Scientific

Planck Constant

Definition

A fundamental physical constant with the exact value 6.62607015 × 10⁻³⁴ J·s, which since 2019 has been used to define the kilogram.

Explanation

The Planck constant relates the energy of a photon to its frequency: E = hν. A photon of visible green light (550 nm, 5.45 × 10¹⁴ Hz) has energy of 3.61 × 10⁻¹⁹ J = 2.25 eV. The kilogram is now defined by fixing h to exactly 6.62607015 × 10⁻³⁴ J·s, measured via a Kibble balance — replacing the 130-year-old platinum-iridium kilogram artifact.

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