Impedance of free space

In electromagnetism, the impedance of free space, $Z 0$ , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is,

Z_{0}={\frac {|\mathbf {E} |}{|\mathbf {H} |}},

where

| E |

is the

magnetic field strength. Its presently accepted value is^[1]

Z 0 = 376.730 313668 (57) Ω

,

where Ω is the

electrical resistance. The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability

μ 0

and the speed of light in vacuum

c 0

. Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the redefinition of the SI base units

that came into force on 20 May 2019, the impedance of free space is subject to experimental measurement because only the speed of light in vacuum

c 0

retains an exactly defined value.

Terminology

The analogous quantity for a

intrinsic impedance of the medium and designated

η

(eta). Hence

Z 0

is sometimes referred to as the intrinsic impedance of free space,^[2] and given the symbol

η 0

.^[3]

It has numerous other synonyms, including:

wave impedance of free space,[4]
the vacuum impedance,^[5]
intrinsic impedance of vacuum,^[6]
characteristic impedance of vacuum,^[7]
wave resistance of free space.^[8]

Relation to other constants

From the above definition, and the plane wave solution to Maxwell's equations,

Z_{0}={\frac {|\mathbf {E} |}{|\mathbf {H} |}}=\mu _{0}c={\sqrt {\frac {\mu _{0}}{\varepsilon _{0}}}}={\frac {1}{\varepsilon _{0}c}},

where

μ 0 \approx 12.566 \times 10 -7

magnetic constant

, also known as the permeability of free space,

ε 0 \approx 8.854 \times 10 -12

electric constant

, also known as the permittivity of free space,

c

is the speed of light in free space,^[9]^[10]

The reciprocal of $Z 0$ is sometimes referred to as the admittance of free space and represented by the symbol $Y 0$ .

Historical exact value

Between 1948 and 2019, the

SI unit the ampere was defined by choosing the numerical value of

μ 0

to be exactly 4

π

× 10⁻⁷ H/m. Similarly, since 1983 the SI metre

has been defined relative to the second by choosing the value of

c 0

to be 299792458 m/s. Consequently, until the 2019 redefinition,

Z_{0}=\mu _{0}c=4\pi \times 29.979\,2458~\Omega

exactly,

or

Z_{0}=\mu _{0}c=\pi \times 119.916\,9832~\Omega

exactly,

or

Z_{0}=376.730\,313\,461\,77\ldots ~\Omega .

This chain of dependencies changed when the ampere was redefined on 20 May 2019.

Approximation as 120π ohms

It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120 $π$ ohms for $Z 0$ . This is equivalent to taking the speed of light $c$ to be precisely 3×10⁸ m/s in conjunction with the then-current definition of $μ 0$ as 4 $π$ × 10⁻⁷

Hertzian dipole

is

R_{r}\approx 80\pi ^{2}\left({\frac {l}{\lambda }}\right)^{2}

(result in ohms; not exact).

This practice may be recognized from the resulting discrepancy in the units of the given formula. Consideration of the units, or more formally dimensional analysis, may be used to restore the formula to a more exact form, in this case to

R_{r}={\frac {2\pi }{3}}Z_{0}\left({\frac {l}{\lambda }}\right)^{2}.

References and notes

^ "2018 CODATA Value: characteristic impedance of vacuum". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-10-31.
ISBN 978-0-521-87565-3
.

^ ^a ^b David K Cheng (1989). Field and wave electromagnetics (Second ed.). New York: Addison-Wesley.
ISBN 0-201-12819-5
.

ISBN 978-981-02-2629-9
.

ISBN 978-0-521-09815-1
.

ISBN 978-0-07-035423-4
.

ISBN 978-1-85233-682-0
.

ISBN 978-0-12-374697-9
.

BIPM have adopted the notation $c 0$ for the speed of light in free space
.

^ "Current practice is to use $c 0$ to denote the speed of light in vacuum according to ISO 31. In the original Recommendation of 1983, the symbol $c$ was used for this purpose." Quote from NIST Special Publication 330, Appendix 2, p. 45. Archived 2016-06-03 at the Wayback Machine.

Further reading

John David Jackson (1998). Classical electrodynamics (Third ed.). New York: Wiley.
ISBN 0-471-30932-X
.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Impedance_of_free_space&oldid=1220612253"

[physconst-Z0-1] "2018 CODATA Value: characteristic impedance of vacuum". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-10-31.

[2] ISBN 978-0-521-87565-3
.

[cheng-3] David K Cheng (1989). Field and wave electromagnetics (Second ed.). New York: Addison-Wesley.
ISBN 0-201-12819-5
.

[4] ISBN 978-981-02-2629-9
.

[5] ISBN 978-0-521-09815-1
.

[6] ISBN 978-0-07-035423-4
.

[7] ISBN 978-1-85233-682-0
.

[8] ISBN 978-0-12-374697-9
.

[9] BIPM have adopted the notation $c 0$ for the speed of light in free space
.

[10] "Current practice is to use $c 0$ to denote the speed of light in vacuum according to ISO 31. In the original Recommendation of 1983, the symbol $c$ was used for this purpose." Quote from NIST Special Publication 330, Appendix 2, p. 45. Archived 2016-06-03 at the Wayback Machine.

[1]

[2]

[3]

[5]

[6]

[7]

[8]

[9]

[10]

Terminology

Relation to other constants

Historical exact value

Approximation as 120π ohms

See also

References and notes

Further reading