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Kelvin Scale

Kelvin Scale

Kelvin scale definition

The Kelvin temperature scale is an absolute temperature scale with zero at absolute zero. Kelvin is the base unit of temperature in the International System of Units (SI). Together with the degree Celsius, the Kelvin degree is one of the two most widely used temperature units worldwide.

Kelvin is the temperature unit of the International System. Kelvin is one of the seven basic units of temperature. Its symbol in the international system of units is K.

The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero, the theoretical absence of energy, is zero (0 K). This unit is named after the British physicist, mathematician and engineer William Thomson (1824 - 1907), who was later named Lord Kelvin. Lord Kelvin wrote about the need for a "thermodynamic temperature scale". Although it is usual to meet the reference degree Kelvin, the reality is that being an absolute scale has no degrees. This is a difference with respect to the Celsius and Farenheit scales that do have degrees.

To express the temperature difference or the interval, the use of kelvin degrees (or kelvins) instead of degrees Celsius helps avoid situations in which these quantities are confused with the centigrade temperature. Kelvin is the unit of temperature suitable for use in derived units, such as W / (m · K) or to have a prefix, such as milli in mK.

Difference between Kelvin and Kelvin degrees

Unlike degrees Fahrenheit and degrees Celsius, Kelvin is not a "degree," nor should it be written with the symbol of degrees. The correct unit name is Kelvin (started with lowercase). Sometimes it can be seen written as Kelvin degree, or with the ° K symbol by analogy with the Celsius degree, but it is not correct. Their units are kelvins and they express with a single K.

Kelvin was defined in 1954 as a Kelvin degree at the 10th General Conference of Weights and Measures. By decision taken in 1967 at the 13th CGPM the name becomes the current Kelvin.

In 1967/1968, Resolution 3 of the 13th General Conference of Weights and Measures changed the name of the unit thermodynamic temperature increase "Kelvin", symbol K, replacing "Kelvin grade", symbol ° K.

Conversion between Kelvin temperature scales

Unlike most conversions between different units of measurement of physical quantities, for which it is sufficient to multiply or divide by a certain factor, conversions between different temperature scales may involve a term that must be added or subtracted in addition to a factor of multiplication This comes from the fact that different temperature scales define a different "zero". In addition, the Kelvin measurement system refers to a particular "zero", which is absolute zero, and corresponds to the lowest temperature which, in theory, can be obtained in any macroscopic system. This causes the temperature measured in "Kelvin" to be called absolute temperature, and for the same reason, the degree symbol (°) is not placed before the Kelvin symbol (K),

Although the Celsius scale and the Kelvin scale have different references, the values ​​of the temperature ranges (that is, the temperature differences) measured with the two scales coincide. This means that, while a temperature T of 25.00 degrees Celsius corresponds to 298.15 k, a temperature difference ΔT of 25.00 ° C corresponds exactly to 25.00 k.


Definition of Kelvin

Kelvin (former degree Kelvin) is defined based on 2 factors,

  • Zero Kelvin corresponds to absolute zero, which is the minimum temperature that can be achieved, and corresponds to the lack of movement at the molecular level. This temperature is equivalent to -273.15 degrees Celsius, and -459.67 degrees Fahrenheit
  • A kelvin is exactly 1 / 273.16 of the thermodynamic temperature of the triple point of water. A difference of a kelvin is equivalent to that of a Celsius degree, currently, the Celsius scale is defined from the kelvin and the Celsius degree is a unit derived from the international system of units.
  • In 2007, the 23rd General Conference of Weights and Measures adopted a resolution clarifying that the water referred to in the definition of Kelvin must have a specific isotropic composition.

    Practical uses of Kelvin

    The Kelvin degree is used in many aspects of physics. However, we present two applications in which Kelvin has a special importance.

    Color temperature

    Kelvin grade is often used to measure the color temperature of light sources. The color temperature is based on the principle that a black body radiator emits light whose color depends on the temperature of the radiator. Black bodies with temperatures below approximately 4000 K appear reddish, while those above approximately 7500 K appear bluish.

    The color temperature is important in the fields of imaging and photography, where a color temperature of approximately 5600 K is required to match the "daylight" film emulsions. In astronomy, the star classification of stars and their place in the Hertzsprung-Russell diagram are based, in part, on their surface temperature, known as the effective temperature. The photosphere of the Sun, for example, has an effective temperature of 5778 K.

    Digital cameras and photographic software often use the color temperature in K in the editing and configuration menus. The simple guide is that the higher the color temperature, the whiter or blue the image will be. The reduction in color temperature will give an image more dominated by reddish, "warmer" colors.

    Kelvin as a measure of noise

    In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to the final noise floor, that is, the temperature of the noise. The so-called Johnson-Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit that uses Friis formulas for noise.

    Kelvin scale history

    The Kelvin scale and its unit have had different development milestones throughout history:

    1848: William Thomson, the first Baron of Kelvin wrote in his diary On the absolute thermometric scale, of the need for a scale where "the infinite cold" (absolute zero) was the null point of the ladder, and which used the degree Celsius for Your increase in unity. This absolute scale is known today as the Kelvin thermodynamic temperature scale. It is notable that the Thomson value of -273.15 actually obtained 0.00366, which was the accepted coefficient of expansion of the gas by degree Celsius relative to the freezing point. The inverse of -0.00366 expressed with five significant digits is -273.22 ° C which is extraordinarily on the verge of the true value of -273.15 ° C.

    1954: Resolution 3 of the 10th CGPM gave the Kelvin scale its modern definition, designating the triple point of water as its second point that defines and assigns its temperature to exactly 273.16 degrees Kelvin.

    1967/1968: Resolution 3 of the 13th CGPM renamed the increase in thermodynamic temperature as kelvin, symbol K, which replaced absolute degree, symbol ° K. In addition, it is useful to define more explicitly the magnitude of the increase, the 13th CGPM also affirms at resolution 4 that Kelvin, a unit of thermodynamic temperature, is equal to the fraction 1 / 273.16 of the thermodynamic temperature of the triple point of water.

    2005: The International Committee of Weights and Measures (CIPM), a CGPM committee, stated that for the purposes of delineating the temperature of the triple point of water, the Kelvin thermodynamic temperature scale definition would refer to water with a composition isotopic specified in the Vienna Standard Mean Ocean Water (VSMOW).

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    Last review: January 28, 2020