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 he 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 1.1. their standard definitions are given in the appendix 1.

Table 1.1

Physical quantity

SI Unit

Symbol

Length

Metre

m

Mass

Kilogram

kg

Time

Second

s

Electric Current

Ampere

A

Thermodynamic

Kelvin

k

Intensity of light

Candela

cd

Amount of substance

Mole

mol

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 (gable 1.2).

Table 1.2

Supplementary units

Physical quantity

SI Unit

Symbol

Plane angel

Radian

Rad

Solid angle

Steradian

sr

Radian

The radian is the plane angel between two radii of a circle which cut off on the circumference an arc, equal in length to the radius, as shown in fig. 1.1 (a).

equal-in-length-to-the-radiusFig. 1.1 (a)

Steradian

The steradina 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. (fig. 1.1b).

square-of-radius-of-the-sphere

Fig. 1.1(b)

Derived Units

SI units fro measuring all other physical quantities are derived from the base and supplementary units. Some of the derived units are given in table. 1.3.

Table 1.3

Physical quantity

Unit

Symbol

In terms of base units

Force Newton N  derived-units-table-1.3-01
Work Joule J  derived-units-table-1.3-02
Power Watt W  derived-units-table-1.3-03
Pressure Pascal Pa  derived-units-table-1.3-04
Electric charge Coulomb C  A s

 

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