Introduction:
The process of comparison of an unknown quantity and a known fixed quantity is called measurement and the known fixed quantity used as a standard of measurement is called unit.
There are two different physical quantities: (i) Base Quantity (ii) Derived Quantity. The units for these quantities are fundamental and derived units respectively.
Fundamental units are those units that measure the physical quantities that are independent of any other physical quantities. For example, mass, time, length, temperature, electric current, intensity of light, and quantity of matter are Fundamental Quantities whereas kilogram, second, meter, Kelvin, Ampere, Candela, Mole are the units of those Fundamental Quantities respectively.
Derived units are those units that measure the derived quantity and can be expressed in terms of fundamental units. For example, Units of speed, force, momentum, etc.
Characteristics of Standard Unit
- A standard unit should be well defined.
- A standard unit should remain constant.
- A standard unit should be easily available and reproducible.
- A standard unit should be universally agreed upon.
System of measurement
MKS system is a system of units where length is measured in meter, mass in kilogram, and time in second.
CGS system is a system of units where length is measured in centimeter, mass in gram, and time in second.
FPS system is a system of units where length is measured in foot, mass in pounds, and time in second.
SI system is the international system of measurement and is an extended and improved version of MKS system.
Units of Fundamental quantities according to SI system
Quantity |
Unit |
Symbol |
Length |
Meter |
M |
Mass |
Kilogram |
Kg |
Time |
Second |
S |
Temperature |
Kelvin |
K |
Current |
Ampere |
A |
Luminous intensity |
Candela |
Cd |
Amount of Substance |
Mole |
Mol |
Plane angle |
Radian |
Rad |
Solid angle |
Steradian |
sr |
Quantities in terms of power of 10
Prefix |
Symbol |
Multiplying Factor |
peta |
P |
10^{15} |
tera |
T |
10^{12} |
giga |
G |
10^{9} |
mega |
M |
10^{6} |
kilo |
k |
10^{3} |
Deci |
d |
10^{-1} |
centi |
c |
10^{-2} |
milli |
m |
10^{-3} |
micro |
µ |
10^{-6} |
nano |
n |
10^{-9} |
pico |
p |
10^{-12} |
femto |
f |
10^{-15} |
atto |
a |
10^{-18} |
Meter: The meter is defined as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.
Second: The second is the duration of 9,192,631,770 periods of radiations corresponding to the transitions between the two hyperfine levels of the ground state of cesium-133 atoms.
Kilogram: The kilogram is the mass equal to the mass of standard platinum-iridium alloy of cylinder (90% platinum and 10% iridium) kept at the International Bureau of Weights and Measures in Paris, France. Its height and radius are equal to 4cm.
Advantages of SI units
- It is a rational system of units.
- It is a coherent system of units.
- It is a decimal system.
- It is highly useful in practical and theoretical work
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