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Relative permittivity
From Citizendium, the Citizens' Compendium
Contents |
In physics, in particular in electrostatics, the relative permittivity (also known by the now obsolescent term dielectric constant) is an intrinsic property of a non-conducting material, a dielectric. It is usually denoted by ε_{r}. For simple dielectrics, using SI units, ε_{r} is related to the proportionality constant between the electric field E and the electric displacement D, namely D = ε_{r} ε_{0} E, where ε_{0} is the electric constant. The relative permittivity describes the ease by which a dielectric medium may be polarized.
A related quantity is the electric susceptibility, denoted by χ_{e}, related to the relative permittivity in SI units by:^{[1]}
Empirically it is observed that the relative permittivity ε_{r} ≥ 1. Only classical vacuum has ε_{r} = 1 (exact); all dielectrics have values larger than one (but some materials, such as non-dense inert gases, have relative permittivities very close to the vacuum value of unity).
Relation to polarization
The electric displacement D is related to the electric polarization of a dielectric medium P as:
In a simple dielectric material, the polarization induced by E is simply proportional to the field and is given in terms of the medium's dielectric susceptibility as:
Consequently, the electric displacement in such a dielectric medium is simply:^{[1]}
Here ε is referred to as the dielectric permittivity.
In implementing these relations, it should be noted that the use of a constant as the relative permittivity of a substance is an approximation, even for quantum vacuum. A more complete representation recognizes that all media exhibit departures from this approximation, in particular, a dependence on field strength, a dependence upon the rate of variation of the field in both time and space, and a dependence upon the direction of the field. In many materials these dependencies are slight; in others, like ferroelectrics, they are pronounced.
Relation to forces
When two electric charges are placed inside a dielectric, the electrostatic force between the charges is changed by a factor 1/ε_{r}.
Some values
An extensive list of values for the relative permittivities of a number of dielectrics is found on the web.^{[2]}
Definition by capacitance of parallel-plate capacitor
An common alternative definition invokes parallel-plate capacitors. In order to make the connection with the definition above, we recall that the electric field above a charged plate of infinite size is independent of the distance from the plate. (This will be shown later in a separate section.) If σ is the charge density on the plate (charge per surface, in SI units C/m^{2}), the strength E of the field E is given by
- E = σ/(2 ε_{0}ε_{r}),
where ε_{0} is the electric constant. In Gaussian units one may take ε_{0} = 1. When the charge density σ is positive the electric field (a vector) E points away from the plate. Of course, plates of infinite size do not exist, but this formula is applicable when the height is much smaller than the dimensions of the plate, so that border effects can be neglected.
In parallel-plate capacitors border effects can usually be ignored and because both plates have the same charge density (but of opposite sign), the electric field inside a capacitor, filled with a dielectric with ε_{r}, is double that of one plate
- E = σ/(ε_{0}ε_{r}).
If the plates have surface area A, they carry a total charge Q = σ A (positive on one plate, negative on the other),
- Q = ε_{0}ε_{r} A E.
Say the distance between the plates is d, then the voltage difference V between the plates is E / d.
The capacitance C of a capacitor is by definition Q / V, so that we find that the capacitance of a parallel-plate capacitor is linear in the relative permittivity ε_{r}:
- C = ε_{r} C_{0} with C_{0} ≡ ε_{0} A / d.
Clearly, C_{0} is the capacity with the reference medium of classical vacuum between the plates, and in principle one may define
In practice, the measurement is compared with some real medium that approximates the ideal reference medium of classical vacuum. For example, one could use an ideal gas as the comparison medium and measure the permittivity as the pressure of this gas was reduced. By fitting a theoretical expression relating the permittivity to the gas pressure, one can extrapolate to zero pressure to approximate the comparison to the ideal classical vacuum.
This comparison definition of relative permittivity is common, and reads in words: ε_{r} is equal to the ratio of the capacitance of a capacitor filled with the dielectric to the capacitance of an identical capacitor in a vacuum without the dielectric material.
Because ε_{r} > 1, the insertion of a dielectric between the plates of a parallel-plate capacitor always increases its capacitance, or ability to store opposite charges on each plate.
This comparative approach to determining relative permittivity is a macroscopic measurement of dielectrics, an averaged property that can be deduced in principle from the underlying electrical behavior of the material on the atomic scale.
Generalizations
The relative permittivity of a dielectric is a function of temperature and, when the dielectric is a gas, also of pressure. (The pressure dependence of solid and liquid dielectrics is usually very weak.)
For low-symmetry dielectrics (solids and liquid crystals) it may happen that the constant is a second rank tensor, which is represented with respect to a Cartesian coordinate system by a 3 × 3 matrix
For frequency-dependent (time-dependent) electric fields the relative permittivity is in general a function of the angular frequency ω. The function ε_{r}(ω) may vary strongly as function of ω, especially for ω-values in the visible or UV part of the electromagnetic spectrum. It may also happen that a dielectric absorbs energy of an electromagnetic wave that hits it (dielectric loss). This absorption is described by extending the permittivity into the complex plane; the real part of ε_{r}(ω) has the meaning given above, while its imaginary part describes the energy absorption.
Electric field above infinite plate
To compute E we let the charged plate be in the x-y plane. It is a priori clear from the symmetry of the problem that the field at a point z on the z-axis has only a z component, the field components parallel to the plate are always compensated by corresponding negative components.
Use that an infinitesimal surface element in cylinder coordinates times the surface charge density σ gives an infinitesimal charge in the plate,
where σ is assumed constant over the plate and for convenience sake we take it positive (hence the field points in the positive z direction). The electric field in the z direction at a point (0,0,z) is by Coulomb's law
Integration over φ gives a factor 2π and
Often one defines the electric displacement by D ≡ ε_{0}ε_{r}E. Cleary D_{z} depends only on the charge density and not on the permittivity,
The fact that E and D do not depend on the distance of the field point to the plate may be compared with the gravitational force, −mg, being independent of height close to the surface of the Earth.
Notes
- ↑ ^{1.0} ^{1.1} For example see Keqian Zhang, Dejie Li (2007). Electromagnetic Theory for Microwaves and Optoelectronics, 2nd ed. Springer, p. 8. ISBN 3540742956.
- ↑ For example, see: Dielectric constant values. Clipper Controls. Retrieved on 2011-04-22.