SI Units

What is a Unit?

In our daily life, we often need to measure various physical quantities. To measure a physical quantity, we compare it with some standard quantity. For example, if we purchase some sugar, we must know how much quantity of sugar we are talking about. Thus, there is a need of some standard quantity for measuring unknown quantity. This standard quantity is called unit. Various standard units have been in use at different times in different parts of the world. With the passage of time, these units were made more precise and acceptable. People especially business communities and scientists of different countries faced problems of converting the units into one another. This problem was solved in a conference of the scientists from all over the world held in Paris. In 1960, the eleventh general conference of International Committee on Weights and Measures recommended that all countries of the world should adopt a system of same kind of standard units. This conference recommended the use of International System of units. It is abbreviated as SI. According to this system, the units of length, mass, time and volume are given in the following table.

Table. 1: SI Units

Physical Quantity	Symbol	Unit	Symbol
Length	l	meter	m
Mass	m	kilogram	kg
Time	t	second	s
Volume	V	cube meter	$m^3$

A practical unit of volume is liter (L). Mostly the liter is used for measuring volume of liquids such as milk, petrol, cooking oil, etc. It is $1/1000th$ part of a cubic meter ($m^3$ ). Therefore $1\, m = 1000 \,L$
Also $1 \,L = 1000 \, \text{milli-litre} = 1000\, \text{cubic centimetre (cc)} $ In the past, the systems of units most commonly used were the English and metric, the English system is based on a single standard, the metric is subdivided into two interrelated standards: The MKS and The CGS
The MKS system uses Meters, Kilograms, and Seconds, while the CGS system uses Centimeters, Grams, and Seconds. Unfortunately, the use of more than one system of units in a world, would introduce unnecessary complications to the basic understanding of any technical data. The need for a standard set of units to be adopted by all nations has become increasingly obvious. The International Bureau of Weights and Measures located at Sevres, France, has been the host for the General Conference of Weights and Measures, attended by representatives from all nations of the world. Since then, SI has been adopted by the Institute of Electrical and Electronic Engineers, Inc. (IEEE) in 1965 and by the United States of America Standards Institute in 1967 as a standard for all scientific and engineering literature.

Prefixes

The main advantage of SI units is that their multiples and sub-multiples can be conveniently expressed using prefixes. Prefixes are based on multiplying and dividing the units by powers of 10. The words or letters added before SI units such as milli (m), centi (c) and kilo (k) are known as prefixes.

• Milli means 1000 part. For example, millimetre (mm) is 1000 part of a metre, i.e., 1/1000 m. It means, 1 m = 1000 mm.
• Centi means 100 part. For example, centimetre (cm) is 100 part of a metre, i.e., 1 cm = 1/100 m. It means 1 m = 100 cm.
• Kilo means 1000 times. For example, kilometre (km) is 1000 times of a metre, i.e., 1 km =1000 m.

Thus, diameter of a thin wire can be written in smaller units of centimeter (cm) or millimetre (mm) instead of metre. Similarly, the longer distance between two cities may be expressed better in a bigger unit of distance, i.e., kilometre (km).