Planck constant

The constant factor which relates the energy of a photon to its frequency. Symbol, h.  According to the 2006 CODATA recommended values, it is 6.626 068 96 × 10−34 joule-seconds, with a standard uncertainty of 0.000 000 33 × 10−34.

photograph of Planck

It is named for the German physicist Max Planck.

It is often encountered as the reduced Planck constant, symbol ħ (pronounced “h bar”), which is h divided by 2 pi, or 1.054 571 628 x 10-34 J s.

At its 24th meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to make the value of the Planck constant a matter of definition, rather than something to be determined experimentally. The new value will be exactly 6.626 06X × 10−34 joule-seconds, where X stands for one or more yet to be determined digits. The new definition will not be adopted before 2014.

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