Kelvin (K)
The kelvin is the SI base unit of thermodynamic temperature, named after the Irish-Scottish physicist William Thomson, 1st Baron Kelvin. Unlike Celsius and Fahrenheit, the kelvin scale starts at absolute zero - the lowest temperature physically possible, where all thermal motion ceases. Scientists, engineers, and technicians worldwide use kelvin for precise temperature measurements in physics, chemistry, astronomy, and materials science.
Definition
The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649 × 10-23 J/K. Absolute zero (0 K) is the lowest possible temperature, equal to −273.15°C or −459.67°F. One kelvin has the same magnitude as one degree Celsius - the scales differ only in their zero point. To convert kelvin to Celsius, subtract 273.15. To convert kelvin to Fahrenheit, multiply by 9/5 and subtract 459.67. The kelvin symbol is K (not °K), and it is written without a degree sign.
History
William Thomson proposed the concept of an absolute temperature scale in 1848, arguing that temperature should be measured from the point at which molecular motion stops entirely. He based his scale on the Celsius degree but shifted the zero point to absolute zero (−273.15°C). The unit was originally called "degree Kelvin" and written °K, but in 1967 the General Conference on Weights and Measures dropped the word "degree" and the ° symbol, making it simply "kelvin" (K). In 2019, the kelvin was redefined by fixing the numerical value of the Boltzmann constant at exactly 1.380649 × 10-23 joules per kelvin, decoupling it from the properties of water and grounding it in a fundamental physical constant.
Common Uses
In physics and chemistry, kelvin is the standard unit for expressing thermodynamic temperatures, reaction kinetics, and gas law calculations (the ideal gas law PV = nRT uses kelvin). Astronomers use kelvin to describe stellar surface temperatures - the Sun's surface is approximately 5,778 K. Color temperature of lighting is specified in kelvin: warm white bulbs are around 2,700 K, daylight bulbs around 5,000-6,500 K. Cryogenics and superconductor research operate in ranges from millikelvins to a few hundred kelvin. Industrial furnaces, semiconductor manufacturing, and calibration laboratories all use kelvin for precision.
Did You Know? Facts About Kelvin
- Absolute zero (0 K) has never been reached in a laboratory, though scientists have cooled atoms to within billionths of a kelvin above it.
- The cosmic microwave background radiation, the afterglow of the Big Bang, has a temperature of about 2.725 K.
- The kelvin is the only SI base unit whose name starts with a lowercase letter when written out, even though its symbol K is uppercase.
- The Sun's core temperature is approximately 15.7 million kelvin, while its surface is a relatively cool 5,778 K.
- LED and monitor color temperatures use kelvin: 2,700 K looks warm and yellowish, 6,500 K looks cool and bluish, and 10,000 K appears almost violet-white.