The laws of Physics are expressed in terms of physical quantities such as distance, speed, time, force, volume, electric current, etc. For measurement, each physical quantity is assigned a unit.

For example, time could be measured in minutes, hours or days. But for the purpose of useful communication among different people, this unit must be compared with a standard unit acceptable to all. In Science, international agreement on the basic units is absolutely essential, otherwise scientists in one part of the world would not understand the results of an investigation conducted in another part.

SI Units

With the need of agreed units in mind, the 14th General Conference on Weights and Measures held in 1971, adopted seven base or fundamental units. These units form the SI system. The system is popularly known as the metric system.

Quantity Unit Symbol
Length metre m
Mass kilogram kg
Time second s
Electric Current ampere A
Temperature kelvin K
Luminous Intensity candela cd
Amount of Substance mole mol