普朗克常数Planck constant

h 出现在量子力学的所有基本关系中：E = hν、p = h/λ、Δx·Δp ≥ ħ/2、角动量量子化 L = ħ√[l(l+1)]。h 极小，所以量子效应在宏观世界难以察觉；若 h 趋于零，量子力学就退化为经典力学。化学中，光谱能级差 ΔE = hν = hc/λ，例如 500 nm 光子能量约 2.48 eV，相当于 239 kJ/mol，与化学键能处于同一量级，这就是可见光和紫外光能引发光化学反应的原因。

h appears in every basic quantum relation: E = hν, p = h/λ, Δx·Δp ≥ ħ/2 and angular-momentum quantisation L = ħ√[l(l+1)]. Because h is so small, quantum effects are hidden in everyday life, and as h → 0 quantum mechanics reduces to classical mechanics. In chemistry, spectral energy gaps are ΔE = hν = hc/λ; a 500 nm photon carries about 2.48 eV, or 239 kJ/mol, comparable to bond energies, which is why visible and UV light can drive photochemistry.

1900 年普朗克为解释黑体辐射引入作用量子 h，最初只将其视为数学技巧。1905 年爱因斯坦用它解释光电效应，1913 年玻尔用它建立原子模型，h 的物理实在性才被广泛接受。1916 年密立根通过光电效应实验精确测定了 h。

Planck introduced the quantum of action h in 1900 to explain black-body radiation, at first regarding it as a mathematical device. Its physical reality was widely accepted only after Einstein used it for the photoelectric effect (1905) and Bohr for his atomic model (1913). Millikan measured h precisely from the photoelectric effect in 1916.

2019 年 5 月 20 日起，国际单位制以固定普朗克常数的数值来定义千克，取代了保存在巴黎的国际千克原器。这一数值主要通过基布尔秤（瓦特秤，将机械功率与电功率比较）和 X 射线晶体密度法测定。

From 20 May 2019 the SI kilogram has been defined by fixing the numerical value of Planck's constant, replacing the International Prototype Kilogram kept near Paris. The value was determined mainly with Kibble (watt) balances, which compare mechanical and electrical power, and by the X-ray crystal-density method.