2.4 PHYSICAL CONSTANTS USED AS ATOMIC UNITS: Difference between revisions

From IUPAC Green Book 5th Edition
(Added first row of table.)
(Added the second row of table.)
Line 14: Line 14:
! style="border-bottom: 1px solid;text-align: left;font-weight: normal;font-style: italic;width: 5%;" | Notes
! style="border-bottom: 1px solid;text-align: left;font-weight: normal;font-style: italic;width: 5%;" | Notes
|-
|-
| <span id="mass"/>[[mass]] || electron mass || $m_{\rm{e}}$ || $\approx 9.1.94 \times 10^{-31}\ \rm{kg}$ ||  
| <span id="mass"/>mass || electron mass || $m_{\rm{e}}$ || $\approx 9.1094 \times 10^{-31}\ \rm{kg}$ ||
|-
| <span id="mass"/>charge || elementary charge || $e$ || $= 1.602\ 176\ 634 \times 10^{-19}\ \rm{C}$ ||  
|-
|-
|}
|}


(note num) <span id="<xx.yy>_Note_<zz>"/>Note text.<br/>
(note num) <span id="<xx.yy>_Note_<zz>"/>Note text.<br/>

Revision as of 10:24, 25 January 2024

Sometimes fundamental physical constants, or other well defined physical quantities, are used as though they were units in certain specialized fields of science. For example, in astronomy it may be more convenient to express the mass of a star in terms of the mass of the sun. In atomic and molecular physics it is similarly more convenient to express masses in terms of the electron mass, $m_{\rm{e}}$, or in terms of the unified atomic mass unit, $1\ \rm{u}$, and to express charges in terms of the elementary charge $e$, and energies in terms of the electronvolt, $\rm{eV}$.

The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of $1\ \rm{V}$ in vacuum, $1\ \rm{e}V = 1.602\ 176\ 634 \times 10^{−19}\ \rm{J}$. The numerical value of a quantity expressed in this unit may be converted into its value when expressed in the SI by multiplication with the value of the physical constant in the SI.

The dalton and the unified atomic mass unit are alternative names for the same unit, therefore $1\ \rm{u} = 1 \rm{Da} \approx 1.6605 \times 10^{−27}\ \rm{kg}$. The dalton may be combined with the SI prefixes to express the masses of large molecules in kilodalton ($\rm{kDa}$) or megadalton ($\rm{MDa}$).


Physical quantity Physical constant Symbol for unit Value in SI units Notes
mass electron mass $m_{\rm{e}}$ $\approx 9.1094 \times 10^{-31}\ \rm{kg}$
charge elementary charge $e$ $= 1.602\ 176\ 634 \times 10^{-19}\ \rm{C}$

(note num) <span id="<xx.yy>_Note_<zz>"/>Note text.