Fundamental Units (Breif)

Fundamental Units 

Physical Quantities

All the quantities in terms of which laws of physics are described, and who's measurement is necessary are called physical quantities. 

Units

A definite amount of a physical quantity is taken as its standard unit. The standard unit should be easily reproducible, internationally accepted. 

Fundamental Units

Those Physical quantities which are independent of each other are called fundamental quantities and their units are called fundamental units.

S. No.	Fundamental Quantities	Fundamental Units	Symbol
1	Length	metre	m
2	Mass	kilogram	kg
3	Time	second	s
4	Temperature	kelvin	K
5	Electric Current	ampere	A
6	Luminous intensity	candela	cd
7	Amount of substance	mole	mol

Supplementary Fundamental Units

Radian and steradian are two supplementary fundamental units. It measures the plane angle and solid angle respectively.

S. No.	Supplementary Fundamental Quantities	Supplementary Unit	Symbol
1	Plane angle	radian	rad
2	Solid angle	steradian	sr

Derived Units

Those physical quantities which are derived from fundamental quantities are called derived quantities and their units are called derived units. eg. velocity, acceleration, force, work, etc.  

Definitions of Fundamental Units

The seven fundamental units of SI have been defined as under:

1. 1 Kilogram: A cylindrical prototype mass made of platinum and iridium alloys of height 39 mm and diameter 39 mm. It is a mass of 5.0188 X 10 ²⁵ Atoms of carbon-12.
2. 1 meter: 1 metre is the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second 1983.
3. 1 Second: 1 second is the time in which cesium atom vibrates 9192631770 times in an atomic clock.
4. 1 Kelvin: 1 Kelvin is the 1/273.16 part of the thermodynamics temperature of the triple point of water.
5. 1 Candela: 1 candela is the luminous intensity in a given direction of a source which emits monochromatic radiation of frequency 540 x 10¹² Hz and whose radiant intensity is 1/683 watt per steradian.
6. 1 ampere: 1 ampere is the electric current which is maintained in two straight parallel conductors of infinite length and of negligible circular cross-section area placed one metre apart in a vacuum would produce between them a force equal to 2 X 10 ^-7 N per metre of length.
7. 1 Mole: 1 Mole is the amount of substance of a system which contains many elementary entities (may be atoms, molecules, ions, electrons or group of particles, as this and atoms in 0.012kg of carbon isotope ₆C¹²

System of Units

A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities. The common system of units which is used in mechanics are given bellow:

1. CGS System: In this system, the unit of length is a centimeter, the unit of mass is gram and the unit of time is second. C: Centimeter, G: Gram, and S: second. 
2. FPS System: In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second. F: Foot, P: pound, and S: second.
3. MKS System: In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second. M: Metre, K: Kilogram, S: Second.