Units and Measurements - Chapter Summary

Measurement

It is a comparison process used to measure a physical quantity to find out how many times the standard amount of that physical quantity is present in the given situation.

Unit

It is an arbitrarily chosen standard which is accepted by the society in terms of which other physical quantities of similar nature can be expressed.

Measurement - Numerical Value X Unit

Physical Quantities

They can be classified into:

• Fundamental Quantities
• Derived Quantities

Fundamental Quantities (Basic)

They are quantities that cannot e resolved to further quantities. There are seven fundamental Quantities

• Length
• Mass
• Time
• Temperature
• Electric Current
• Luminous Intensity
• Amount of Matter

Derived Quantities

They are the physical quantities that can be derived from the seven fundamental quantities.
Eg: Volume, Area, Pressure, Power, Work, Force, etc.

Systems of Units

C.G.S - Centimetre, Gram, Seconds
F.P.S -  Foot, Pound, Second
M.K.S - Metre, Kilogram, Second
S.I - System International

Quantity	Symbol	Unit	Unit Symbol
Length	l, L	Metre	M
Mass	M	Kilogram	kg
Time	t	Second	S
Temperature	T	Kelvin	K
Electric Current	I	Ampere	A
Luminous Intensity	Io	Candela	Cd
Amount of Matter	m	Mole	Mol

Supplementary Units

1. Plain Angle - Unit is Radian denoted by rd
2. Solid Angle - Unit is Steradian denoted by sr

Advantages of S.I System

1. It is a coherent system - all the derived units of the S.I system can be obtained by just multiplying or dividing the fundamental units and no numerical factors are involved in it.
2. It is a rational system - In this S.I system, one physical quantity has only one unit which may be in any form.
3. It is a metric system - All the subunits of the S.I system can be expressed as the powers of 10.
4. It is an Internationally accepted system

Practical Units for Large Distances 

• Astronomical Unit (AU) - It is the mean distance between the centres of the sun and the planets.      
• Light Year (ly) - It is the distance travelled by light in vacuum in one year.                        
• Parallactic Second (parsec) - It is the distance covered by an arc of length 1 AU subtends an angle of 1II  

1 AU = 1.496 x 1011 m1 ly = 9.46 x 1015 m 1 parsec = 3.08 x 1016 m

Practical Units for Small Distances

• 1 Micron (um) = 10-6 m
• 1 Nano (nm) = 10-9 m
• 1 Angstram = 10-10 m
• 1 Fermi = 10-15 m
• 1 Pico = 10-12 m

Practical Units for Heavy Masses

• 1 tonne = 1000 kg
• 1 quintel - 100 kg
• 1 Csl (Chandrashakar Limit) = 1.4 x Mass of Sun (1030 kg)

Practical Units for very Small Masses

• 1 AMU (Atomic Mass Unit) = 1.66 x 10-27 kg

Practical Units for Large Time

Solar Day - Time taken by the Earth to complete a rotation on its own axis.

Solar Year - Time taken by Earth to complete one revolution around the sun.

Lunar Month - It is the time taken by the moon to complete one revolution around the Earth.

• 1 solar day = 24 hours
• 1 solar year = 165.25 solar days
• 1 lunar month = 27 solar days

Practical Units for very small Time

• 1 Shake = 10-8 seconds