### Natural and atomic units

In some cases, the values of quantities are expressed in terms of fundamental constants of nature or so-called natural units. The use of these units occur when it is necessary for the most effective communication of information. In such cases, the specific natural units that are used must be identified. Examples of some physical quantities used as natural units are given in the table. While theoretical results intended primarily for other theorists may be left in natural units, if they are also intended generally, they must also be given in acceptable units.

### Some Fundamental Physical Constants

 Physical Quantity Symbol Value in SI Units speed of light in vacuum c, co 299 792 458 m s-1 elementary charge e 1.602 176 53(14) x 10-19 C Planck constant h 6.626 0693(11) x 10-34 J s Avogadro constant L, NA 6.022 1415(10) x 1023 mol-1 electron mass me 9.109 3826(16) x 10-31 kg proton mass mp 1.672 621 71(29) x 10-27 kg electronvolt eV 1.602 176 53(14) x 10-19 J Faraday constant F 9.648 533 83(83) x 104 C mol-1 Hartree energy Eh 4.359 744 17(75) x 10-18 J Bohr radius ao 5.291 772 108(18) x 10-11 m Bohr magneton µB 9.274 009 49(80) x 10-24 J T-1 nuclear magneton µN 5.050 783 43(43) x 10-27 J T-1 Rydberg constant R∞ 10 973 731.568 527(73) m-1 molar gas constant R 8.314 472(15) J mol-1 K-1 Boltzmann constant k, kB 1.380 650 5(24) x 10-23 J K-1 gravitational constant G 6.6742(10) x 10-11 m3 kg-1 s-2 standard acceleration of gravity gn 9.806 65 m s-2 triple point of water Ttp(H20) 273.16 K zero of Celsius scale T(0oC) 273.15 K molar volume of ideal gas (273.15 K, 100 kPa) Vm 22.710 981(40) x 10-3 m3 mol-1 magnetic constant (permeability of vacuum) µo 4p x 10-7 = 12.566 370 614 x 10-7 N A-2 electric constant (permittivity of vacuum) eo 8.854 187 817 x 10-12 F m-1
Table 10. Some Fundamental Physical Constants.

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#### Convert between Different Units of Measurement

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To use the measurement conversion calculators simply enter a number value into the desired field and click calculate. All results shown will be equivalent values. Values are given to seven significant figures ( the odd result may show 9's or 0's over running). Values of 10 000 or greater will be displayed in the e-format e.g. 2.3456e7 which equals 2.3456 x 107 or 23 456 000. Values lower than 0.001 will be displayed in the e-format e.g. 2.3456e-5 which equals 2.3456 x 10-5 or 0.000 023456. The calculators require that your browser has java script enabled. View all measurement unit conversion calculators on the same page (may not work with all browsers; requires Iframe).

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