Physical constant

In science, a physical constant is a physical quantity whose numerical value is fixed. It can be constrasted to a mathematical constant which is a fixed number that does not directly involve a physical measurement.

There are many such constants used in science, some of the most famous of which being: Planck's constant, the gravitational constant and Avogadro's constant (better known as Avogadro's number). Constants can take many forms; some, such as the Planck length represents a fundamental physical distance, others such as the speed of light signifies the maximun speed limit of the universe, yet others are dimensionless quantities such as the fine-structure constant which embodies the interaction between electrons and photons.

Below is a list of physical constants:

Quantity	Symbol	Value	Ref.
speed of light in vacuum	c	299 792 458 m·s-1 (defined)	a
permeability of vacuum	μ0	4π × 10-7 N A-2 (defined)	a
		12.566 370 614... × 10-7 N A-2	a
permittivity of vacuum	ε0 = 1/(μ0c2)	8.854 187 817 ... × 10-12 F·m-1	a
characteristic impedance of vacuum	Z0 = μ0c	376.730 313 461... Ω (defined)	a
gravitational constant	G	6.672 59(85) × 10-11 m3·kg-1·s-2	?
Planck's constant	h	6.626 068 76(52) × 10-34 J·s	a
Dirac's constant	h = h / (2π)	1.054 571 596(82) × 10-34 J·s	a
Planck mass	mp = (hc / G)1/2	2.1767(16) × 10-8 kg	a
Planck length	lp= (hG / c3) 1/2	1.6160(12) × 10-35 m	a
Planck time	tp = (hG / c5)1/2	5.3906(40) × 10-44 s	a
elementary charge	e	1.602 176 462(63) × 10-19 C	a
electron rest mass	me	9.109 381 88(72) × 10-31 kg	a
proton rest mass	mp	1.672 621 58(13) × 10-27 kg	a
neutron rest mass	mn	1.674 927 16(13) × 10-27 kg	a
atomic mass constant[?], (unified atomic mass unit)	mu = 1 u	1.660 538 73(13) × 10-27 kg	a
Avogadro constant	L, NA	6.022 141 99(47) × 1023 mol-1	a
Boltzmann constant	k	1.380 6503(24) × 10-23 J·K-1	a
Faraday constant	F	9.648 534 15(39) × 104 C·mol-1	a
gas constant	R	8.314 472(15) J·K-1·mol-1	a
zero of the Celsius scale		273.15 K (defined)	?
molar volume, ideal gas, p = 1 bar, θ = 00C		22.710 981(40) L·mol-1	a
standard atmosphere	atm	101 325 Pa (defined)	a
fine structure constant	α = μ0e2c / (2h)	7.297 352 533(27) × 10-3	a
	α-1	137.035 999 76(50)	a
Bohr radius	a0	5.291 772 083(19) × 10-11 m	a
Hartree energy	Eh	4.359 743 81(34) × 10-18 J	a
Rydberg constant	R∞	1.097 373 156 8549(83) × 107 m-1	a
Bohr magneton	μB	9.274 008 99(37) × 10-24 J·T-1	a
electron magnetic moment[?]	μe	-9.284 763 62(37) × 10-24 J·T-1	a
Lande g-factor for free electron	ge	2.002 319 304 386(20)	?
nuclear magneton	μN	5.050 786 6(17) × 10-27 J·T-1	?
proton magnetic moment[?]	μp	1.410 607 61(47) × 10-26 J·T-1	?
proton magnetogyric ratio	γp	2.675 221 28(81) × 108 s-1·T-1	?
magnetic moment of protons in H20, μ'p	μ'p / μB	1.520 993 129(17) × 10-3	?
proton resonance frequency per field in H20	γ'p / (2π)	42.576 375 (13) M·Hz·T-1	?
Stefan-Boltzmann constant	σ	5.670 400(40) × 10-8 W·m-2·K-4	a
first radiation constant	c1	3.741 774 9(22) × 10-16 W·m2	?
second radiation constant	c2	1.438 769 (12) × 10-2 m·K	?
standard acceleration of free fall	gn	9.80665 m·s-2 (defined)	?

References

^aPeter J. Mohr and Barry N. Taylor, "CODATA Recommended Values of the Fundamental Physical Constants: 1998," Journal of Physical and Chemical Reference Data, Vol. 28, No. 6, 1999 and Reviews of Modern Physics, Vol. 72, No. 2, 2000.[[1] (http://physics.nist.gov/cuu/Constants/)]