Boltzmann constant
The fundamental constant, written k, that links temperature to the energy per atom.
A high temperature means atoms and molecules are moving violently. Multiplied by the absolute temperature, the Boltzmann constant gives a measure of the average energy per atom. Its value is 1.380649×10⁻²³ joules per kelvin.
It is named after Boltzmann, but it was Planck in 1900 who wrote it as k and first found its value[1]. Fitting the blackbody formula to measurements fixed it together with Planck's constant h, and from it he derived Avogadro's number and the charge of a single electron more accurately than direct measurements of the time.
Since 2019 this value has been fixed exactly by definition and is used to define the kelvin, the unit of temperature[2].
Related terms
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
- EntropyA quantity measuring how many ways a thing can be spread around. In a system that exchanges nothing with its surroundings, entropy can increase but never decrease.
- Blackbody radiationLight emitted by an ideal body that absorbs all light falling on it. Its spectrum depends only on temperature, not on what the body is made of.