SI units

SI Units

SI Units (pronounced as /ɛs aɪ 'juːnɪts/), also known as the International System of Units, is the modern form of the metric system and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, and mole.

Etymology

The term "SI" is derived from the French language, Système International d'Unités (pronounced as /sistɛm ɛ̃tɛʁnasjɔnal dynite/). The system was first proposed by the French physicist Gabriel Mouton in 1670 and was officially adopted in 1960 by the 11th General Conference on Weights and Measures (CGPM).

Base Units

The SI system is based on seven base units, which by convention are regarded as dimensionally independent. The base units are:

• Metre (m) - The metre is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum to be 299,792,458 when expressed in the unit m·s⁻¹.
• Kilogram (kg) - The kilogram is the SI unit of mass. It is defined by setting the Planck constant to 6.62607015×10⁻³⁴ J·s.
• Second (s) - The second is the SI unit of time. It is defined by setting the Cesium frequency, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to exactly 9,192,631,770 Hz.
• Ampere (A) - The ampere is the SI unit of electric current. It is defined by setting the elementary charge to 1.602176634×10⁻¹⁹ C.
• Kelvin (K) - The kelvin is the SI unit of thermodynamic temperature. It is defined by setting the Boltzmann constant to 1.380649×10⁻²³ J·K⁻¹.
• Mole (mol) - The mole is the SI unit of amount of substance. It is defined by setting the Avogadro constant to 6.02214076×10²³ mol⁻¹.
• Candela (cd) - The candela is the SI unit of luminous intensity. It is defined by setting the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz to exactly 683 lm·W⁻¹.

Related Terms

• Metric system
• Measurement
• Physical quantity
• Dimensional analysis
• Units of measurement

External links

• Medical encyclopedia article on SI units
• Wikipedia's article - SI units

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