Planck constant

The constant factor which relates the energy of a photon to its frequency. Symbol, h. According to the 2014 CODATA recommended values, it is 6.626 070 040 × 10⁻³⁴ joule-seconds, with a standard uncertainty of 0.000 000 081 × 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 1.054 571 800 x 10⁻³⁴ J s.

Setting a fixed value for the Planck constant

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.

Replacing the International Prototype of the Kilogram with h

The Planck constant is given in joule seconds. The joule, a unit of energy, is

a fraction, meter squared times kilogram over seconds squared.

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