What is the value of the absolute temperature

Kelvin

Lexicon> Letter K> Kelvin

Definition: a unit of temperature commonly used in physics

English: Kelvin

Categories: units, heat and cold

Author: Dr. Rüdiger Paschotta

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Original creation: November 3rd, 2010; last change: 15.05.2021

URL: https://www.energie-lexikon.info/kelvin.html

This is a unit commonly used in physics and technology for temperature and, in particular, for temperature differences Kelvin, named after Lord Kelvin. In the past one spoke of Degrees Kelvin (° K), today only from Kelvin.

Absolute zero temperature can never be reached exactly; however, it can be approximated very closely with modern methods.

The zero point of the Kelvin scale corresponds to this absolute zero point of temperature, which is around −273.15 ° C on the Celsius scale. At this temperature there would be no microscopic heat movement of the matter; H. the internal energy would assume the minimum possible value. However, according to the third law of thermodynamics, it is in principle impossible to reach this temperature exactly. At least it succeeds with methods of so-called laser cooling (laser cooling) To achieve temperatures of just a few nanokelvin (billionths of a Kelvin) above absolute zero.

By definition, the step size of the Kelvin scale corresponds exactly to that of the Celsius scale. So you get the temperature in Kelvin from the temperature in degrees Celsius (° C) by adding 273.15 to the numerical value. For example, 100 ° C corresponds to approx. 373.15 K.

Details of the absolute temperature

For many physical formulas, temperature values ​​in Kelvin must be used.

Since the Kelvin degrees indicate the distance to absolute temperature zero, they are very important in thermodynamics. For example, in the formula for the Carnot efficiency of a heat engine

Enter the two temperatures as absolute temperatures in Kelvin. Likewise, Kelvin temperatures must always be entered in formulas for Boltzmann distributions.

Information for temperature differences

Temperature differences are to be stated in Kelvin, not in degrees Celsius. However, the corresponding numerical values ​​are the same.

According to the standard (recommendation according to DIN 1345) should also Temperature differences are always given in Kelvin, not in ° C. For example, boiling water (at normal pressure) is 100 K warmer than ice water.

Temperature differences are often relevant in energy technology. For example, the density of the heat flow through thermal insulation depends on the temperature difference between the two sides, which is specified in Kelvin. This is why the heat transfer coefficient (U-value) of a component or a house wall is expressed in units of W / (m2 K) (watts per square meter and Kelvin) and not in W / (m2 ° C). The temperature spread in a central heating system is also given in Kelvin.