By international agreement a small number of physical quantities such as length, time etc. are chosen and assigned standards. These quantities are called ‘base quantities’ and their units as ‘base units’. All other physical quantities are expressed in terms of these ‘base quantities’. The units of these dependent quantities are called ‘derived units’.
The standard for a unit should have the following characteristics.
(a) It should be well defined.
(b) It should be invariable (should not change with time)
(c) It should be convenient to use
(d) It should be easily accessible
The 14th general conference on weights and measures (in France) picked seven quantities as base quantities, thereby forming the International System of Unitsabbreviated as SI (System de International) system.
Base quantities and their units
The seven base quantities and their units are
|
Base quantity
|
Unit
|
Symbol
|
|
Length
|
Metre
|
M
|
|
Mass
|
Kilogram
|
Kg
|
|
Time
|
Second
|
Sec
|
|
Electric current
|
Ampere
|
A
|
|
Temperature
|
Kelvin
|
K
|
|
Luminous intensity
|
Candela
|
Cd
|
|
Amount of substance
|
Mole
|
Mole
|
Derived units
We can define all the derived units in terms of base units. For example, speed is defined to be the ratio of distance to time.
Unit of Speed = (unit of distance (length))/(unit of time)
= m/s = ms-1 (Read as metre per sec.)
SOME DERIVED SI UNITS AND THEIR SYMBOLS
|
Quantity
|
Unit
|
Symbol
|
Express in base units
|
|
Force
|
newton
|
N
|
Kg-m/sec2
|
|
Work
|
joules
|
J
|
Kg-m2/sec2
|
|
Power
|
watt
|
W
|
Kg-m2/sec3
|
|
Pressure
|
pascal
|
Pa
|
Kg m-1/S2
|
|