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INTERNATIONAL SYSTEM OF UNITS (SI) The International System of Units, abbreviated as SI (from the French name Le Système International d’Unités ), was estab- lished in 1960 by the 11th General Conference on Weights and Measures (CGPM) as the modern metric system of measurement. The core of the SI is the seven base units for the physical quantities length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. These base units are: Base quantity SI base unit Name Symbol length meter m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K amount of substance mole mol luminous intensity candela cd The SI base units are defined as follows: meter: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. kilogram: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. second: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. ampere: The ampere is that constant current which, if main- tained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conduc- tors a force equal to 2∙10 –7 newton per meter of length. kelvin: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. mole: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. candela: The candela is the luminous intensity, in a given di- rection, of a source that emits monochromatic radiation of frequency 540∙10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. SI derived units Derived units are units which may be expressed in terms of base units by means of the mathematical symbols of multiplication and division (and, in the case of °C, subtraction). Certain derived units have been given special names and symbols, and these special names and symbols may themselves be used in combination with those for base and other derived units to express the units of other quantities. The next table lists some examples of derived units ex- pressed directly in terms of base units: SI derived unit Physical quantity Name Symbol area square meter m 2 volume cubic meter m 3 speed, velocity meter per second m/s acceleration meter per second squared m/s 2 wave number reciprocal meter m -1 density, mass density kilogram per cubic meter kg/m 3 specific volume cubic meter per kilogram m 3 /kg current density ampere per square meter
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