Measurement and system of units

How a unit of a physical quantity should be?

• The measurement of a unit should be definite and unambiguous.


• The unit should be such that its measure does not change.


• The prototype (replica) of a unit should be easily reproducible.


• The replica of a unit should be easily available.

Units of a physical quantities and systems of units:

A limited number of physical quanitities called fundamental quantities or base quantities of which units should be fixed and with the help of them the units of all other quantities can be fixed. The units of fundamental quantities are called as fundamental or base units. The units of all other physical quantities can be expressed as a combination of base units. Such physical quantities are called derived quantities and their units are called derieved units.

The different systems of units are as under:

• British (FPS) system (foot, pound, second system)


• CGS system (centimeter, gram, second system)


• MKS system ( Meter, Kilogram, second system)


• MKSA system ( Meter, kilogram, second, Ampere system)


• SI system

International system of units:

Sr. No.	Physical Quantity	Name of unit	Symbol
1	Length	meter	m
2	Mass	kilogram	Kg
3	Time	second	s
4	Electric current	ampere	A
5	Thermodynamic Temperature	Kelvin	K
6	Luminous intensity	candela	cd
7	Quanitity of matter	mole	mol