SI base unit

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SI base units

Unit relations in the old SI

Unit relations in the new SI

SI base units are the fundamental units of measurement defined by the International System of Units (SI) for the seven basic quantities of a physical nature. These units are the building blocks of the SI system and all other units of measurement are derived from them. The SI base units and their corresponding physical quantities are: the meter (m) for length, the kilogram (kg) for mass, the second (s) for time, the ampere (A) for electric current, the kelvin (K) for thermodynamic temperature, the mole (mol) for the amount of substance, and the candela (cd) for luminous intensity.

Definition of SI Base Units

Each of the SI base units is defined by a specific physical phenomenon or a fundamental physical property, ensuring that they are universally accessible and can be consistently reproduced in laboratories around the world.

• The meter is defined by the distance light travels in vacuum in 1/299,792,458 of a second.
• The kilogram is defined by the Planck constant, h, which is exactly 6.62607015×10^−34 kg⋅m^2⋅s^−1.
• The second is defined by the transition frequency of cesium-133 atoms, Δν_Cs.
• The ampere is defined by the elementary charge, e, which is 1.602176634×10^−19 coulombs.
• The kelvin is defined by the Boltzmann constant, k, which is 1.380649×10^−23 J⋅K^−1.
• The mole is defined by the Avogadro constant, N_A, which is 6.02214076×10^23 entities per mole.
• The candela is defined by the luminous efficacy of monochromatic radiation of frequency 540×10^12 Hz, K_cd, which is 683 lm/W.

Importance of SI Base Units

The SI base units play a crucial role in science, technology, and commerce by providing a universal language for measurement. This standardization allows for the precise and accurate exchange of information globally, facilitating international collaboration and innovation. The definitions of these units are periodically reviewed and updated by the International Committee for Weights and Measures (CIPM) to incorporate advancements in measurement science and technology.

Recent Changes to SI Base Units

In 2019, a significant revision of the SI base units took place, where all units were redefined in terms of physical constants. This shift from artifact-based definitions (e.g., the kilogram being defined by a physical object) to constant-based definitions ensures greater stability and universality of the measurement system.

See Also

• International System of Units
• Physical quantity
• Unit of measurement
• Metric system

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