## 2007-09-19

### Nota Fizik: SI UNITS In DeTAIL

The International System of Units, universally abbreviated SI (from the French Le Système International d'Unités), is the modern metric system of measurement. The SI was established in 1960 by the 11th General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures). The CGPM is the international authority that ensures wide dissemination of the SI and modifies the SI as necessary to reflect the latest advances in science and technology.

Definitions of the SI base units
The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent

Base quantity Name Symbol

length meter m

Description
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.

mass kilogram kg

Description
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram - Pavillon de Breteuil (Sèvres).

time second s

Description
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.

electric current ampere A

Description
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

thermodynamic temperature kelvin K

Description
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

amount of substance mole mol

Description
1. 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; its symbol is "mol."

2. 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.

luminous intensity candela cd

Description
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.