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The physical constant ** ε_{0}** (pronounced as “epsilon nought” or “epsilon zero”), commonly called the

A **physical constant**, sometimes **fundamental physical constant** or **universal constant**, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant, which has a fixed numerical value, but does not directly involve any physical measurement.

In electromagnetism, **absolute permittivity**, often simply called **permittivity**, usually denoted by the Greek letter ε (epsilon), is the measure of capacitance 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. Permittivity is the measure of a material's ability to store an electric field in the polarization of the medium.

- Value
- Redefinition of the SI units
- Terminology
- Historical origin of the parameter ε0
- Rationalization of units
- Determination of a value for ε0
- Permittivity of real media
- See also
- Notes

*ε*_{0}= 187817...×10^{−12}F⋅m^{−1}8.854 ( farad s per metre ).^{ [1] }

It is the capability of the vacuum to permit electric field lines. This constant relates the units for electric charge to mechanical quantities such as length and force.^{ [2] } For example, the force between two separated electric charges (in the vacuum of classical electromagnetism) is given by Coulomb's law:

An **electric field** is a vector field surrounding an electric charge that exerts force on other charges, attracting or repelling them. Mathematically the electric field is a vector field that associates to each point in space the force, called the Coulomb force, that would be experienced per unit of charge by an infinitesimal test charge at that point. The units of the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m). Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature.

**Electric charge** is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges; *positive* and *negative*. Like charges repel and unlike attract. An object with an absence of net charge is referred to as *neutral*. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.

**Coulomb's law**, or **Coulomb's inverse-square law**, is a law of physics for quantifying Coulomb's force, or electrostatic force. Electrostatic force is the amount of force with which stationary, electrically charged particles either repel, or attract each other. This force and the law for quantifying it, represent one of the most basic forms of force used in the physical sciences, and were an essential basis to the study and development of the theory and field of classical electromagnetism. The law was first published in 1785 by French physicist Charles-Augustin de Coulomb.

The value of the constant fraction is approximately 9 × 10^{9} N⋅m^{2}⋅C^{−2}, *q*_{1} and *q*_{2} are the charges, and *r* is the distance between them. Likewise, *ε*_{0} appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources.

**Maxwell's equations** are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematical model for electric, optical, and radio technologies, such as power generation, electric motors, wireless communication, lenses, radar etc. Maxwell's equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields. One important consequence of the equations is that they demonstrate how fluctuating electric and magnetic fields propagate at the speed of light. Known as electromagnetic radiation, these waves may occur at various wavelengths to produce a spectrum from radio waves to γ-rays. The equations are named after the physicist and mathematician James Clerk Maxwell, who between 1861 and 1862 published an early form of the equations that included the Lorentz force law. He also first used the equations to propose that light is an electromagnetic phenomenon.

A **magnetic field** is a vector field that describes the magnetic influence of electrical currents and magnetized materials. In everyday life, the effects of magnetic fields are often seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field varies with location. As such, it is an example of a vector field.

In physics, **electromagnetic radiation** refers to the waves of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.

The value of *ε*_{0} is currently *defined* by the formula^{ [3] }

where *c* is the defined value for the speed of light in classical vacuum in SI units,^{ [4] } and *μ*_{0} is the parameter that international Standards Organizations call the "magnetic constant" (commonly called vacuum permeability). Since *μ*_{0} has the *defined* value 4π × 10^{−7} H/m,^{ [5] } and *c* has the *defined* value 299792458 m⋅s^{−1},^{ [6] } it follows that *ε*_{0} can be expressed numerically as

The **speed of light** in vacuum, commonly denoted * c*, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second. It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relativity,

The **International System of Units** is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.

The **henry** is the SI derived unit of electrical inductance. If a current of 1 ampere flowing through the coil produces flux linkage of 1 weber turn, the coil has a self inductance of 1 henry. The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

- (or A
^{2}⋅s^{4}⋅kg^{−1}⋅m^{−3}in SI base units, or C^{2}⋅N^{−1}⋅m^{−2}or C⋅V^{−1}⋅m^{−1}using other SI coherent units).^{ [7] }^{ [8] }

The historical origins of the electric constant *ε*_{0}, and its value, are explained in more detail below.

Under the accepted proposals to redefine the ampere as a fixed number of elementary charges per second in 2019,^{ [9] } the electric constant will no longer have an exact fixed value. The value of the electron charge becomes a defined number, not measured, making *μ*_{0} a measured quantity. Consequently, *ε*_{0} will not be exact. As before, it will be defined by the equation *ε*_{0} = 1/(*μ*_{0}*c*^{2}), and is thus determined by the value of *μ*_{0}, the magnetic vacuum permeability which in turn is determined by the experimentally determined dimensionless fine-structure constant *α*:

The **ampere**, often shortened to "amp", is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The **elementary charge**, usually denoted by * e* or sometimes

The physical constant *μ*_{0},, commonly called the **vacuum permeability**, **permeability of free space**, **permeability of vacuum**, or **magnetic constant**, is the magnetic permeability in a classical vacuum. *Vacuum permeability* is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum.

with *e* the exact elementary charge, *h* the exact Planck constant, and *c* the exact speed of light in vacuum.

The relative uncertainty in the value of *ε*_{0} therefore would be the same as that for the dimensionless fine-structure constant, 1/137.035999046(27) according to a new measurement.^{ [10] }

Historically, the parameter *ε*_{0} has been known by many different names. The terms "vacuum permittivity" or its variants, such as "permittivity in/of vacuum",^{ [11] }^{ [12] } "permittivity of empty space",^{ [13] } or "permittivity of free space"^{ [14] } are widespread. Standards Organizations worldwide now use "electric constant" as a uniform term for this quantity,^{ [7] } and official standards documents have adopted the term (although they continue to list the older terms as synonyms).^{ [15] }^{ [16] } In the new SI system, the permittivity of vacuum will not be a constant anymore, but a measured quantity, related to the (measured) dimensionless fine structure constant.

Another historical synonym was "dielectric constant of vacuum", as "dielectric constant" was sometimes used in the past for the absolute permittivity.^{ [17] }^{ [18] } However, in modern usage "dielectric constant" typically refers exclusively to a relative permittivity *ε*/*ε*_{0} and even this usage is considered "obsolete" by some standards bodies in favor of relative static permittivity.^{ [16] }^{ [19] } Hence, the term "dielectric constant of vacuum" for the electric constant *ε*_{0} is considered obsolete by most modern authors, although occasional examples of continuing usage can be found.

As for notation, the constant can be denoted by either or , using either of the common glyphs for the letter epsilon.

As indicated above, the parameter *ε*_{0} is a measurement-system constant. Its presence in the equations now used to define electromagnetic quantities is the result of the so-called "rationalization" process described below. But the method of allocating a value to it is a consequence of the result that Maxwell's equations predict that, in free space, electromagnetic waves move with the speed of light. Understanding why *ε*_{0} has the value it does requires a brief understanding of the history.

The experiments of Coulomb and others showed that the force *F* between two equal point-like "amounts" of electricity, situated a distance *r* apart in free space, should be given by a formula that has the form

where *Q* is a quantity that represents the amount of electricity present at each of the two points, and *k*_{e} is Coulomb's constant. If one is starting with no constraints, then the value of *k*_{e} may be chosen arbitrarily.^{ [20] } For each different choice of *k*_{e} there is a different "interpretation" of *Q*: to avoid confusion, each different "interpretation" has to be allocated a distinctive name and symbol.

In one of the systems of equations and units agreed in the late 19th century, called the "centimetre–gram–second electrostatic system of units" (the cgs esu system), the constant *k*_{e} was taken equal to 1, and a quantity now called "gaussian electric charge" *q*_{s} was defined by the resulting equation

The unit of gaussian charge, the statcoulomb, is such that two units, a distance of 1 centimetre apart, repel each other with a force equal to the cgs unit of force, the dyne. Thus the unit of gaussian charge can also be written 1 dyne^{1/2} cm. "Gaussian electric charge" is not the same mathematical quantity as modern (MKS and subsequently the SI) electric charge and is not measured in coulombs.

The idea subsequently developed that it would be better, in situations of spherical geometry, to include a factor 4π in equations like Coulomb's law, and write it in the form:

This idea is called "rationalization". The quantities *q*_{s}′ and *k*_{e}′ are not the same as those in the older convention. Putting *k*_{e}′ = 1 generates a unit of electricity of different size, but it still has the same dimensions as the cgs esu system.

The next step was to treat the quantity representing "amount of electricity" as a fundamental quantity in its own right, denoted by the symbol *q*, and to write Coulomb's Law in its modern form:

The system of equations thus generated is known as the rationalized metre–kilogram–second (rmks) equation system, or "metre–kilogram–second–ampere (mksa)" equation system. This is the system used to define the SI units.^{ [21] } The new quantity *q* is given the name "rmks electric charge", or (nowadays) just "electric charge". Clearly, the quantity *q*_{s} used in the old cgs esu system is related to the new quantity *q* by

One now adds the requirement that one wants force to be measured in newtons, distance in metres, and charge to be measured in the engineers' practical unit, the coulomb, which is defined as the charge accumulated when a current of 1 ampere flows for one second. This shows that the parameter *ε*_{0} should be allocated the unit C^{2}⋅N^{−1}⋅m^{−2} (or equivalent units – in practice "farads per metre").

In order to establish the numerical value of *ε*_{0}, one makes use of the fact that if one uses the rationalized forms of Coulomb's law and Ampère's force law (and other ideas) to develop Maxwell's equations, then the relationship stated above is found to exist between *ε*_{0}, *μ*_{0} and *c*_{0}. In principle, one has a choice of deciding whether to make the coulomb or the ampere the fundamental unit of electricity and magnetism. The decision was taken internationally to use the ampere. This means that the value of *ε*_{0} is determined by the values of *c*_{0} and *μ*_{0}, as stated above. For a brief explanation of how the value of *μ*_{0} is decided, see the article about *μ*_{0}.

By convention, the electric constant *ε*_{0} appears in the relationship that defines the electric displacement field **D** in terms of the electric field **E** and classical electrical polarization density **P** of the medium. In general, this relationship has the form:

For a linear dielectric, **P** is assumed to be proportional to **E**, but a delayed response is permitted and a spatially non-local response, so one has:^{ [22] }

In the event that nonlocality and delay of response are not important, the result is:

where *ε* is the permittivity and *ε*_{r} the relative static permittivity. In the vacuum of classical electromagnetism, the polarization **P** = **0**, so *ε*_{r} = 1 and *ε* = *ε*_{0}.

- ↑ "CODATA Value: electric constant".
*The NIST Reference on Constants, Units, and Uncertainty*. US National Institute of Standards and Technology. June 2015. Retrieved 2015-09-25.2014 CODATA recommended values

- ↑ "electric constant".
*Electropedia: International Electrotechnical Vocabulary (IEC 60050)*. Geneva: International Electrotechnical Commission. Retrieved 2015-03-26.. - ↑ The approximate numerical value is found at: "Electric constant, ε
_{0}".*NIST reference on constants, units, and uncertainty: Fundamental physical constants*. NIST. Retrieved 2012-01-22. This formula determining the exact value of*ε*_{0}is found in Table 1, p. 637 of PJ Mohr; BN Taylor; DB Newell (April–June 2008). "__Table 1: Some exact quantities relevant to the 2006 adjustment__*in*CODATA recommended values of the fundamental physical constants: 2006" (PDF).*Rev Mod Phys*.**80**(2): 633–729. arXiv: 0801.0028 . Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633. - ↑ Quote from NIST: "The symbol
*c*is the conventional symbol for the speed of light in vacuum. " See NIST*Special Publication 330*, p. 18 - ↑ See the last sentence of the NIST definition of ampere.
- ↑ See the last sentence of the NIST definition of meter.
- 1 2 Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006" (PDF).
*Reviews of Modern Physics*.**80**(2): 633–730. arXiv: 0801.0028 . Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633. Archived from the original (PDF) on 2017-10-01. Direct link to value .. - ↑ A summary of the definitions of
*c*,*μ*_{0}and*ε*_{0}is provided in the 2006 CODATA Report: CODATA report, pp. 6–7 - ↑ "Resolution 1 of 24th meeting of the General Conference on Weights and Measures".
*On the possible future revision of the International System of Units, the SI*(PDF). Sèvres, France: International Bureau for Weights and Measures. 21 Oct 2011. It is not expected to be adopted until some prerequisite conditions are met, and in any case not before 2014. See "Possible changes to the international system of units".*IUPAC Wire*. International Union of Pure and Applied Chemistry.**34**(1). January–February 2012. - ↑ Parker, Richard H.; Yu, Chenghui; Zhong, Weicheng; Estey, Brian; Müller, Holger (2018-04-13). "Measurement of the fine-structure constant as a test of the Standard Model".
*Science*.**360**(6385): 191–195. doi:10.1126/science.aap7706. ISSN 0036-8075. PMID 29650669. - ↑ SM Sze & Ng KK (2007). "Appendix E".
*Physics of semiconductor devices*(Third ed.). New York: Wiley-Interscience. p. 788. ISBN 0-471-14323-5. - ↑ RS Muller, Kamins TI & Chan M (2003).
*Device electronics for integrated circuits*(Third ed.). New York: Wiley. Inside front cover. ISBN 0-471-59398-2. - ↑ FW Sears, Zemansky MW & Young HD (1985).
*College physics*. Reading, Mass.: Addison-Wesley. p. 40. ISBN 0-201-07836-8. - ↑ B. E. A. Saleh and M. C. Teich,
*Fundamentals of Photonics*(Wiley, 1991) - ↑ International Bureau of Weights and Measures (2006). "The International System of Units (SI)" (PDF). p. 12.
- 1 2 Braslavsky, S.E. (2007). "Glossary of terms used in photochemistry (IUPAC recommendations 2006)" (PDF).
*Pure and Applied Chemistry*.**79**(3): 293–465, see p. 348. doi:10.1351/pac200779030293. - ↑ "Naturkonstanten". Freie Universität Berlin.
- ↑ King, Ronold W. P. (1963).
*Fundamental Electromagnetic Theory*. New York: Dover. p. 139. - ↑ IEEE Standards Board (1997). "IEEE Standard Definitions of Terms for Radio Wave Propagation" (PDF). p. 6.
- ↑ For an introduction to the subject of choices for independent units, see John David Jackson (1999). "Appendix on units and dimensions".
*Classical electrodynamics*(Third ed.). New York: Wiley. pp. 775*et seq.*. ISBN 0-471-30932-X. - ↑ International Bureau of Weights and Measures. "The International System of Units (SI) and the corresponding system of quantities".
- ↑ Jenö Sólyom (2008). "Equation 16.1.50".
*Fundamentals of the physics of solids: Electronic properties*. Springer. p. 17. ISBN 3-540-85315-4.

The **centimetre–gram–second system of units** is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism.

An **electromagnetic field** is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces of nature.

The **wave impedance** of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields. For a transverse-electric-magnetic (TEM) plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the impedance of free space. The symbol *Z* is used to represent it and it is expressed in units of ohms. The symbol *η* (eta) may be used instead of *Z* for wave impedance to avoid confusion with electrical impedance.

An **electric potential** is the amount of work needed to move a unit of positive charge from a reference point to a specific point inside the field without producing an acceleration. Typically, the reference point is the Earth or a point at infinity, although any point beyond the influence of the electric field charge can be used.

In physics, **Gauss's law**, also known as **Gauss's flux theorem**, is a law relating the distribution of electric charge to the resulting electric field. The surface under consideration may be a closed one enclosing a volume such as a spherical surface.

In physics, **screening** is the damping of electric fields caused by the presence of mobile charge carriers. It is an important part of the behavior of charge-carrying fluids, such as ionized gases, electrolytes, and charge carriers in electronic conductors . In a fluid, with a given permittivity *ε*, composed of electrically charged constituent particles, each pair of particles interact through the Coulomb force as

**Classical electromagnetism** or **classical electrodynamics** is a branch of theoretical physics that studies the interactions between electric charges and currents using an extension of the classical Newtonian model. The theory provides a description of electromagnetic phenomena whenever the relevant length scales and field strengths are large enough that quantum mechanical effects are negligible. For small distances and low field strengths, such interactions are better described by quantum electrodynamics.

In electromagnetism, **displacement current density** is the quantity ∂* D*/∂

**Gaussian units** constitute a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs (centimetre–gram–second) units. It is also called the **Gaussian unit system**, **Gaussian-cgs units**, or often just **cgs units**. The term "cgs units" is ambiguous and therefore to be avoided if possible: cgs contains within it several conflicting sets of electromagnetism units, not just Gaussian units, as described below.

In plasmas and electrolytes, the **Debye length**, named after Peter Debye, is a measure of a charge carrier's net electrostatic effect in solution and how far its electrostatic effect persists. A **Debye sphere** is a volume whose radius is the Debye length. With each Debye length, charges are increasingly electrically screened. Every Debye‐length , the electric potential will decrease in magnitude by 1/e. Debye length is an important parameter in plasma physics, electrolytes, and colloids. The corresponding Debye screening wave vector for particles of density , charge at a temperature is given by in Gaussian units. Expressions in MKS units will be given below. The analogous quantities at very low temperatures are known as the Thomas-Fermi length and the Thomas-Fermi wave vector. They are of interest in describing the behaviour of electrons in metals at room temperature.

In physics, the **electric displacement field**, denoted by **D**, is a vector field that appears in Maxwell's equations. It accounts for the effects of free and bound charge within materials. "**D**" stands for "displacement", as in the related concept of displacement current in dielectrics. In free space, the electric displacement field is equivalent to flux density, a concept that lends understanding to Gauss's law. In SI, it is expressed in units of coulomb per metre squared (C⋅m^{−2}).

The **Coulomb constant**, the **electric force constant**, or the **electrostatic constant** (denoted *k*_{e}, *k* or *K*) is a proportionality constant in electrodynamics equations. In SI units, it is exactly equal to 8987551787.3681764 N·m^{2}·C^{−2}, or roughly equaling 8.99×10^{9} N·m^{2}·C^{−2}. It was named after the French physicist Charles-Augustin de Coulomb (1736–1806) who introduced Coulomb's law.

The **electromagnetic wave equation** is a second-order partial differential equation that describes the propagation of electromagnetic waves through a medium or in a vacuum. It is a three-dimensional form of the wave equation. The homogeneous form of the equation, written in terms of either the electric field **E** or the magnetic field **B**, takes the form:

**Lorentz–Heaviside units** constitute a system of units within CGS, named for Hendrik Antoon Lorentz and Oliver Heaviside. They share with CGS-Gaussian units the property that the electric constant *ε*_{0} and magnetic constant *µ*_{0} do not appear, having been incorporated implicitly into the unit system and electromagnetic equations. Lorentz–Heaviside units may be regarded as normalizing *ε*_{0} = 1 and *µ*_{0} = 1, while at the same time revising Maxwell's equations to use the speed of light *c* instead.

In particle physics and physical cosmology, **Planck units** are a set of units of measurement defined exclusively in terms of five universal physical constants, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units.

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