भौतिक नियतांक

भौतिक नियतांक (physical constant) उस भौतिक राशि (physical quantity) को कहते हैं जिसके बारे में ऐसा विश्वास किया जाता है कि वह राशि प्रकृति में सार्वत्रिक (universal) है तथा समय के साथ अपरिवर्तनशील या नियत है। भौतिक नियतांक, गणितीय नियतांक से इस मामले में भिन्न हैं कि गणितीय नियतांक संख्यात्मक दृष्टि से तो नियत होते हैं किन्तु उनका किसी मापन से सम्बन्ध नहीं होता।

विज्ञान में बहुत से भौतिक नियतांक हैं जिनमें से प्रमुख हैं - शून्य में प्रकाश का वेग c, गुरुत्वाकर्षण नियतांक G, प्लांक नियतांक h, निर्वात का विद्युत नियतांक ε₀, तथा एलेक्ट्रान का आवेश e.

विस्तृत जानकारी के लिये मूलभूत भौतिक नियतांक देखें।

Quantity	Symbol	Value	Relative Standard Uncertainty
speed of light in vacuum	${\displaystyle c\,}$	299 792 458 m·s−1	defined
Newtonian constant of gravitation	${\displaystyle G\,}$	6.67428(67)×१०−11 m3·kg−1·s−2	1.0 × 10−4
Planck constant	${\displaystyle h\,}$	6.626 068 96(33) × 10−34 J·s	5.0 × 10−8
reduced Planck constant	${\displaystyle \hbar =h/(2\pi )}$	1.054 571 628(53) × 10−34 J·s	5.0 × 10−8

Quantity	Symbol	Value[1] (SI units)	Relative Standard Uncertainty
magnetic constant (vacuum permeability)	${\displaystyle \mu _{0}\,}$	4π × 10−7 N·A−2 = 1.256 637 061... × 10−6 N·A−2	defined
electric constant (vacuum permittivity)	${\displaystyle \varepsilon _{0}=1/(\mu _{0}c^{2})\,}$	8.854 187 817... × 10−12 F·m−1	defined
characteristic impedance of vacuum	${\displaystyle Z_{0}=\mu _{0}c\,}$	376.730 313 461... Ω	defined
Coulomb's constant	${\displaystyle \kappa =1/4\pi \varepsilon _{0}\,}$	8.987 551 787... × 109 N·m²·C−2	defined
elementary charge	${\displaystyle e\,}$	1.602 176 487(40) × 10−19 C	2.5 × 10−8
Bohr magneton	${\displaystyle \mu _{B}=e\hbar /2m_{e}}$	927.400 915(23) × 10−26 J·T−1	2.5 × 10−8
conductance quantum	${\displaystyle G_{0}=2e^{2}/h\,}$	7.748 091 7004(53) × 10−5 S	6.8 × 10−10
inverse conductance quantum	${\displaystyle G_{0}^{-1}=h/2e^{2}\,}$	12 906.403 7787(88) Ω	6.8 × 10−10
Josephson constant	${\displaystyle K_{J}=2e/h\,}$	4.835 978 91(12) × 1014 Hz·V−1	2.5 × 10−8
magnetic flux quantum	${\displaystyle \phi _{0}=h/2e\,}$	2.067 833 667(52) × 10−15 Wb	2.5 × 10−8
nuclear magneton	${\displaystyle \mu _{N}=e\hbar /2m_{p}}$	5.050 783 43(43) × 10−27 J·T−1	8.6 × 10−8
von Klitzing constant	${\displaystyle R_{K}=h/e^{2}\,}$	25 812.807 557(18) Ω	6.8 × 10−10

Quantity		Symbol	Value[1] (SI units)	Relative Standard Uncertainty
Bohr radius		${\displaystyle a_{0}=\alpha /4\pi R_{\infty }\,}$	5.291 772 108(18) × 10−11 m	3.3 × 10−9
classical electron radius		${\displaystyle r_{e}=e^{2}/4\pi \varepsilon _{0}m_{e}c^{2}\,}$	2.817 940 2894(58) × 10−15 m	2.1 × 10−9
electron mass		${\displaystyle m_{e}\,}$	9.109 382 15(45) × 10−31 kg	5.0 × 10−8
Fermi coupling constant		${\displaystyle G_{F}/(\hbar c)^{3}}$	1.166 39(1) × 10−5 GeV−2	8.6 × 10−6
fine-structure constant		${\displaystyle \alpha =\mu _{0}e^{2}c/(2h)=e^{2}/(4\pi \varepsilon _{0}\hbar c)\,}$	7.297 352 537 6(50) × 10−3	6.8 × 10−10
Hartree energy		${\displaystyle E_{h}=2R_{\infty }hc\,}$	4.359 744 17(75) × 10−18 J	1.7 × 10−7
proton mass		${\displaystyle m_{p}\,}$	1.672 621 637(83) × 10−27 kg	5.0 × 10−8
quantum of circulation		${\displaystyle h/2m_{e}\,}$	3.636 947 550(24) × 10−4 m² s−1	6.7 × 10−9
Rydberg constant		${\displaystyle R_{\infty }=\alpha ^{2}m_{e}c/2h\,}$	10 973 731.568 525(73) m−1	6.6 × 10−12
Thomson cross section		${\displaystyle (8\pi /3)r_{e}^{2}}$	6.652 458 73(13) × 10−29 m²	2.0 × 10−8
weak mixing angle		${\displaystyle \sin ^{2}\theta _{W}=1-(m_{W}/m_{Z})^{2}\,}$	0.222 15(76)	3.4 × 10−3

Quantity		Symbol	Value[1] (SI units)	Relative Standard Uncertainty
atomic mass unit (unified atomic mass unit)		${\displaystyle m_{u}=1\ u\,}$	1.660 538 86(28) × 10−27 kg	1.7 × 10−7
Avogadro's number		${\displaystyle N_{A},L\,}$	6.022 141 5(10) × 1023 mol−1	1.7 × 10−7
Boltzmann constant		${\displaystyle k=k_{B}=R/N_{A}\,}$	1.380 6504(24) × 10−23 J·K−1	1.8 × 10−6
Faraday constant		${\displaystyle F=N_{A}e\,}$	96 485.3383(83)C·mol−1	8.6 × 10−8
first radiation constant		${\displaystyle c_{1}=2\pi hc^{2}\,}$	3.741 771 18(19) × 10−16 W·m²	5.0 × 10−8
	for spectral radiance	${\displaystyle c_{1L}\,}$	1.191 042 82(20) × 10−16 W·m² sr−1	1.7 × 10−7
Loschmidt constant	at ${\displaystyle T}$=273.15 K and ${\displaystyle p}$=101.325 kPa	${\displaystyle n_{0}=N_{A}/V_{m}\,}$	2.686 777 3(47) × 1025 m−3	1.8 × 10−6
gas constant		${\displaystyle R\,}$	8.314 472(15) J·K−1·mol−1	1.7 × 10−6
molar Planck constant		${\displaystyle N_{A}h\,}$	3.990 312 716(27) × 10−10 J·s·mol−1	6.7 × 10−9
molar volume of an ideal gas	at ${\displaystyle T}$=273.15 K and ${\displaystyle p}$=100 kPa	${\displaystyle V_{m}=RT/p\,}$	2.2710 981(40) × 10−2 m³·mol−1	1.7 × 10−6
	at ${\displaystyle T}$=273.15 K and ${\displaystyle p}$=101.325 kPa		2.2413 996(39) × 10−2 m³·mol−1	1.7 × 10−6
Sackur-Tetrode constant	at ${\displaystyle T}$=1 K and ${\displaystyle p}$=100 kPa	${\displaystyle S_{0}/R={\frac {5}{2}}}$ ${\displaystyle +\ln \left[(2\pi m_{u}kT/h^{2})^{3/2}kT/p\right]}$	−1.151 704 7(44)	3.8 × 10−6
	at ${\displaystyle T}$=1 K and ${\displaystyle p}$=101.325 kPa		−1.164 867 7(44)	3.8 × 10−6
second radiation constant		${\displaystyle c_{2}=hc/k\,}$	1.438 775 2(25) × 10−2 m·K	1.7 × 10−6
Stefan-Boltzmann constant		${\displaystyle \sigma =(\pi ^{2}/60)k^{4}/\hbar ^{3}c^{2}}$	5.670 400(40) × 10−8 W·m−2·K−4	7.0 × 10−6
Wien displacement law constant		${\displaystyle b=(hc/k)/\,}$ 4.965 114 231...	2.897 768 5(51) × 10−3 m·K	1.7 × 10−6

Quantity		Symbol	Value (SI units)	Relative Standard Uncertainty
conventional value of Josephson constant[2]		${\displaystyle K_{J-90}\,}$	4.835 979 × 1014 Hz·V−1	defined
conventional value of von Klitzing constant[3]		${\displaystyle R_{K-90}\,}$	25 812.807 Ω	defined
molar mass	constant	${\displaystyle M_{u}=M(\,^{12}{\mbox{C}})/12}$	1 × 10−3 kg·mol−1	defined
	of carbon-12	${\displaystyle M(\,^{12}{\mbox{C}})=N_{A}m(\,^{12}{\mbox{C}})}$	1.2 × 10−2 kg·mol−1	defined
standard acceleration of gravity (gee, free-fall on Earth)		${\displaystyle g_{n}\,\!}$	9.806 65 m·s−2	defined
standard atmosphere		${\displaystyle {\mbox{atm}}\,}$	101 325 Pa	defined

Using dimensional analysis, it is possible combine fundamental physical constants to produce basic units of measurement. Depending on the choice and arrangement of constants used, the resulting natural units may have physical meaning. For example, Planck units use ${\displaystyle c,G,\hbar ,\varepsilon _{0}}$ and ${\displaystyle k}$ to derive constants relevant to unified theories, including quantum gravity.

Name	Dimension	Expression	Value[1] (SI units)
Planck length	Length (L)	${\displaystyle l_{\text{P}}={\sqrt {\frac {\hbar G}{c^{3}}}}}$	1.616 252(81) × 10−35 m
Planck mass	Mass (M)	${\displaystyle m_{\text{P}}={\sqrt {\frac {\hbar c}{G}}}}$	2.176 44(11) × 10−8 kg
Planck time	Time (T)	${\displaystyle t_{\text{P}}={\sqrt {\frac {\hbar G}{c^{5}}}}}$	5.391 24(27) × 10−44 s
Planck charge	Electric charge (Q)	${\displaystyle q_{\text{P}}={\sqrt {4\pi \varepsilon _{0}\hbar c}}}$	1.875 545 870(47) × 10−18 C
Planck temperature	Temperature (Θ)	${\displaystyle T_{\text{P}}={\sqrt {\frac {\hbar c^{5}}{Gk^{2}}}}}$	1.416 785(71) × 1032 K

• साँचा:CODATA2006
• Barrow, John D., The Constants of Nature; From Alpha to Omega - The Numbers that Encode the Deepest Secrets of the Universe. Pantheon Books, 2002. ISBN 0-375-42221-8.

• भौतिक राशि

• Sixty Symbols, University of Nottingham
• IUPAC - Gold Book