Explain International System of Units?
International System of Units
In 1960, an international committee agreed on a set of definitions and standard to describe the physical quantities. The system that was established is called the System International (SI).
Due to the simplicity and convenience with which the units in this system are amenable to arithmetical manipulation, it is in universal use by the world’s scientific community and by most nations. The system international (SI) is built up from three kinds of units: base units, supplementary units and derived units
- Base Units. There are seven base units for various physical quantities namely; length, mass, time, temperature, electric current luminous intensity and amount of a substance (with special reference to the number of particles). The names of base units for these physical quantities together with symbols are listed in Table on the right:
- Derived Units. SI units for measuring all other physical quantities are derived from the base and supplementary units. Some of the derived units are given in Table below:
- Supplementary Units. The General Conference on Weights and Measures has not yet classified certain units of the SI under either base units or derived units; These SI units are called supplementary units. For the time being this class contains only two units of purely geometrical quantities, which are plane angle and the solid angle as given in the table below:
The radian is the plane angle between two radii of a circle which cut off on the circumference an arc, equal in length Ito the radius, as shown in Fig below
The steradians is the solid angle (three-dimensional angle) subtended at the centre of a sphere by an area of its surface equal to the square of radius of the sphere.