Kelvin

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvins is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins, approximately the melting point of water under ordinary conditions.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin.

Typographical conventions

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike degrees Fahrenheit, or degrees Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104).

Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units.

Unicode contains a legacy code for a kelvin symbol (K) to accommodate some old code pages in certain Oriental languages; it is not recommended for use any more. In all languages, the symbol should be the Roman letter Unicode K for current usage.

Conversion factors

Kelvins and Celsius

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.

• kelvins to °C

  <math>\mathrm{K} – 273.15<math>

Temperature and energy

In a thermodynamic system, the energy carried by the particles is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the temperature of particles with a certain energy; or to calculate the energy of particles at a certain temperature:

• electron volts to kelvins
  

  <math>\mathrm{eV} \times 11,\!605<math>

• kelvins to electron volts
  

  <math>\frac{\mathrm{K}}{11,\!605}<math>

See also

• ITS-90 International Temperature Scale

External link

• BIPM brochure on the kelvin