UNIT

Unit: The chosen standard used for measuring a physical quantity is called unit.

Unit should be:

(i)                  Well defined

(ii)                Easy to reproduce

(iii)               Easy to compare

(iv)              Independent of changes in physical conditions

(v)                Internationally accepted

Units are of two types – (i) Fundamental Unit; (i) Derived Unit

System of units _ Unites depend on choice. Each choice of units leads to a new system (set) of units. The internationally accepted systems are (i) CGS system, (ii) MKS System (iii) FPS system (iv) SI Units.

In SI Units, there are seven fundamental units given in the following

Physical Quantity	Si Unit	Symbol
Length	Metre	m
Mass	Kilogram	kg
Time	Second	s
Electric current	Ampere	A
Temperature	Kelvin	K
Luminuous intensity	Candela	Cd
Amount of substance	mole	mol

Besides these seven fundamental units, two supplementary units are also defined, viz, radian [rad] for plane angle and steradian (sr) for solid angle.

All the units which are defined / expressed in terms of fundamental units are called derived units.

Some important derived units.

S. No	Physical Quantity	Cgs units	SI unit	Relation
1.	Force	dyne	newton	1 newton = 10⁵ dyne
2.	work	erg	joule	1 joule = 10⁷erg