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Planck's constant has units of energy multiplied by time, which are the units of action. These units may also be written as momentum times distance ( N· m· s), which are the units of angular momentum.
A closely-related quantity is the reduced Planck constant (sometimes called Dirac's constant):
where π is the constant pi. This constant is pronounced as "h-bar".
The figures cited here are the 2002 CODATACODATA (Committee on Data for Science and Technology) was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. It seeks to improve the compilation, c-recommended values for the constants and their uncertainties. The 2002 CODATA results were made available in December 2003 and represent the best-known, internationally-accepted values for these constants, based on all data available through 31 December 2002. New CODATA figures are scheduled to be published approximately every four years.
Planck's constant is used to describe quantization, a phenomenon occurring in microscopic particleA particle is # In particle physics, a basic unit of matter or energy. In ecology, a small object of nonbiological kind. In linguistics, a grammatical particle. In computer graphics, an element of a particle system (simulation). In common use: a very smals such as electronThe electron (also called negatron commonly represented as e&minus is a subatomic particle. In an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. Electrons have the smallest electrical charge and when they movs and photonFor the Japanese anime video, see Photon (anime). In physics, the photon (from Greek φοτος, meaning light is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by qus in which certain physical properties occur in fixed amounts rather than assuming a continuous range of possible values. For instance, the energy E carried by a beam of lightLight is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. The three basic dimensions of light (and of all electromagnetic radiation with constant frequencyFrequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the l ν can only take on the values
It is sometimes more convenient to use the angular frequencyIn physics (specifically mechanics and electrical engineering), angular frequency ω (also called angular speed is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity . The term angular freque ω=2πν, which gives
Many such "quantization conditions" exist. A particularly interesting condition governs the quantization of angular momentum. Let J be the total angular momentum of a system with rotational invariance, and Jz the angular momentum measured along any given direction. These quantities can only take on the values
Thus, may be said to be the "quantum of angular momentum".
Planck's constant also occurs in statements of Heisenberg's uncertainty principle. The uncertainty (more precisely: the standard deviation) in any position measurement, Δx, and the uncertainty in a momentum measurement along the same direction, Δp, obeys
There are a number of other such pairs of physically measurable values which obey a similar rule.
On some browsers, the Unicode symbol ℎ (ℎ) is rendered as Planck's constant, and the symbol ℏ (ℏ) is rendered as Dirac's constant.
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