Niche it!
BobbyGs Info
Physics > Physical Constants

Physics Help

Physical Constants

From Wikipedia the free encyclopedia, by MultiMedia

Back | Home | Up | Next


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 ms-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 Fm-1 a
characteristic impedance of vacuum Z0 = μ0c 376.730 313 461... Ω (defined) a
gravitational constant G 6.672 59(85) 10-11 m3kg-1s-2 ?
Planck's constant h 6.626 068 76(52) 10-34 Js a
Dirac's constant h = h / (2π) 1.054 571 596(82) 10-34 Js 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's number L, NA 6.022 141 99(47) 1023 a
Boltzmann constant k 1.380 6503(24) 10-23 JK-1 a
Faraday constant F 9.648 534 15(39) 104 Cmol-1 a
gas constant R 8.314 472(15) JK-1mol-1 a
zero of the Celsius scale   273.15 K (defined) ?
molar volume, ideal gas, p = 1 bar, θ = 00C   22.710 981(40) Lmol-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 JT-1 a
electron magnetic moment μe -9.284 763 62(37) 10-24 JT-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 JT-1 ?
proton magnetic moment μp 1.410 607 61(47) 10-26 JT-1 ?
proton magnetogyric ratio γp 2.675 221 28(81) 108 s-1T-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) MHzT-1 ?
Stefan-Boltzmann constant σ 5.670 400(40) 10-8 Wm-2K-4 a
first radiation constant c1 3.741 774 9(22) 10-16 Wm2 ?
second radiation constant c2 1.438 769 (12) 10-2 mK ?
standard acceleration of free fall gn 9.80665 ms-2 (defined) ?

Some "constants" are really artifacts of the unit system used, like mks or cgs. In natural units, some of these supposedly physical constants turn out to be mere conversion factors.

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.


Home | Up | Physical Laws | Physical Constants | International System | Unit Conversions | Physical Units

Physics Help, made by MultiMedia | Free content and software

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.