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Concept
Kelvin
Summary
The 'kelvin', symbol K, is a unit of measurement for temperature. The Kelvin scale is an absolute scale, which is defined such that 0 K is absolute zero and a change of thermodynamic temperature T by 1 kelvin corresponds to a change of thermal energy kT by 1.380649e−23J. The Boltzmann constant was exactly defined in the 2019 redefinition of the SI base units such that the triple point of water is 273.16K. The kelvin is the base unit of temperature in the International System of Units (SI), used alongside its prefixed forms. It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).
Historically, the Kelvin scale was developed from the Celsius scale, such that 273.15 K was 0 °C (the approximate melting point of ice) and a change of one kelvin was exactly equal to a change of one degree Celsius. This relationship remains accurate, but the Celsius, Fahrenheit, and Rankine scales are now defined in terms of the Kelvin scale. The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries the Celsius scale remains the dominant scale outside of these fields. In the United States, outside of the physical sciences, the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature.
Thermodynamic temperature#History
During the 18th century, multiple temperature scales were developed, notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including atomic theory and the kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale) the melting point of water served as such a starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that
The freezing point of water is 0 degrees.
Official source
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