Kelvin

Categories: Accuracy disputes | SI base units | Units of temperature

For other uses, see Kelvin (disambiguation).

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops. It is also common to give a temperature relative to the reference temperature of 273.15 K, approximately the melting point of water under ordinary conditions; this convention is the Celsius temperature scale.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

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 includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

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 degrees Celsius

  <math>

\mathrm{C} = \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:

<math>

\overline{\mathrm{E_{kin}}} = \frac{3}{2} \cdot k_B \cdot \mathrm{T} </math>

• electron volts to kelvins

<math>

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

• kelvins to electron volts

<math>

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

SI multiples

See also

• ITS-90 International Temperature Scale

External link

• BIPM brochure on the kelvin

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