What is Systemé international d’ Unités (SI Units) Definition

 

What is Systemé international d’ Unités (SI Units) Definition

SI Units

SI is the abbreviation for “Systemé international d’ Unités” in French, and is the modern form of the metric system introduced at the Eleventh International Conference of Weights and Measures, 1960. Systemé international d’ Unités system of units possesses features that make it logically superior to any other system and also more convenient, as it is coherent, rational and comprehensive.

A system of Units is said to be coherent if the product of the quotient of any two unit quantities in the system is the unit of the resultant quantity without the introduction of any numerical factor. For example, unit velocity will result when unit length is divided by unit time.

In 1956, India, by an Act of Parliament No. 89, switched over to the metric system of weights and measures. The definitions of various units  given in the Act conform to the definitions of the SI units.

The SI units are based on seven base units with a unit symbol assigned to each of them as given in the following table.

Definition

The definition of these base units are as follows :

1. Length : The metre is the length equal to 1650760.73 wavelengths, in vacuum, of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.
2. Mass : The kilogram is equal to the mass of the international prototype kilogram stored Sevrés, France.
3. Time : The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
4. Electric Current : The ampere is the current which, if maintained in two straight parallel conductors of infinite length and of negligible circular cross section and placed one metre apart in vacuum, would produced between these conductors a force equal to 2 x 10-7 newton per meter of length (N/m).
5. Thermodynamic temperature : The kelvin is the 1/273.16 fraction of the thermodynamic temperature of the triple point of water.
6. Amount of Substance : The mole is the amount of substance in a system which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12.
7. Luminous intensity : The candela is the luminous is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 m2of a blackbody at the temperature of freezing platinum (2046 K), under a pressure of 101 325 newtons per square meter.

Some Derived Unites

The two dimensionless quantities, plane angle and solid angle, are treated as independent quantities with SI units radian (rad) and steradian (sr), respectively. These are known as supplymentary units. The radian is the plane angle between two such radii of a circle which cuts off, in the circumference, an arc equal to the length of the radius, Thus,

 

Ө (in radians) = arc/radius

The steradian is the solid angle which, with its vertex at the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square having sides equal to the radius of the sphere. Thus, if S is the area cut off on the surface of a sphere of radius r, the solid angle at the centre of the sphere is

 

Ω (in steradian) = S/r2

 

All other units are known as compound or derived SI units, some of which may have special names as given in the above table. The SI units cover all the fields of physics and engineering.