Fundamental Units
Introduction
Physics is that branch which deals with the study of nature and natural phenomenon. The word physics comes from the Greek word ‘fusis’ meaning nature. In this unit we will discuss some of the important aspect of measurement in physics. We will also discuss why we need a unit to measure a physical quantity.
In the measurement of any physical quantity, we require some ‘reference standard’. This reference standard of measurement is called a unit. These are independent quantities i.e. they do not need any other quantity to represent them. Let us consider three physical quantities mass, length and time. These quantities are independent of each other. So, three separate units are required for the measurement of these quantities. Thus, it becomes important to establish a system of units.
Measurement in Physics
Fundamental Units
Measurement of a physical quantity involves:
- The standard or unit in which the quantity is being measured
- The numerical value representing the number of times the quantity contains that unit.
The physical quantities which do not depend upon other quantities are called fundamental quantities. In M.K.S. system the fundamental quantities are mass, length and time, while in more general Standard International (S.I.) system the Fundamental quantities are mass, length, time, temperature, luminous intensity, current and amount of substance. The units of fundamental quantities are called fundamental units and are discussed below.
Derived Units
The units of physical quantities which may be derived from fundamental units are called derived units, for example:
Unit of area:
area = length × breadth
unit of area = unit of length × unit of breadth
= m × m = m2
Unit of Velocity:
velocity = Displacement/Time
unit of velocity =Unit of Displacement/Unit of Time
= m/s = ms-1
Hence m2 and ms-1 are derived units.
Systems of Units :
There are following principal system of units:
1. C.G.S System :
length → centimetre (cm),
mass → gram (g)
time → second (s).
2. F.P.S System :
length → foot (ft),
mass → pound (lb),
time → second (s).
3. M.K.S. System:
length → metre (m),
mass → kilogram (kg),
time → second (s).
4. S.I. System :
It has SEVEN fundamental units.
Length → metre (m),
Mass → kilogram (kg),
Time → second (s).
Temperature → kelvin (K),
Luminous intensity → candela (cd),
Electric current → ampere (A),
Amount of substance → mole (mol).
In S.I. system there are two supplementary units.
P Radian (rad) : Unit of plane angle
P Steradian (st) : Unit of solid angle