SI Base Units and Their Defining Constants
| Quantity | Unit | Symbol | Defined By | Constant Used |
|---|---|---|---|---|
| Length | meter | m | Distance light travels in 1/299,792,458 seconds | Speed of light $c$ |
| Time | second | s | 9,192,631,770 cycles of Cs-133 microwave radiation | Caesium frequency |
| Mass | kilogram | kg | Defined via Planck constant using Kibble balance | Planck constant $h$ |
| Electric current | ampere | A | Defined via elementary charge | Elementary charge $e$ |
| Temperature | kelvin | K | Defined via Boltzmann constant | Boltzmann constant $k$ |
| Amount of substance | mole | mol | Number of entities in 12 g of carbon-12 | Avogadro constant $N_A$ |
| Luminous intensity | candela | cd | Luminous efficacy at 540 THz | Luminous efficacy $K_{\text{cd}}$ |
All SI base units are now defined using fixed values of fundamental physical constants.
Constants Explained
- $h$ (Planck constant): $6.62607015 \times 10^{-34}$ $J \cdot S$ — links energy to frequency.
- $e$ (Elementary charge): $1.602176634 \times 10^{-19}$ $C$ — charge of one proton/electron.
- $k$ (Boltzmann constant): $1.380649 \times 10^{-23}$ $J/K$ — relates temperature to energy.
- $N_A$ (Avogadro constant): $6.02214076 \times 10^{23}$ $mol^{-1}$ — particles per mole.
- $K_{\text{cd}}$ (Luminous efficacy): 683 $lm/W$ — lumens per watt at $540 THz$