International System of Units

International System of Units

The international system of units (SI) are divided into three classes:

1. Base units
2. Derived units
3. Supplementary units.

The international system of units abbreviated as SI has been universally accepted for international use in all fields of engineering and day-to-day requirements. Therefore, all business and even household transactions are conducted in SI units. SI system offers the following advantages over another system of units.

1. There is one and only one unit for each physical quantity. Therefore, a table of conversions from one unit to another is not required.
2. The system is coherent with the derived units. The conversion factor from the original unit to the derived unit is simply a multiplication or division by 1.
3. There are no conversions between electrical and mechanical systems e.g. a motor or an automobile engine is now rated as in kW rather than horsepower. Energy is now expressed in watt-sec rather than in Joule etc.

There are a large number of quantities (more than thirty) which an electrical engineer deals in. However, it is not necessary to assign a standard unit to each quantity as these quantities are functionally related through experiments, mathematical derivation, or definitions. The minimum number of quantities required to express the units of all other quantities is known as fundamental quantities. The following are the considerations for the selection of fundamental quantities.

1. A minimum number of constants should be required to establish a relationship between the various quantities involved in the study of the given discipline.
2. The measuring units shall be of a practical size. There are seven fundamental units which are listed below with their name, quantity symbol, and unit symbol.

Unit Definitions

Length-metre, 1, m

It is defined in terms of wavelengths of a particular radiation from krypton 86.

Mass-kilogram, m kg

It is defined equal to the mass of the international prototype kept in Sevres, France.

Time-seconds, t, s

It is defined in terms of the duration of a specific number of periods of a particular radiation from the cesium-133 atom.

Current-ampere, I, A

It is defined as the constant electric current in two infinite parallel conductors separated from each other by 1 m, produce a force of 2 x 10^-9 N/m.

Base Units

SI Base Units

Quantity	Name	Symbol
length	metre	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
luminous intensity	candela	cd
amount of substance	mole	mol

Derived Units

SI Derived Units

Name	Unit	Quantity Symbol	Unit Symbol	Other Units
Electric Capacitance	farad	C	F	C/V
Electric Conductance	siemens	G	S	A/V
Electric Inductance	henry	L	H	Wb/A
Electric Potential Difference	volt	V or E	V	W/A
Electric Resistance	ohm	R	$\Omega$	V/A
Energy	joule	W	J	N.m
Force	newton	F	N	kg.m/s2
Frequency	hertz	f	Hz	s-1

Supplementary Units

The SI units assigned to a third class called “Supplementary units” may be regarded either as base units or as derived units.

SI Supplementary Units

Quantity	Name	Symbol
plane angle	radian	rad
solid angle	steradian	sr