International system of units
International System of Units (SI)
The International System of Units (SI, from the French Système international) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world.
Pronunciation: /ˌɪntərˈnæʃənəl ˈsɪstəm əv ˈjuːnɪts/
Etymology: The term "International System of Units" is a translation of the French Système international d'unités, the name given to the system by the International Bureau of Weights and Measures (BIPM) in 1960.
Overview
The SI is a coherent system, which means that its units are chosen in such a way that they are interrelated and can be defined in terms of each other. The SI is divided into seven base units, which are the second (s), meter (m), kilogram (kg), ampere (A), kelvin (K), mole (mol), and candela (cd).
Base Units
- Second (s): The second is the SI base unit of time. It is defined by taking the fixed numerical value of the caesium frequency, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s⁻¹.
- Meter (m): The meter is the SI base unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s⁻¹.
- Kilogram (kg): The kilogram is the SI base unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10⁻³⁴ when expressed in the unit J s, which is equal to kg m² s⁻¹.
- Ampere (A): The ampere is the SI base unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10⁻¹⁹ when expressed in the unit C, which is equal to A s.
- Kelvin (K): The kelvin is the SI base unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10⁻²³ when expressed in the unit J K⁻¹, which is equal to kg m² s⁻² K⁻¹.
- Mole (mol): The mole is the SI base unit of amount of substance. It is defined by taking the fixed numerical value of the Avogadro constant N_A to be 6.022 140 76 × 10²³ when expressed in the unit mol⁻¹.
- Candela (cd): The candela is the SI base unit of luminous intensity. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd, to be 683 when expressed in the unit lm W⁻¹, which is equal to cd sr W⁻¹.
See Also
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