The Stefan–Boltzmann constant (also Stefan's constant), a physical constant denoted by the Greek letter σ (sigma), is the constant of proportionality in the Stefan–Boltzmann law: "the total intensity radiated over all wavelengths increases as the temperature increases", of a black body which is proportional to the fourth power of the thermodynamic temperature. The theory of thermal radiation lays down the theory of quantum mechanics, by using physics to relate to molecular, atomic and sub-atomic levels. Slovenian physicist Josef Stefan formulated the constant in 1879, and it was later derived in 1884 by Austrian physicist Ludwig Boltzmann. The equation can also be derived from Planck's law, by integrating over all wavelengths at a given temperature, which will represent a small flat black body box. "The amount of thermal radiation emitted increases rapidly and the principal frequency of the radiation becomes higher with increasing temperatures". The Stefan–Boltzmann constant can be used to measure the amount of heat that is emitted by a blackbody, which absorbs all of the radiant energy that hits it, and will emit all the radiant energy. Furthermore, the Stefan–Boltzmann constant allows for temperature (K) to be converted to units for intensity (W⋅m−2), which is power per unit area.
In cgs units the Stefan–Boltzmann constant is
The Stefan–Boltzmann constant is defined in terms of other fundamental constants as
- kB is the Boltzmann constant,
- h is the Planck constant,
- ħ is the reduced Planck constant, and
- c is the speed of light in vacuum,
- R is the universal gas constant
- NA is the Avogadro constant
- R∞ is the Rydberg constant
- Ar(e) is the "relative atomic mass" of the electron
- Mu is the molar mass constant (1 g/mol by definition)
- α is the fine-structure constant.
Dimensional formula: M1T−3Θ−4
A related constant is the radiation constant (or radiation density constant) a which is given by
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