Electronvolt
The electronvolt (symbol: eV) is a non-SI unit of energy that has been accepted for use with the SI by the CIPM.[1] It is defined as the kinetic energy gained by an electron on passing through a potential difference of one volt in vacuum:[1] as such, its value in SI units is determined by the measured value of the elementary charge. The 2006 CODATA recommended value is 1 eV = 1.602 176 487(40) × 10−19 J.[2][note 1]
In chemistry, it is often useful to express the electronvolt as a molar quantity, whose value in SI units is determined by the measured value of the Faraday constant. The 2006 CODATA recommended value is 1 eV = 96.485 3399(24) kJ mol−1.[2][note 1]
In particle physics, multiples of the electronvolt are also used as a units of mass based on the principle of mass–energy equivalence. The dimensionally correct unit of mass for use with the SI would be eV/c2, although the division by the square of the speed of light is often assumed[note 2] and masses quoted simply in megaelectronvolts (MeV, 106 eV) or gigaelectronvolts (GeV, 109 eV). The 2006 CODATA recommended values are 1 GeV/c2 = 1.073 544 188(27) u = 1.782 661 758(44) × 10-27 kg.[2][note 1] The electronvolt can also be used to express frequency (particularly of X-rays and γ-rays), wavenumber and even temperature.[3]
Notes and references
Notes
- ↑ 1.0 1.1 1.2 The values of the fundamental physical constants used for these converstions are (CODATA, 2006):
- Elementary charge: 1.602 176 487(40) × 10−19 C Direct link to value
- Faraday constant: 96 485.3399(24) C mol−1 Direct link to value
- Avogadro constant: 6.022 141 79(30) × 1023 mol−1 Direct link to value
- Speed of light: 299 792 458 m s−1 (by definition) Direct link to value
- Planck constant: 6.626 068 96(33) × 10−34 J s Direct link to value
- ↑ In the system of natural units, the speed of light is taken as dimensionless and equal to unity and so the electronvolt is a valid unit of mass in its own right.
References
- ↑ 1.0 1.1 The International System of Units (SI), 8th ed.; International Bureau of Weights and Measures: Sèvres, France, 2006; pp 125–26. ISBN 92-822-2213-6, <http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf>.
- ↑ 2.0 2.1 2.2 Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>.
- ↑ Quantities, Units and Symbols in Physical Chemistry, 2nd ed.; Blackwell Science: Oxford, 1993. ISBN 0-63203-5838, <http://old.iupac.org/publications/books/gbook/green_book_2ed.pdf>.
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