SI Units & Fundamental Physical Constants
The International System of Units (SI), maintained by the Bureau International des Poids et Mesures (BIPM), defines seven base physical quantities: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). In 2019, the General Conference on Weights and Measures (CGPM) implemented a landmark redefinition of SI units, fixing all base units to invariant fundamental physical constants rather than physical artifacts. Key fundamental constants include Planck's constant ($h = 6.626 \times 10^{-34} \text{ J}\cdot\text{s}$), the speed of light in vacuum ($c = 299,792,458 \text{ m/s}$), the elementary electric charge ($e$), and the Avogadro constant ($N_A = 6.022 \times 10^{23} \text{ mol}^{-1}$).
Key Concepts & Examination Highlights
- The 2019 SI redefinition officially tied the kilogram to Planck's constant ($h$) using the Kibble balance, retiring the International Prototype of the Kilogram.
- There are seven fundamental base SI units: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).
- The speed of light in a vacuum is defined as an exact universal constant of 299,792,458 meters per second.