kelvin (unit)
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| kelvin (unit) | |
|---|---|
| Name | kelvin |
| Namedafter | William Thomson, 1st Baron Kelvin |
| Quantity | Thermodynamic temperature |
| Unitsystem | SI |
kelvin (unit). The kelvin, symbol K, is the SI base unit of thermodynamic temperature. It is defined by fixing the numerical value of the Boltzmann constant to exactly 1.380649×10⁻²³ when expressed in the unit J·K⁻¹, which is equal to kg·m²·s⁻²·K⁻¹. This definition directly links temperature to energy, making the kelvin a fundamental unit for scientific measurement across disciplines from physics to meteorology.
Definition and scale
The kelvin is defined using the Boltzmann constant, a fundamental constant of nature that relates the average kinetic energy of particles in a gas to the thermodynamic temperature. One kelvin is equal to a change in thermodynamic temperature that results in a change of thermal energy kT by exactly 1.380649×10⁻²³ joule. The Kelvin scale is an absolute temperature scale, meaning its zero point, absolute zero, is the theoretical point at which particles have minimal thermal motion. This contrasts with relative scales like the Celsius scale or Fahrenheit scale, which are defined relative to the freezing point of water and the average human body temperature, respectively. The size of one kelvin increment is precisely equal to one degree on the Celsius scale, making conversion between the two straightforward.
History
The scale is named for the British physicist and engineer William Thomson, 1st Baron Kelvin, who first proposed an absolute thermometric scale in 1848. His work built upon earlier concepts from Sadi Carnot and Benoît Paul Émile Clapeyron regarding the efficiency of heat engines. For many years, the unit was defined by two fixed points: the triple point of water and absolute zero. The General Conference on Weights and Measures adopted the name "kelvin" as the SI unit in 1967, replacing the earlier term "degree Kelvin." A major redefinition occurred in 2019 as part of a broader overhaul of the SI base units, shifting the definition from a material property to a fixed fundamental constant, the Boltzmann constant, thereby ensuring the unit's stability and universality.
Usage conventions
In scientific writing, the unit is simply "kelvin" (plural: kelvins) and is never preceded by the word "degree" or the degree symbol (°). This convention distinguishes it from the degree Celsius and degree Fahrenheit. The symbol is always an uppercase K. When expressing temperature intervals or differences, such as in calorimetry or reporting a climate change anomaly, the kelvin is used exclusively within the SI system. For example, a temperature increase of 10 K is equivalent to an increase of 10 °C. The International Bureau of Weights and Measures provides strict guidelines on unit usage to maintain global consistency in fields like metrology and standardization.
Related units
The most directly related unit is the degree Celsius, where a temperature in degrees Celsius is defined as the temperature in kelvins minus 273.15. The degree Rankine is the absolute temperature unit in the Imperial system, with one degree Rankine equal to ⁵⁄₉ of a kelvin. In some specialized scientific contexts, such as plasma physics or astrophysics, temperatures may be expressed in electronvolts (eV) using the Boltzmann constant as a conversion factor. Other historical temperature scales, like the Réaumur scale or the Delisle scale, are now largely obsolete but provide context for the development of thermometry.
Practical applications
The kelvin is crucial in fields requiring precise temperature measurement and control. In cryogenics, researchers working with superconductivity or Bose–Einstein condensates routinely deal with temperatures within a few millikelvins of absolute zero. The Cosmic Microwave Background radiation, a remnant of the Big Bang, has been measured by satellites like the Cosmic Background Explorer to be approximately 2.725 K. In climate science, global temperature anomalies are often tracked in kelvins to avoid ambiguity. The unit is also fundamental in defining the International Temperature Scale of 1990, a practical calibration standard used by national institutes like the National Institute of Standards and Technology and National Physical Laboratory (United Kingdom).