Physical constant relating temperature to energy, appearing in entropy expressions.The Boltzmann constant (symbol: k or kB) is a fundamental physical constant that connects the microscopic world of atoms and molecules to the macroscopic world of everyday temperatures and energies. It is the proportionality factor between the average kinetic energy of particles in a gas and the absolute temperature of that gas. More deeply, it appears in the statistical definition of entropy, S = k ln W, where W is the number of microscopic configurations consistent with a macroscopic state. This makes it the bridge between thermodynamics and statistical mechanics, giving a quantitative meaning to the idea that temperature is a measure of the average random motion of particles.
The constant was introduced by Max Planck in 1900 while deriving the blackbody radiation law, and he named it after Ludwig Boltzmann, who had pioneered statistical mechanics but never explicitly used the constant in its modern form. The value was first measured accurately by Planck using radiation data.
In kinetic theory, the average translational kinetic energy of a particle in an ideal gas is (3/2)kT, so the constant converts temperature (in kelvin) into energy (in joules). In statistical mechanics, the Boltzmann factor exp(-E/kT) gives the relative probability of a system being in a state of energy E at temperature T. The constant sets the energy scale for thermal fluctuations: at room temperature, kT is about 1/40 of an electronvolt, which is why chemical reactions and biological processes operate at that energy scale.
It is used in every calculation involving thermal noise, from the Johnson–Nyquist noise in electronic circuits to the design of infrared detectors and the calibration of thermometers. It also defines the kelvin itself, since 2019 the kelvin is defined by fixing the value of k exactly.
The Boltzmann constant is the only fundamental constant that has a dedicated unit (joules per kelvin) named after a person, and its value was chosen in 2019 so that the kelvin is defined by setting k = 1.380649 × 10⁻²³ J/K exactly.
| S | variable | entropy of the system — entropy joule per kelvin |
| Ω | variable | number of microstates corresponding to the macrostate — number of microstates |
| k_B | constant | relates temperature to energy per degree of freedom — boltzmann constant = 1.380649e-23 joule per kelvin |
| ln | operator | natural logarithm — natural logarithm |
| S | variable | Entropy — entropy joule per kelvin |
| W | variable | Number of microstates — number of states |
| k | constant | Relates temperature to energy in statistical mechanics — boltzmann constant = 1.380649e-23 joule per kelvin |
| \leq | operator | Indicates an upper bound — less than or equal |
| \ln | operator | Computes the logarithm to base e — natural logarithm |
| ⟨hν⟩ | variable | Average photon energy — energy joule |
| T | variable | Temperature of the blackbody — temperature kelvin |
| c | constant | A numerical factor that depends on the frequency integration limits and the specific definition of 'average' (e.g., over the full spectrum). — proportionality constant = 2.701 |
| k_B | constant | Fundamental constant converting temperature to energy scale — boltzmann constant = 1.380649e-23 joule per kelvin |
| ⟨ ⟩ | operator | Statistical average over the photon distribution — ensemble average |
| T | variable | Hawking temperature of the black hole — temperature kelvin |
| M | variable | Mass of the black hole — mass kilogram |
| \hbar | constant | Reduced Planck constant, fundamental quantum of action — reduced planck constant = 1.054571817e-34 joule second |
| c | constant | Speed of light in vacuum, universal speed limit — speed of light = 299792458 metre per second |
| G | constant | Newtonian constant of gravitation, sets strength of gravity — gravitational constant = 6.67430e-11 cubic metre per kilogram per second squared |
| k_B | constant | Boltzmann constant, relates temperature to energy — boltzmann constant = 1.380649e-23 joule per kelvin |
| S | variable | Entropy of the black hole — entropy joule per kelvin |
| A | variable | Area of the event horizon — area square metre |
| k | constant | Boltzmann constant — boltzmann constant = 1.380649e-23 joule per kelvin |
| c | constant | Speed of light in vacuum — speed of light = 299792458 metre per second |
| G | constant | Newtonian gravitational constant — gravitational constant = 6.67430e-11 cubic metre per kilogram per second squared |
| ħ | constant | Reduced Planck constant — reduced planck constant = 1.054571817e-34 joule second |
| c_s | variable | adiabatic sound speed — sound speed metre per second |
| T | variable | temperature — temperature kelvin |
| m | variable | mean particle mass — mass kilogram |
| γ | constant | ratio of specific heats (Cp/Cv) of the gas — adiabatic index = varies (e.g., 5/3 for monatomic ideal ga |
| k_B | constant | relates temperature to kinetic energy of particles — boltzmann constant = 1.380649e-23 joule per kelvin |




