Physical constants: the 25 most written about

Speed of light

Speed of electromagnetic waves

The speed at which electromagnetic waves travel. Light moves more slowly inside materials such as water or glass, and fastest in a vacuum.

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Planck constant

Physical constant in quantum mechanics

Also called Planck's constant and written h, a fundamental physical constant central to quantum mechanics. A photon's energy equals its frequency times the Planck constant, and a particle's momentum equals the wavenumber of its matter wave times the same constant.

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Gravitational constant

Physical constant for the strength of gravity induced by a mass

An empirical physical constant giving how strong the gravitational field induced by a mass is. Calculations of gravitational effects use it both in general relativity, Albert Einstein's theory, and in the law of universal gravitation of Isaac Newton.

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Avogadro constant

Conversion constant for amount of substance

A conversion constant of chemistry, usually written N_A: the ratio between an amount of substance and how many particles it contains. The particles can be any chosen elementary entity, for instance atoms, molecules, ions or ion pairs.

Wikidata · Photo: 胡景翰教授, CC BY-SA 4.0

Boltzmann constant

Physical constant relating particle kinetic energy with temperature

The factor of proportionality linking the average relative thermal energy of a gas's particles to the gas's thermodynamic temperature. It appears in the definitions of the molar gas constant and the kelvin (K), in Boltzmann's entropy formula and Planck's law of black-body radiation, and in calculations of thermal noise in resistors.

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Gas constant

Physical constant equivalent to the Boltzmann constant, but in different units

Also called the molar gas constant and written R, the molar counterpart of the Boltzmann constant: its units are energy per amount of substance per temperature increment, where the Boltzmann constant's are per particle. It also combines the constants of Boyle's law, Charles's law, Gay-Lussac's law and Avogadro's law.

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Elementary charge

Charge carried by one electron

Usually written e, the size of the charge carried by an electron: a fundamental physical constant fixed at exactly e = 1.602176634×10⁻¹⁹ C. It measures how strongly the electromagnetic force couples, and is one of the seven constants that define the International System of Units (SI).

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Cosmological constant

Value representing energy density of space

Also called Einstein's cosmological constant, a coefficient that Albert Einstein first added to the field equations of general relativity and later took out. Much later it was brought back in physical cosmology to express the energy density of space, the vacuum energy that arises in quantum mechanics.

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Faraday constant

Physical constant: Electric charge of one mole of electrons

A physical constant of physical chemistry, written F and sometimes styled ℱ. For any sample of matter it is the total electric charge (q) divided by how many elementary charge carriers (n) the sample holds: F = q/n. Measured in coulombs per mole (C/mol), it amounts to the "molar elementary charge", the electric charge carried by one mole of elementary carriers such as protons.

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Chandrasekhar limit

Maximum mass of a stable white dwarf star

The greatest mass a stable white dwarf star can have. Such stars hold off gravitational collapse mainly through electron degeneracy pressure, whereas main sequence stars do so through thermal pressure.

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Fine-structure constant

Dimensionless number that quantifies the strength of the electromagnetic interaction

Also called the Sommerfeld constant and usually written α, a fundamental physical constant that measures how strong the electromagnetic interaction is between charged elementary particles.

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Bohr radius

Unit of length about the size of a hydrogen atom

A physical constant roughly equal to the most likely distance between the electron and the nucleus of a hydrogen atom in the ground state. It bears the name of Niels Bohr because of its part in his model of the atom.

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Solar constant

Intensity of sunlight or solar constant

A mathematical equation giving the average energy that reaches a given area one astronomical unit from the Sun. Put simply, it measures how much solar energy per second per square metre arrives at the distance of Earth's orbit.

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Rydberg constant

Physical constants of energy and wavenumber

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Vacuum permittivity

Absolute dielectric permittivity of free space

Usually written ε₀, the absolute dielectric permittivity of a classical vacuum. It is also called the electric constant, the permittivity of free space or the distributed capacitance of the vacuum.

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Bohr magneton

Unit of magnetic moment

A physical constant of atomic physics and the natural unit for the magnetic moment an electron has because of its spin or orbital angular momentum.

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Vacuum permeability

Physical constant

The magnetic permeability of a classical vacuum. This physical constant is usually written μ₀, and its value is close to 4π × 10⁻⁷ H/m.

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Tolman–Oppenheimer–Volkoff limit

Upper bound to the mass of cold, nonrotating neutron stars

The upper limit on the mass of neutron stars that are cold and not rotating, the counterpart for them of the Chandrasekhar limit for white dwarfs. A star heavier than this TOV limit collapses into a black hole, or possibly into a quark star.

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Nuclear magneton

Physical constant

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Standard gravity

Standard gravitational acceleration on Earth

Also called the standard acceleration of gravity or of free fall, the nominal acceleration due to gravity of an object in a vacuum close to the Earth's surface. ISO standard 80000 defines it as the constant 9.80665 m/s², usually written ɡ₀.

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Classical electron radius

Physical constant providing length scale to interatomic interactions

A combination of fundamental physical quantities setting a length scale for problems in which an electron interacts with electromagnetic radiation. Picture a classical charged sphere whose electric field holds as much energy as the electron's rest mass energy: its radius would be the classical electron radius.

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Hartree

Unit of energy in the atomic units system

Also called the Hartree energy and written Eₕ, the unit of energy in the system of atomic units, named after the British physicist Douglas Hartree. CODATA recommends a value of 27.211386245981(30) eV, equal to 4.3597447222060(48)×10⁻¹⁸ J.

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Loschmidt constant

Number of particle in a given volume of an ideal gas

Also called Loschmidt's number and written n₀, the count of an ideal gas's particles (atoms or molecules) in a unit of volume, its number density, usually given at standard temperature and pressure. The value CODATA recommended in 2018 is 2.686780111...×10²⁵ m⁻³, taken at 1 atm and 0 °C.

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Impedance of free space

Physical constant; ratio of electric to magnetic field strength in a vacuum

Written Z₀, a physical constant of electromagnetism: for electromagnetic radiation moving through free space, it relates the strength of the electric field to that of the magnetic field.

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Magnetic flux quantum

Quantized unit of magnetic flux

The magnetic flux through a contour is the magnetic field multiplied by the area of the loop. In certain physical systems, superconductors among them, that flux can take only particular fixed values.

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