Unit & Measurement

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Unit & Measurement
Measurement- It is a process of finding an unknown quantity with the standard reference one.

Need of measurements  
  1. If all the quantities measured are clearly defined then there is no chance of confusion between two persons.
  2. The aim of measurement is determined whether the product is manufactured as per the requirement.
  3. To identify various law.
  4. Measurement plays a key role in development of engineering & science.
  5. To assure the quality of the product.

Physical quantity - A characteristics(property) which can be measured & which follows laws of physics is called a physical quantity. E.g. - Mass, length, Time, velocity, acceleration etc.

Types of physical quantity-
(i)Fundamental or base quantity - The physical quantity which do not depend on the other physical quantity are known as fundamental or Base quantity.
There are 7 fundamental quantities
a) Mass                     b) Temperature                    c) Electric current                   d) length  
e) Time                      f) luminous Intensity            g) Amount of substance.

(ii)Derived quantity - The physical quantity which are derived from fundamental quantity, are called derived physical quantity.
e.g. -   Area = length x length, 
           velocity – length/ Time  

(iii)Supplementary quantity - There are also two physical quantities, which are neither fundamental nor derived. These quantities are called supplementary quantity. These are plane angles & solid angle.

Unit- To measure a physical quantity; a standard value of same physical quantity is used. This standard value of the physical quantity is known as its unit.
E.g.  F = 10 N
Here "N" is unit of a physical quantity which is force.
“10” indicates that "N” is used 10 times to measure this force.
& F is measured physical quantity.

Types of unit
i) Fundamental or Base unit. The unit of fundamental quantity is called fundamental or Base unit.
ii) Derived Unit -The unit of derived quantity is called derived unit
iii) Supplementary Unit - The unit of supplementary quantity is called supplementary Unit.

Requirement of a standard unit
A unit selected for measuring a physical quantity fulfills the following requirements
  1. It should be well defined.
  2. It should be suitable in size.
  3. It should not change with time and from place to place.
  4. It should be easily reproduces at all places.
  5. It should not change in its physical condition such as temperature & pressure.
  6. It should be easily available.
  7. it is universally accepted.

System of units
i) C.G.S system - In this system the units of length, mass & time are centimeter, gram & sec respectively. This system is also known as French system
(ii) F.P.S system - In this system the units of length, mass & time are foot, pound & sec respectively. This system is also known as British system.
(iii) MKS system - In this system the units of length, mass & time are meter, kilogram & second respectively. This system is also known as French system.
(iv) SI system - This system is known as international system of unit. This system is modified form of MKS system. Hence it is also known as Rationalized MKS system.
 In this system there are seven base unit & two supplementary unit.
Base Unit -
Fundamental
quantity
Unit
Symbol
1. Length
meter
m
2. Mass
kilogram
kg
3. Time
second
s
4. Temp.
kelvin
K
5. Electric current
ampere
A
6. Amount of substance
mole
mol
7. luminous Intensity
candela
cd
Note-All symbols are represented by small letters excepts those which started with the name of Scientist in order to give him respect. Like K where Kelvin's name started from K.

Supplementary unit –
quantity
Unit
Symbol
1.    Plane angle
radian
rad
2.    solid angle
steradian
sr

Application of SI unit
  1. It has 7 base units & 2 supplementary unit; which covers all the physical quantity.
  2. SI unit is rational i. e; it gives one unit for one physical quantity
  3. SI unit is coherent i.e; every unit can be expressed in terms of fundamental unit.
  4. SI unit is metric i.e; can be expressed in multiple of 10.
  5. Accepted in almost all countries of world.
  6. The symbols fixed by SI unit are not subjected to change. E.g.  "Kg" has to be written like this in every language. 
  • Prefix of multiples & sub-multiples of units