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System of Units
System of units : A complete set of units, both fundamental and derived for all kinds of physical quantities is called system of units. The common systems are given below
(1) CGS system : The system is also called Gaussian system of units. In it length, mass and time have been chosen as the fundamental quantities and corresponding fundamental units are centimeter (cm), gram (g) and second (s) respectively.
(2) MKS system : The system is also called Giorgi system. In this system also length, mass and time have been taken as fundamental quantities, and the corresponding fundamental units are metre, kilogram and second.
(3) FPS system : In this system foot, pound and second are used respectively for measurements of length, mass and time. In this system force is a derived quantity with unit poundal.
(4) S. I. system : It is known as International system of units, and is infact extended system of units applied to whole physics. There are seven fundamental quantities in this system. These quantities and their units are given in the following table
Quantity |
Name of Unit |
Symbol |
Length |
meter |
M |
Mass |
kilogram |
Kg |
Time |
second |
S |
Electric Current |
ampere |
A |
Temperature |
Kelvin |
K |
Amount of Substance |
mole |
Mol |
Luminous Intensity |
candela |
Cd |
Besides the above seven fundamental units two supplementary units are also defined
Radian (rad) for plane angle and Steradian (sr) for solid angle.
Note : Apart from fundamental and derived units we also use very frequently practical units. These may be fundamental or derived units
e.g., light year is a practical unit (fundamental) of distance while horse power is a practical unit (derived) of power.
Practical units may or may not belong to a system but can be expressed in any system of units
e.g., 1 mile = 1.6 km = 1.6 10^{3}m.