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Planck's constant, denoted h, named after the physicist Max Planck, is a physical constant which appears in all quantum mechanical equations. Its value is approximately
Planck's constant can be seen as a conversion factor between frequency and energy, especially for photons. The unicode symbol ℎ (ℎ) can be used for Planck's constant.
The abbreviation
where π is Pi, is commonly encountered. It is pronounced as "hbar". The constant <math>\hbar</math> is sometimes referred to as Dirac's constant after Paul Dirac. The unicode symbol ℏ (ℏ) can be used for this on some browsers.
<math>\hbar</math> is the quantum of angular momentum, including spin. The angular momentum of any system, measured against any particular choice of axis, is always an integer multiple of this value. <math>\hbar</math> also occurs in Heisenberg's uncertainty principle. It has therefore been argued that <math>\hbar</math> is more fundamental than h. <math>\hbar</math> is used to define the Planck units.
see also: Electromagnetic radiation, Schrödinger equation, Heisenberg's uncertainty principle, Waveparticle duality, Quantum Hall effect
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