# ISO 31-5

ISO 31-5 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to electricity and magnetism. It is superseded by ISO 80000-6.

Some of its definitions are below, with values taken from NIST values of the constants:

Name Symbol Definition Value
Speed of light in vacuum c0 299 792 458 m s−1 299 792 458 m s−1
Magnetic constant μ0 4π × 10−7 N A−2 12.566 370 614... x 10−7 N A−2
Electric constant ε0 ${\displaystyle {\begin{matrix}{\frac {1}{\mu _{0}{c_{0}}^{2}}}\end{matrix}}}$ 8.854 187 817... x 10−12 F m−1
Characteristic impedance of vacuum Z0 μ0 c0 376.730 313 461...Ω
ISO/IEC 80000

ISO 80000 or IEC 80000 is an international standard promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

The standard introduces the International System of Quantities (ISQ). It is a style guide for the use of physical quantities and units of measurement, formulas involving them, and their corresponding units, in scientific and educational documents for worldwide use. In most countries, the notations used in mathematics and science textbooks at schools and universities follow closely the guidelines in this standard.The ISO/IEC 80000 family of standards was completed with the publication of Part 1 in November 2009.

ISO 31

ISO 31 (Quantities and units, International Organization for Standardization, 1992) is a deprecated international standard for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents. It is superseded by ISO/IEC 80000.

Impedance of free space

The impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, Z0 = |E|/|H|, where |E| is the electric field strength and |H| is the magnetic field strength. It currently has an exactly defined value

${\displaystyle Z_{0}=(119.916~983~2)\pi ~\Omega =376.730~313~461~77\ldots ~\Omega .}$

The impedance of free space (more correctly, the wave impedance of a plane wave in free space) equals the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Since the values of these constants are exact (they are given in the definitions of the ampere and the metre respectively), the value of the impedance of free space is likewise exact. However, with the redefinition of the SI base units which are going into force on May 20, 2019, this value is subject to experimental measurement.

Index of physics articles (I)

The index of physics articles is split into multiple pages due to its size.

Optical medium

An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. The medium has an intrinsic impedance, given by

${\displaystyle \eta ={E_{x} \over H_{y}}}$

where ${\displaystyle E_{x}}$ and ${\displaystyle H_{y}}$ are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to:

${\displaystyle \eta ={\sqrt {\mu \over \varepsilon }}\ .}$

For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted Z0, and

${\displaystyle Z_{0}={\sqrt {\mu _{0} \over \varepsilon _{0}}}\ .}$

Waves propagate through a medium with velocity ${\displaystyle c_{w}=\nu \lambda }$, where ${\displaystyle \nu }$ is the frequency and ${\displaystyle \lambda }$ is the wavelength of the electromagnetic waves. This equation also may be put in the form

${\displaystyle c_{w}={\omega \over k}\ ,}$

where ${\displaystyle \omega }$ is the angular frequency of the wave and ${\displaystyle k}$ is the wavenumber of the wave. In electrical engineering, the symbol ${\displaystyle \beta }$, called the phase constant, is often used instead of ${\displaystyle k}$.

The propagation velocity of electromagnetic waves in free space, an idealized standard reference state (like absolute zero for temperature), is conventionally denoted by c0:

${\displaystyle c_{0}={1 \over {\sqrt {\varepsilon _{0}\mu _{0}}}}\ ,}$
where ${\displaystyle \varepsilon _{0}}$ is the electric constant and ${\displaystyle ~\mu _{0}\ }$ is the magnetic constant.

For a general introduction, see Serway For a discussion of synthetic media, see Joannopoulus.

Vacuum permittivity

The physical constant ε0 (pronounced as “epsilon nought” or “epsilon zero”), commonly called the vacuum permittivity, permittivity of free space or electric constant or the distributed capacitance of the vacuum, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum. It has an exactly defined value that can be approximated as

ε0 = 8.854187817...×10−12 F⋅m−1 (farads per metre).

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. For example, the force between two separated electric charges (in the vacuum of classical electromagnetism) is given by Coulomb's law:

${\displaystyle \ F_{\text{C}}={\frac {1}{4\pi \varepsilon _{0}}}{\frac {q_{1}q_{2}}{r^{2}}}}$

The value of the constant fraction is approximately 9 × 109 N⋅m2⋅C−2, q1 and q2 are the charges, and r is the distance between them. Likewise, ε0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources.

ISO standards by standard number
1–9999
10000–19999
20000+

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