisotropic material, the relative permittivity may be a tensor, causing "birefringence. The actual permittivity is then calculated by multiplying the relative permittivity by ε_{0}:
where χ (frequently written χ_{e}) is the electric susceptibility of the material.
The susceptibility is defined as the constant of proportionality (which may be a "tensor) relating an "electric field E to the induced "dielectric "polarization density P such that
where ε_{0} is the "electric permittivity of free space.
The susceptibility of a medium is related to its relative permittivity ε_{r} by
So in the case of a vacuum,
The susceptibility is also related to the "polarizability of individual particles in the medium by the "ClausiusMossotti relation.
The "electric displacement D is related to the polarization density P by
The permittivity ε and "permeability µ of a medium together determine the "phase velocity v = c/n of "electromagnetic radiation through that medium:
The capacitance of a capacitor is based on its design and architecture, meaning it will not change with charging and discharging. The formula for capacitance is written as
where is the area of one plate, is the distance between the plates, and is the permittivity of the medium between the two plates. For a capacitor with relative permittivity , it can be said that
Permittivity is connected to electric flux (and by extension electric field) through "Gauss' Law. Gauss' Law states that for a closed "Gaussian surface, s
where is the net electric flux passing through the surface, is the charge enclosed in the Gaussian surface, is the electric field vector at a given point on the surface, and is a differential area vector on the Gaussian surface.
If the Gaussian surface uniformly encloses an insulated, symmetrical charge arrangement, the formula can be simplified to
where represents the angle between the electric field vector and the area vector.
If all of the electric field lines cross the surface at 90°, the formula can be further simplified to
Because the surface area of a sphere is , the electric field a distance away from a uniform, spherical charge arrangement is
where is "Coulomb's constant (). This formula applies to the electric field due to a point charge, outside of a conducting sphere or shell, outside of a uniformly charged insulating sphere, or between the plates of a spherical capacitor.
In general, a material cannot polarize instantaneously in response to an applied field, and so the more general formulation as a function of time is
That is, the polarization is a "convolution of the electric field at previous times with timedependent susceptibility given by χ(Δt). The upper limit of this integral can be extended to infinity as well if one defines χ(Δt) = 0 for Δt < 0. An instantaneous response would correspond to a "Dirac delta function susceptibility χ(Δt) = χδ(Δt).
It is convenient to take the "Fourier transform with respect to time and write this relationship as a function of frequency. Because of the "convolution theorem, the integral becomes a simple product,
This frequency dependence of the susceptibility leads to frequency dependence of the permittivity. The shape of the susceptibility with respect to frequency characterizes the "dispersion properties of the material.
Moreover, the fact that the polarization can only depend on the electric field at previous times (i.e. effectively χ(Δt) = 0 for Δt < 0), a consequence of "causality, imposes "Kramers–Kronig constraints on the susceptibility χ(0).
As opposed to the response of a vacuum, the response of normal materials to external fields generally depends on the "frequency of the field. This frequency dependence reflects the fact that a material's polarization does not change instantaneously when an electric field is applied. The response must always be causal (arising after the applied field), which can be represented by a phase difference. For this reason, permittivity is often treated as a complex function of the "(angular) frequency ω of the applied field:
(since complex numbers allow specification of magnitude and phase). The definition of permittivity therefore becomes
where
The response of a medium to static electric fields is described by the lowfrequency limit of permittivity, also called the static permittivity ε_{s} (also ε_{DC}):
At the highfrequency limit, the complex permittivity is commonly referred to as ε_{∞}. At the "plasma frequency and below, dielectrics behave as ideal metals, with electron gas behavior. The static permittivity is a good approximation for alternating fields of low frequencies, and as the frequency increases a measurable phase difference δ emerges between D and E. The frequency at which the phase shift becomes noticeable depends on temperature and the details of the medium. For moderate field strength (E_{0}), D and E remain proportional, and
Since the response of materials to alternating fields is characterized by a complex permittivity, it is natural to separate its real and imaginary parts, which is done by convention in the following way:
where
The choice of sign for timedependence, e^{−iωt}, dictates the sign convention for the imaginary part of permittivity. The signs used here correspond to those commonly used in physics, whereas for the engineering convention one should reverse all imaginary quantities.
The complex permittivity is usually a complicated function of frequency ω, since it is a superimposed description of "dispersion phenomena occurring at multiple frequencies. The dielectric function ε(ω) must have poles only for frequencies with positive imaginary parts, and therefore satisfies the "Kramers–Kronig relations. However, in the narrow frequency ranges that are often studied in practice, the permittivity can be approximated as frequencyindependent or by model functions.
At a given frequency, the imaginary part of ε̂ leads to absorption loss if it is positive (in the above sign convention) and gain if it is negative. More generally, the imaginary parts of the "eigenvalues of the anisotropic dielectric tensor should be considered.
In the case of solids, the complex dielectric function is intimately connected to band structure. The primary quantity that characterizes the electronic structure of any crystalline material is the probability of "photon absorption, which is directly related to the imaginary part of the optical dielectric function ε(ω). The optical dielectric function is given by the fundamental expression:^{[7]}
In this expression, W_{c,v}(E) represents the product of the "Brillouin zoneaveraged transition probability at the energy E with the joint "density of states,^{[8]}^{[9]} J_{c,v}(E); φ is a broadening function, representing the role of scattering in smearing out the energy levels.^{[10]} In general, the broadening is intermediate between "Lorentzian and "Gaussian;^{[11]}^{[12]} for an alloy it is somewhat closer to Gaussian because of strong scattering from statistical fluctuations in the local composition on a nanometer scale.
According to the "Drude model of magnetized plasma, a more general expression which takes into account the interaction of the carriers with an alternating electric field at millimeter and microwave frequencies in an axially magnetized semiconductor requires the expression of the permittivity as a nondiagonal tensor.^{[13]} (see also "Electrogyration).
If ε_{2} vanishes, then the tensor is diagonal but not proportional to the identity and the medium is said to be a uniaxial medium, which has similar properties to a "uniaxial crystal.
"ε_{r}″/"ε_{r}′  "Current "conduction  "Field "propagation 

0  "perfect dielectric lossless medium 

≪ 1  lowconductivity material poor conductor 
lowloss medium good dielectric 
≈ 1  lossy conducting material  lossy propagation medium 
≫ 1  highconductivity material good conductor 
highloss medium poor dielectric 
∞  "perfect conductor 
Materials can be classified according to their complexvalued permittivity ε, upon comparison of its real ε′ and imaginary ε″ components (or, equivalently, "conductivity, σ, when accounted for in the latter). A "perfect conductor has infinite conductivity, σ = ∞, while a "perfect dielectric is a material that has no conductivity at all, σ = 0; this latter case, of realvalued permittivity (or complexvalued permittivity with zero imaginary component) is also associated with the name lossless media.^{[14]} Generally, when σ/ωε′ ≪ 1 we consider the material to be a lowloss dielectric (although not exactly lossless), whereas σ/ωε′ ≫ 1 is associated with a good conductor; such materials with nonnegligible conductivity yield a large amount of "loss that inhibit the propagation of electromagnetic waves, thus are also said to be lossy media. Those materials that do not fall under either limit are considered to be general media.
In the case of a lossy medium, i.e. when the conduction current is not negligible, the total current density flowing is:
where
The size of the "displacement current is dependent on the "frequency ω of the applied field E; there is no displacement current in a constant field.
In this formalism, the complex permittivity is defined as:^{[15]}
In general, the absorption of electromagnetic energy by dielectrics is covered by a few different mechanisms that influence the shape of the permittivity as a function of frequency:
The above effects often combine to cause nonlinear effects within capacitors. For example, dielectric absorption refers to the inability of a capacitor that has been charged for a long time to completely discharge when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage, a phenomenon that is also called soakage or battery action. For some dielectrics, such as many polymer films, the resulting voltage may be less than 1–2% of the original voltage. However, it can be as much as 15–25% in the case of "electrolytic capacitors or "supercapacitors.
In terms of "quantum mechanics, permittivity is explained by "atomic and "molecular interactions.
At low frequencies, molecules in polar dielectrics are polarized by an applied electric field, which induces periodic rotations. For example, at the "microwave frequency, the microwave field causes the periodic rotation of water molecules, sufficient to break "hydrogen bonds. The field does work against the bonds and the energy is absorbed by the material as "heat. This is why microwave ovens work very well for materials containing water. There are two maxima of the imaginary component (the absorptive index) of water, one at the microwave frequency, and the other at far ultraviolet (UV) frequency. Both of these resonances are at higher frequencies than the operating frequency of microwave ovens.
At moderate frequencies, the energy is too high to cause rotation, yet too low to affect electrons directly, and is absorbed in the form of resonant molecular vibrations. In water, this is where the absorptive index starts to drop sharply, and the minimum of the imaginary permittivity is at the frequency of blue light (optical regime).
At high frequencies (such as UV and above), molecules cannot relax, and the energy is purely absorbed by atoms, exciting "electron energy levels. Thus, these frequencies are classified as "ionizing radiation.
While carrying out a complete "ab initio (that is, firstprinciples) modelling is now computationally possible, it has not been widely applied yet. Thus, a phenomenological model is accepted as being an adequate method of capturing experimental behaviors. The "Debye model and the "Lorentz model use a firstorder and secondorder (respectively) lumped system parameter linear representation (such as an RC and an LRC resonant circuit).
The relative permittivity of a material can be found by a variety of static electrical measurements. The complex permittivity is evaluated over a wide range of frequencies by using different variants of "dielectric spectroscopy, covering nearly 21 orders of magnitude from 10^{−6} to 10^{15} "hertz. Also, by using "cryostats and ovens, the dielectric properties of a medium can be characterized over an array of temperatures. In order to study systems for such diverse excitation fields, a number of measurement setups are used, each adequate for a special frequency range.
Various microwave measurement techniques are outlined in Chen et al..^{[16]} Typical errors for the HakkiColeman method employing a puck of material between conducting planes are about 0.3%.^{[17]}
At infrared and optical frequencies, a common technique is "ellipsometry. "Dual polarisation interferometry is also used to measure the complex refractive index for very thin films at optical frequencies.