The Planck constant (denoted h, also called Planck's constant) is a physical constant that is the quantum of action in quantum mechanics. The Planck constant was first described as the proportionality constant between the energy (E) of a photon and the frequency (ν) of its associated electromagnetic wave. This relation between the energy and frequency is called the Planck relation:
Since the frequency , wavelength λ, and speed of light c are related by λν = c, the Planck relation for a photon can also be expressed as
The above equation leads to another relationship involving the Planck constant. Given p for the linear momentum of a particle, the de Broglie wavelength λ of the particle is given by
In applications where frequency is expressed in terms of radians per second ("angular frequency") instead of cycles per second, it is often useful to absorb a factor of 2π into the Planck constant. The resulting constant is called the reduced Planck constant or Dirac constant. It is equal to the Planck constant divided by 2π, and is denoted ħ ("h-bar"):
The energy of a photon with angular frequency ω, where ω = 2πν, is given by
The reduced Planck constant is the quantum of angular momentum in quantum mechanics.
The Planck constant is named after Max Planck, one of the founders of quantum theory, who discovered it in 1900. Classical statistical mechanics requires the existence of h (but does not define its value). Planck discovered that physical action could not take on any indiscriminate value. Instead, the action must be some multiple of a very small quantity (later to be named the "quantum of action" and now called Planck's constant). This inherent granularity is counterintuitive in the everyday world, where it is possible to "make things a little bit hotter" or "move things a little bit faster". This is because the quanta of action are very, very small in comparison to everyday macroscopic human experience. Hence, the granularity of nature appears smooth to us.
Thus, on the macroscopic scale, quantum mechanics and classical physics converge at the classical limit. Nevertheless, it is impossible, as Planck discovered, to explain some phenomena without accepting the fact that action is quantized. In many cases, such as for monochromatic light or for atoms, this quantum of action also implies that only certain energy levels are allowed, and values in-between are forbidden. In 1923, Louis de Broglie generalized the Planck relation by postulating that the Planck constant represents the proportionality between the momentum and the quantum wavelength of not just the photon, but the quantum wavelength of any particle. This was confirmed by experiments soon afterwards.
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... are based on fixed "conventional values" of the Josephson constant and the von Klitzing constant, KJ–90 and RK–90 respectively ... such measurements is that they are also a direct measurement of the Planck constant h The principle of the "electronic kilogram" would be to define the value of the Planck constant in the same way that the ... measure masses to the same accuracy as they currently measure the Planck constant ...
... defined in terms of universal physical constants in such a manner that some chosen physical constants take on the numerical value of one when expressed in terms of a particular set of natural units ... units are chosen so that the speed of light and the gravitational constant are set equal to unity ... Planck units are a form of geometric units obtained by also setting Boltzmann's constant, the Coulomb force constant and the reduced Planck constant to ...
... The electron rest mass in kilograms is calculated from the definition of the Rydberg constant R∞ where α is the fine structure constant and h is the Planck constant ... value, is due entirely to the uncertainty in the value of the Planck constant ... is an adjusted parameter in the CODATA set of fundamental physical constants, while the electron rest mass in kilograms is calculated from the values of the Planck constant, the fine structure constant ...
... New SI definitions Basic concepts of quantum mechanics Planck units Stigler's law Wave–particle duality ...
... institutes to measure the Avogadro constant by the X-ray crystal density method to a relative uncertainty of 2×10−8 or less ... is part of the efforts to redefine the kilogram in terms of a universal physical constant, rather than the International Prototype Kilogram, and ... System of Units (SI), a measurement of the Avogadro constant is an indirect measurement of the Planck constant The measurements use highly polished spheres of silicon with a mass of one kilogram ...
Famous quotes containing the word constant:
“Liberalism, austere in political trifles, has learned ever more artfully to unite a constant protest against the government with a constant submission to it.”
—Alexander Herzen (18121870)