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Planck's constant

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

h = 6.6261 × 10-34 Js

Planck's constant can be seen as a conversion factor between frequency and energy, especially for photons. The unicode symbol &#8462; (ℎ) can be used for Planck's constant.

The abbreviation

$\hbar = \frac{h}{2\pi}$

where π is Pi, is commonly encountered. It is pronounced as "h-bar". The constant $\hbar$ is sometimes referred to as Dirac's constant after Paul Dirac. The unicode symbol &#8463; (ℏ) can be used for this on some browsers.

$\hbar$ 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. $\hbar$ also occurs in Heisenberg's uncertainty principle. It has therefore been argued that $\hbar$ is more fundamental than h. $\hbar$ is used to define the Planck units.