Encyclopedia > Physical constant
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 | 1.054 571 596(82) × 10-34 J·s | a | |
| Planck mass | mp = ( |
2.1767(16) × 10-8 kg | a |
| Planck length | lp= ( |
1.6160(12) × 10-35 m | a |
| Planck time | tp = ( |
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) | ? |
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/)]
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