International System of Units
International System of Units
The international system of units (SI) are divided into three classes[1]:
- Base units
- Derived units
- 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[2]. Therefore, all business and even household transactions are conducted in SI units. SI system offers the following advantages over another system of units.
- There is one and only one unit for each physical quantity. Therefore, a table of conversions from one unit to another is not required.
- 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.
- 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.
- A minimum number of constants should be required to establish a relationship between the various quantities involved in the study of the given discipline.
- 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
| 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
| 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 | 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.
| Quantity | Name | Symbol |
|---|---|---|
| plane angle | radian | rad |
| solid angle | steradian | sr |
