- SI System Definition: The SI system (International System of Units) is defined as a globally accepted standard for measuring physical quantities.
- Advantages of SI Units: SI units form a coherent and rational metric system, simplifying international scientific, technical, and industrial work.
- Fundamental Units: Fundamental units in the SI system include meters for length and kilograms for mass, serving as the building blocks for all other units.
- Derived Units: Derived units, such as meters per second for speed, are combinations of fundamental units.
- Disadvantages of SI Units: One disadvantage of SI units is that they can dilute the importance of other units and may require inconvenient conversions, such as when measuring area in square feet.
What is the Si System of Units?
The SI system is the International System of Units, the worldwide standard for measuring physical quantities. It gives one agreed unit for each quantity so a length in metres and a mass in kilograms mean the same thing in every country that uses the system.
Without that standard, the same quantity can be written in many units. Length can be metres, centimetres or feet. Mass can be kilograms or grams. Switching among those units needs conversion factors and invites arithmetic mistakes.
A single standard unit for each quantity removes most of that conversion work from scientific, technical and industrial measurement.
That chosen unit is the standard unit for the quantity. Most measurements are then reported in that unit, so results can be compared without a local conversion table.
SI System of Units
SI units take their name from the International System of Units. The 11th General Conference on Weights and Measures adopted the SI in 1960. The 14th CGPM added the mole in 1971. Since 20 May 2019 the seven base units follow from seven defining constants fixed by the 26th CGPM. The system is used worldwide for scientific, technical, industrial and commercial measurement.
A unit chosen for the system should have the following qualities:
- It must have a suitable size.
- It should be accurately defined.
- It must have an easy access.
- It must be time independent.
- It should not change with the change in physical quantities.
The SI units that stand on their own, such as the metre and the kelvin, are the base units of the base quantities. Units built from those base units, such as metre per second, are derived units.
The seven SI base quantities and their units are the first seven rows in the table. Radian and steradian are SI derived units with special names, not base units.
SI Base Units
| Quantity | Unit | Symbol |
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Electric current | ampere | A |
| Luminous intensity | candela | cd |
| Plane angle | radian | rad |
| Solid angle | steradian | sr |
Advantages of SI System of Units
- It is a coherent system of units.
- It is a rational system of units.
- SI is a metric system.
Conclusion of SI System of Units
SI units are widely used because they form one coherent metric set. The practical limit is that some trades still use customary units, so a value may still need converting for local use.
House area is often stated in square feet. That figure must be converted if the work needs square metres. The same conversion step appears in other trades. The SI set is still the usual choice for science and engineering because one coherent system outweighs those local conversions.
