Planck constant
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The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalence, the relationship between mass and frequency. Specifically, a photon's energy is equal to its frequency multiplied by the Planck constant. The constant is generally denoted by ℎ. The reduced Planck constant, or Dirac constant, equal to ℎ divided by 2𝝅, is denoted by ℏ.
In metrology it is used, together with other constants, to define the kilogram, the SI unit of mass. The SI units are defined in such a way that, when the Planck constant is expressed in SI units, it has the exact value:
- ℎ = 6.62607015×10−34 J⋅Hz−1
It is often used with units of eV, which corresponds to the SI unit per elementary charge.
