The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.Historically, the Kelvin scale was developed by shifting the starting point of the much older Celsius scale down from the melting point of water to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the Boltzmann constant k to be exactly 1.380649×10−23 J⋅K−1. Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380649×10−23 J. The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15, meaning that a change or difference in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K.
The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius remains the dominant scale outside of these fields and in the United States the Fahrenheit scale (also now defined using the kelvin) is still widespread.
Unlike the degree Fahrenheit and degree Celsius, the kelvin dates to the mid 19th century rather than the early to mid 18th, and it is never referred to nor written as a degree, nor typically capitalized except as the unit symbol K (e.g., "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside").
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