Kelvin

The kelvin ( unit symbol: K) is the SI base unit of thermodynamic temperature and at the same time legal unit of temperature; It is also used to indicate temperature differences. In Germany, Austria, Switzerland and other European countries, the degree Celsius ( unit symbol: ° C ) is considered a legal unit for expressing Celsius temperatures and their differences. It refers to 0 ° C converted 273.15 K.

The temperature difference specification degree ( deg ) has been replaced by the Kelvin. Until 1967 was the Unit Name degrees Kelvin, the unit symbol was ° K.

The kelvin is according to William Thomson, later Lord Kelvin, named, who introduced the thermodynamic temperature scale at age 24.

Definition

The Kelvin was the CGPM for the first time in 1954 - again defined and in as it stands in 1968 and established as a SI base unit - then as degrees Kelvin:

What is meant here is pure water having the isotopic composition should be based on the Vienna Standard Mean Ocean Water ( VSMOW ) and its triple point is at ( 0.01 ° C).

By this setting has been reached, that the difference between two temperature values ​​by a Kelvin and degrees Celsius, are of equal size and can be used interchangeably.

The zero point of the Kelvin scale is absolute zero -273.15 ° C. This temperature is, however, according to the Nernst heat theorem neither measurable nor achievable, since particles have no kinetic energy at 0 K ( the remaining energy - zero energy - is a result of the Heisenberg uncertainty principle ).

The improvements in measurement technology made ​​the reproducibility of the two fixed points ' freezing and boiling point of water to the problem: in particular, the boiling point is highly dependent on the air pressure, which in turn depends on the height above sea level and the weather. The triple point of a substance, however, is a ( always and everywhere ) constant material property - that is, when the agent is at its triple point, it always has the same temperature and pressure. However, It is not only on their chemical purity, but also on their isotopic composition.

One reason 1954 was transferred to the temperature calibration only has a single fixed point, the triple point of water to make. This is easily and clearly reproducible, since water only at the triple point at the same firm, is liquid and gaseous. Is relevant to the practice that the triple point of water is one of the temperature fixed points, which are best known and can be represented most accurately. The pressure at the triple point 6.105 mbar, the temperature of 0.01 ° C. May, together with the absolute zero be defined by the absolute temperature scale:

Properties

The kelvin is commonly used, especially in thermodynamics, heat transfer and in science and technology for specifying temperatures and temperature differences.

With the Kelvin definition is also the temperature of the triple point of water, especially the VSMOW determined, on the value of 273.16 K. The melting point temperature of the water in normal conditions, however, is shifted by about 0.01 K, it rests on the Kelvin scale at about 273,15 K.

The temperature is linked by this definition of the power, that is, the energy content of a body or system and is therefore called thermodynamic temperature. Contains a physical object no energy, then it has the temperature 0 K and thus is at absolute zero. When the numerical value is a temperature on the Kelvin Scale times as large as that of a different temperature, the energy content is twice as high as that of ( in contrast, see the Celsius scale). Atomistic in view, one can say that in the Kelvin scale the average kinetic energy of the particles (atoms or molecules) is proportional to the temperature, that is a double kinetic energy corresponds to a double temperature ( in Kelvin). A further connection is derived from the Maxwell -Boltzmann distribution: a doubling of the temperature on the Kelvin scale leads to an increase of the ideal gas particle velocity by a factor of the root mean square.

History

The divisions proposed by William Thomson absolute temperature scale first carried the designation ° A (for absolute). In the SI was 1948-1968 ° K the (degrees Kelvin, and 1954 also " degree absolute ") as the temperature unit. In addition, during the same period were temperature differences - expressed in deg ( degrees) - in contrast to temperature data. The use of these old units is no longer allowed in Germany today. As early as 1948 by the CGPM was an absolute thermodynamic scale with the triple point of water defined as the only fundamental benchmark, but not yet linked to the temperature. The steadily reduced uncertainties in the measurement of the temperature of the Wassertripelpunktes made ​​it possible in the 21st century, the influence of the isotopic composition on the triple point of water to determine (on the order of about 10 mK). The necessary clarification of the definition of the Kelvin took place in 2005 at the 94th Meeting of the CIPM, which should be used as a reference standard purified ocean water; The wording of the Kelvin - definition however, has not changed. The triple point is the calibration of temperature measurement instruments for other temperature ranges unwieldy. But since 1990 there is the ITS -90 ( " International Temperature Scale of 1990 "). It lists more towards distributed over a wide temperature range reference values ​​, for example, well-defined melting points; the triple point of water is a central point of reference here.

Aimed at redefining

As desired in all SI units, is also defined in the future regardless of Kelvin materials are thus attributed to physical constants, such as for example, when m is now the case.

Therefore, working on a redefinition of the kelvin, which could be based for example on the determination of the Boltzmann constant. The results of this international project, in which for example the Physikalisch- Technische Bundesanstalt is leading the work package for primary thermometry for low temperatures, in the framework of international conferences such as the TEMPMEKO 2013 and 2016 and a workshop on this topic, which is scheduled for 2014 to expect.

Color temperature

The color temperature is given in Kelvin. It is important in the photograph and characterization of light sources. The color temperature is the spectral radiance distribution of a black body (see Stefan- Boltzmann law ), which has the temperature = color temperature. For incandescent lamps with wavelength-dependent emissivity or from non- thermal light sources, the color temperature of the temperature of the radiator differs.

According to Wien 's displacement law, the wavelength shift of the spectral radiation peak is proportional to the temperature change in Kelvin.

Ratio pyrometers use this context from the temperature measurement of a body whose emissivity - independent temperature measurement. Requirement is that it is in the reception area is a " gray" spotlights, ie that it has the same emissivity at two receiving wavelengths.

Temperature and energy

It is often important to know whether an energy barrier can be overcome solely due to thermal fluctuations. The probability to overcome the barrier gives the Boltzmann distribution:

The Boltzmann constant. A barrier is virtually never overcome, at it is easily overcome and when the barrier is almost not noticed.

For simplicity, therefore, we are often energies in Kelvin or temperatures in energy units such as joules or electron volts (eV ). The conversion factors are then:

This will be illustrated using the example of the hydrogen molecule:

• At which temperature rotates the hydrogen molecule?

• At which temperature the hydrogen atoms vibrate against each other?

→ See also: Arrhenius equation

Tables

Temperature conversion

Temperature comparison

Prefixes

When temperatures are prefixes are relatively uncommon. For small values ​​mK, nK μK and can be used other derivatives hardly come before.