Planck Constant

The quantum of action linking a photon's energy to its frequency, fixed exactly to define the kilogram.

Specifications & Metrics

symbol

h

status

Exact — SI defining constant (2019)

value

6.62607015 × 10⁻³⁴ J·Hz⁻¹

Relative Scale in Constants

Core Principles & Analysis

• Introduced by Max Planck in 1900 to explain blackbody radiation, the act that began quantum theory.
• Relates a photon's energy to its frequency through E = hf.
• Fixed exactly in the 2019 SI redefinition, making the Kibble balance the kilogram's realization.

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