Physical Constants
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A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, Planck's constant h, the electric constant ε0, and the elementary charge e. Physical constants can take many dimensional forms: the speed of light signifies a maximum speed limit of the universe and is expressed dimensionally as length divided by time; while the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, is dimensionless.
Table of universal constants
| Quantity | Symbol | Value | Relative Standard Uncertainty |
|---|---|---|---|
| speed of light in vacuum | 299 792 458 m·s−1 | defined | |
| Newtonian constant of gravitation | 1.0 × 10−4 | ||
| Planck constant | 6.626 068 96(33) × 10−34 J·s | 5.0 × 10−8 | |
| reduced Planck constant | 3.313 034 48π-1 × 10−34 J·s = 1.054 571 628(53) × 10−34 J·s | 5.0 × 10−8 |
Table of Other Accepted & Useful Constants
| Quantity | Symbol | Value<ref name="concise" /> (SI units) | Relative Standard Uncertainty | |
|---|---|---|---|---|
| Stefan-Boltzmann constant | 5.670 400(40) × 10−8 W·m−2·K−4 | 7.0 × 10−6 | ||
| Wien displacement law constant | 2.897 768 5(51) × 10−3 m·K | 1.7 × 10−6 | ||
| atomic mass unit (unified atomic mass unit) | 1.660 538 86(28) × 10−27 kg | 1.7 × 10−7 | ||
| Avogadro's number | 6.022 141 5(10) × 1023 mol−1 | 1.7 × 10−7 | ||
| Boltzmann constant | 1.380 650 388 238 137 546 253 272 195 613 5(24) × 10−23 J·K−1 | 1.8 × 10−6 | ||
| Faraday constant | 96 485.337 716 389 95(83)C·mol−1 | 8.6 × 10−8 | ||
| Bohr radius | 0.529 177 2108(18) × 10−10 m | 3.3 × 10−9 | ||
| classical electron radius | 2.817 940 299 579 513 654 416 052 301 942(58) × 10−15 m | 2.1 × 10−9 | ||
| electron mass | 9.109 382 15(45) × 10−31 kg | 5.0 × 10−8 | ||
| Fermi coupling constant | 1.166 39(1) × 10−5 GeV−2 | 8.6 × 10−6 | ||
| fine-structure constant | 7.297 352 537 6(50) × 10−3 | 6.8 × 10−10 | ||
| Hartree energy | 4.359 744 17(75) × 10−18 J | 1.7 × 10−7 | ||
| proton mass | 1.672 621 637(83) × 10−27 kg | 5.0 × 10−8 | ||
| quantum of circulation | 3.636 947 550(24) × 10−4 m² s−1 | 6.7 × 10−9 | ||
| Rydberg constant | 10 973 731.568 525(73) m−1 | 6.6 × 10−12 | ||
| Thomson Cross section | 6.652 458 73(13) × 10−29 m² | 2.0 × 10−8 | ||
| Weinberg angle | 0.222 15(76)</td>
<td>3.4 × 10−3 | |||
Table of electromagnetic constants
| Quantity | Symbol | Value<ref name="concise">The values are given in the so-called concise form; the number in brackets is the standard uncertainty, which is the value multiplied by the relative standard uncertainty.</ref> (SI units) | Relative Standard Uncertainty |
|---|---|---|---|
| magnetic constant (vacuum permeability) | 4π × 10−7 N·A−2 = 1.256 637 061... × 10−6 N·A−2 | defined | |
| electric constant (vacuum permittivity) | 2.781 625 140 134 046 080 435 224 912 12π-1 × 10-11 F·m−1 = 8.854 187 817... × 10−12 F·m−1 | defined | |
| characteristic impedance of vacuum | 119.916 983 2π Ω = 376.730 313 461... Ω | defined | |
| Coulomb's constant | 8.987 551 787 368 176 4 × 109 N·m²·C−2 | defined | |
| elementary charge | 1.602 176 487(40) × 10−19 C | 2.5 × 10−8 | |
| Bohr magneton | 927.400 915(23) × 10−26 J·T−1 | 2.5 × 10−8 | |
| conductance quantum | 7.748 091 717 914 392 775 819 594 884 104 2(53) × 10−5 S | 6.8 × 10−10 | |
| inverse conductance quantum | 12 906.403 749 556 760 396 515 369 018 534(88) Ω | 6.8 × 10−10 | |
| Josephson constant | 4.835 978 91(12) × 1014 Hz·V−1 | 2.5 × 10−8 | |
| magnetic flux quantum | 2.067 833 667(52) × 10−15 Wb | 2.5 × 10−8 | |
| nuclear magneton | 5.050 783 43(43) × 10−27 J·T−1 | 8.6 × 10−8 | |
| von Klitzing constant | 25 812.807 499 113 520 793 030 738 037 068(18) Ω | 6.8 × 10−10 |
