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Units & Dimensions – Notes – 1


To measure a physical quantity we need some standard unit of that quantity. An elephant is heavier than a goat but exactly how many times ? This question can be easily answered if we have chosen a standard mass calling it a unit mass. If the elephant is 200 times the unit mass and the goat is 20 times we know that the elephant is 10 times heavier than the goat.

SI system of units :

Fundamental quantities :

Quantity Name of the Unit Symbol
Length metre m
Mass kilogram  kg
Time second s
Electric Current ampere             A
Thermodynamic Temperature kelvin K
Amount of Substance mole      mol
Luminous Intensity candela            cd

Supplementary quantities :

Quantity Name of the Unit Symbol
Plane angle  radian  rad
Solid angle  steradian  sr

Besides the seven fundamental units two supplementary units are defined.

SI Prefixes :

Power of 10          Prefix    Symbol
18 exa E
15 peta P
12 tera T
9 gigs G
6 mega M
3 kilo k
2 hecto h
1 deka da
-1 deci d
-2 centi c
-3 milli m
-6 micro μ
-9 nano n
-12 pico p
-15 femto f
-18 atto a

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