CLASS 11 PHYSICS — CHAPTER 2
Units and Measurements ( इकाइयाँ और मापन) | Premium Colourful Revision Notes
1. INTRODUCTION TO PHYSICAL QUANTITIES
Physics is an experimental science based on precise measurements. Anything that can be measured directly or
indirectly and expressed in numbers is called a Physical Quantity ( भौतिक राशि).
Fundamental Quantities ( मूल राशियाँ) Derived Quantities ( व्युत्पन्न राशियाँ)
Quantities that are independent of other quantities Quantities that are derived from combination of
and cannot be defined in terms of other physical fundamental quantities.
quantities. Examples: Velocity ( Length / Time), Force (Mass ×
Examples: Mass, Length, Time, Temperature, Electric Acceleration), Work, Pressure, Power.
Current, Amount of Substance, Luminous Intensity.
Fundamental Derived Quantities Structure
• Mass [M] ⁻
Speed = Length / Time [L T ¹]
• Length [L] ⁻
Force = Mass × Acc. [M L T ²]
• Time [T] ⁻ ⁻
Pressure = Force / Area [M L ¹ T ²]
Key Rule of Measurement
To express any physical quantity Q, we require a numerical value (n) and a unit (u):
Q = n × u = \text{Constant} \implies n_1 u_1 = n_2 u_2
Class 11 Physics • Chapter 2 Notes Page 1 of 10
, 2. THE INTERNATIONAL SYSTEM OF UNITS (SI UNITS)
An international standard system of measurement is needed for universal scientific communication. The SI System
(Le Système International d'Unités) was accepted internationally in 1971.
SI Unit
Base Quantity Symbol Standard Definition Highlights
Name
Length Meter m Distance traveled by light in vacuum in 1/299,792,458 second.
Defined by Planck's constant h = 6.62607015 \times 10^{-34} \text{ J
Mass Kilogram kg
s}
.
Time Second s Duration of 9,192,631,770 periods of radiation of Cesium-133 atom.
Defined by elementary charge e = 1.602176634 \times 10^{-19} \text{
Electric Current Ampere A
C}.
Thermo. Defined by Boltzmann constant k = 1.380649 \times 10^{-23} \text{
Temperature
Kelvin K
J/K}.
Amount of
Substance
Mole mol Contains exactly 6.02214076 \times 10^{23} elementary entities.
Luminous efficacy of monochromatic radiation of frequency 540 ×
Luminous Intensity Candela cd
10¹² Hz.
SUPPLEMENTARY UNITS
1. Plane Angle ( d\theta) 2. Solid Angle ( d\Omega)
Unit: Radian (rad) Unit: Steradian (sr)
Formula: d\theta = \frac{\text{Arc}}{\text{Radius}} = Formula: d\Omega = \frac{\text{Area}}{r^2} = \frac{dA}
\frac{ds}{r} {r^2}
Full Circle = 2π rad = 360° Complete Sphere = 4π sr
dA
ds
dΩ
dθ
Plane Angle (rad) Solid Angle (sr)
Class 11 Physics • Chapter 2 Notes Page 2 of 10
Units and Measurements ( इकाइयाँ और मापन) | Premium Colourful Revision Notes
1. INTRODUCTION TO PHYSICAL QUANTITIES
Physics is an experimental science based on precise measurements. Anything that can be measured directly or
indirectly and expressed in numbers is called a Physical Quantity ( भौतिक राशि).
Fundamental Quantities ( मूल राशियाँ) Derived Quantities ( व्युत्पन्न राशियाँ)
Quantities that are independent of other quantities Quantities that are derived from combination of
and cannot be defined in terms of other physical fundamental quantities.
quantities. Examples: Velocity ( Length / Time), Force (Mass ×
Examples: Mass, Length, Time, Temperature, Electric Acceleration), Work, Pressure, Power.
Current, Amount of Substance, Luminous Intensity.
Fundamental Derived Quantities Structure
• Mass [M] ⁻
Speed = Length / Time [L T ¹]
• Length [L] ⁻
Force = Mass × Acc. [M L T ²]
• Time [T] ⁻ ⁻
Pressure = Force / Area [M L ¹ T ²]
Key Rule of Measurement
To express any physical quantity Q, we require a numerical value (n) and a unit (u):
Q = n × u = \text{Constant} \implies n_1 u_1 = n_2 u_2
Class 11 Physics • Chapter 2 Notes Page 1 of 10
, 2. THE INTERNATIONAL SYSTEM OF UNITS (SI UNITS)
An international standard system of measurement is needed for universal scientific communication. The SI System
(Le Système International d'Unités) was accepted internationally in 1971.
SI Unit
Base Quantity Symbol Standard Definition Highlights
Name
Length Meter m Distance traveled by light in vacuum in 1/299,792,458 second.
Defined by Planck's constant h = 6.62607015 \times 10^{-34} \text{ J
Mass Kilogram kg
s}
.
Time Second s Duration of 9,192,631,770 periods of radiation of Cesium-133 atom.
Defined by elementary charge e = 1.602176634 \times 10^{-19} \text{
Electric Current Ampere A
C}.
Thermo. Defined by Boltzmann constant k = 1.380649 \times 10^{-23} \text{
Temperature
Kelvin K
J/K}.
Amount of
Substance
Mole mol Contains exactly 6.02214076 \times 10^{23} elementary entities.
Luminous efficacy of monochromatic radiation of frequency 540 ×
Luminous Intensity Candela cd
10¹² Hz.
SUPPLEMENTARY UNITS
1. Plane Angle ( d\theta) 2. Solid Angle ( d\Omega)
Unit: Radian (rad) Unit: Steradian (sr)
Formula: d\theta = \frac{\text{Arc}}{\text{Radius}} = Formula: d\Omega = \frac{\text{Area}}{r^2} = \frac{dA}
\frac{ds}{r} {r^2}
Full Circle = 2π rad = 360° Complete Sphere = 4π sr
dA
ds
dΩ
dθ
Plane Angle (rad) Solid Angle (sr)
Class 11 Physics • Chapter 2 Notes Page 2 of 10
