Kelvin

The Kelvin (symbol K , of the name of Lord Kelvin) is the unit IF of Température Thermodynamique. By convention, the names of unit are common nouns and are written into tiny (“Kelvin” and not “Kelvin”).

The Kelvin is fraction 1/273,16 of the thermodynamic temperature of the Point triples Eau (HO), and a temperature variation of 1 K is equivalent to a variation of 1 °C. However, unlike the Degree Celsius, the Kelvin is an absolute measurement of the temperature which was introduced thanks to the Third principle of thermodynamics. The temperature of 0 K is equal to -273.15 °C and corresponds to the Absolute zero (the triple point of water is thus at the temperature +0,01°C).

Not being a relative measurement, the Kelvin is never preceded by the word “degree” nor of the symbol “  °  ”, contrary to the degrees Centigrade or Fahrenheit. In the beginning, in 1954 (10th CGPM, resolution 3, CR 79), the Kelvin was called the “Kelvin degree”, and was written °K then. The “degree” was removed at the time of 13th CGPM, in 1967 (resolution 3, CR 104) and its symbol became K .

Conversion towards the other units

  • Kelvins in degrees Celsius: K = °C + 273,15
  • degrees Celsius in Kelvins: °C = K - 273,15
  • Kelvins in degrees Fahrenheit: K = (°F + 459,97)/1,8 gtp
  • degrees Fahrenheit in Kelvins: °F = K × 1,8 − 449,67.

Construction and consequence of the unit Kelvin

Fraction 1/273,16 is due to the choice of the triple point of water like point of reference, and to the will to define a unit of temperature making it possible to find the usual intervals of temperatures associated on old temperature scales. Though the official definition of the degree Celsius is based on that of the Kelvin, this last was established subsequently.

Historically the points of reference chosen to build the scales of temperature were the temperature of congelation of the water, which defines the zero, and the boiling point, fixed at 100. These two points thus defined a scale centigrade whose step is a hundredth of the difference in temperature between these two points. This scale of temperature was confused a long time with the Celsius scale.

Concept of temperature thermodynamic, and implicitly that of absolute temperature, introduced the concept of absolute zero, making useless the reference to two points. Only one fixed point of reference is enough. The Point triples water, i.e. the conditions in which coexist the three states (liquid, solid and gas) of water, is a point of invariant temperature and pressure (null variance). It thus constitutes a fundamental fixed point of reference by specifying the isotopic composition of the water whose triple point is used:

  • 0,000  155  76 mole of {{exp|2}} H per mole of {{exp|1}} H
  • 0,000  379  9 mole of 17 O per mole of 16 O
  • 0,002  005  2 mole of 18 O per mole of 16 O

This composition is that of material of reference of the International Atomic Energy Agency (IAEA) known under the name of " Standard Vienna Mean Ocean Toilets " (VSMOW), as recommended by the International union of chemistry pure and applied (UICPA) in J.R. of Laeter, J.K. Böhlke, P. Of Beaver, H. Hidaka, H.S. Peiser, K.J.R. Rosman, and P.D.P. Taylor (2003), '' Atomic Weights off the Elements: Review 2000 (IUPAC Technical Carryforward) '', Pure and Applied Chemistry, vol. 75, No 6, pp. 683– 800.

Multiples of the Kelvin

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