Physics for Everyone: Guide to Basic Physics #3 The International System of Units

The 11th General Conference on Weights and Measures, held in Paris from 11 to 20 October 1960, adopted the six units that should serve as a basis for the establishment of a practical measurement system for international relations.

DECIDED:

1. The metric system based on the six basic units above is designated by the name of the International System of Units
2. The international abbreviation of that system is SI

The International System is based on the adoption of seven fundamental values: the four of the rationalized mksA system, or the Giorgi system, and that is the length, the
mass, time intervals, light intensity and amount of substance. The corresponding measuring units are defined as follows:

Electric current:
The Ampere (A), that is, the constant electrical current, which, when flowing in two straight, parallel, indefinably long, circular cross sectional, spaced 1 m in the void, determines between them a force of 2 *(10^(-7)) N for each meter of conductor.

Length:
The meter (m) or the distance traveled by the light in the time interval of (1/299792458) s.

Mass:
Kg (kg) or the mass of the platinum-iridium prototype deposited at the Bureau International des Poids et Mesures in the subterranean halls of the Breteuil Pavilion in Sevres.

Time:
The second (s), ie the duration of 9 192 631 770 oscillations of the radiation emitted by the cesium 133 (133Cs) atom in the fundamental state 2S 1/2 in the transition from the iperfine level F = 4, M = 0 to the iperfine level F = 3, M = 0.

Temperature:

The Kelvin (k), or fraction of 1 / 273.16 of the thermodynamic temperature of the triple point of water.

Light intensity:
The candle (cd) is the luminous intensity in a given direction of a source that emits monochromatic radiation of 540 * (10 ^ 12) Hz and whose energy intensity in this direction is (1/683) W / sr.

Substance:

The mole (mol), or the amount of substance of a system that contains so many elementary units as are the atoms in 0.012 Kg of carbon 12 (12C).

Beside the seven basic units, two additional units, the radiant and the steradian are defined in SI:

Plane angle:

The radian (rad), that is, the plane corner with the vertex in the center of the circumference which underlies an arc of length equal to the radius.

Solid angle:
The steradian (sr) or that solid corner with the vertex in the center of the sphere which underlies a spherical shell, the area of which is the same as that of a square with sides equal to the radius of the sphere.

See you soon with Chepter #4 Dimensional Analysis