CHAPTER 1: UNITS AND MEASUREMENT

Chapter 1: Units and Measurement, covering Sections 1.1 and 1.2 from Class 11 CBSE Physics:

📘 CHAPTER 1: UNITS AND MEASUREMENT

🔹 1.1 INTRODUCTION

Measurement is the foundation of physics and other sciences. Any physical quantity is measured by comparing it with a standard reference known as a unit. The result of a measurement always consists of two parts:

• A numerical value (how many times the unit is contained in the quantity measured)

• A unit (the standard used for measurement)

➤ Fundamental vs Derived Quantities:

• Physical quantities appear numerous but are interrelated.

• We need only a limited number of base quantities to define all others.

• The units are classified into:

  • Fundamental (Base) Units: Units for quantities like length, mass, time, etc., which are independent of others.

  • Derived Units: Units for other physical quantities (like speed, force, etc.) that are combinations of base units.

➤ System of Units:

• A complete set of units including both base and derived units is called a System of Units.

🔹 1.2 THE INTERNATIONAL SYSTEM OF UNITS (SI Units)

➤ Earlier Systems of Units:

Different countries historically used different unit systems:

System	Length	Mass	Time
CGS	cm	gram	second
FPS	foot	pound	second
MKS	metre	kilogram	second

This caused confusion and inconsistency in scientific communication.

➤ Need for SI System:

To unify global scientific measurements, an international standard was developed — SI units.

➤ What is SI?

• SI stands for Système Internationale d’Unités (French for International System of Units).

• Formally adopted in 1971 by the Bureau International des Poids et Mesures (BIPM).

• Revised recently in November 2018 by the General Conference on Weights and Measures.

• It uses the decimal system, making conversions easy (multiples of 10).

➤ The Seven SI Base Units:

Physical Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temp.	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

➤ Supplementary Units:

Two additional quantities often used in physics:

Quantity	Definition	Unit	Symbol
Plane angle	Ratio of arc length (ds) to radius (r)	radian	rad
Solid angle	Ratio of spherical surface area (dA) to square of radius (r²)	steradian	sr

These are dimensionless, even though they have specific names and symbols.

Note: When using mole, the type of elementary entities (atoms, ions, etc.) must be specified.

🔹 Derived Units in SI

• Many physical quantities (e.g. speed, force, energy) are defined using combinations of base quantities.

• These combinations form Derived Units.

• Some of these derived units are given special names and symbols (e.g. newton, joule, pascal).

➤ Examples:

Quantity	Unit Name	Symbol	In terms of SI base units
Force	newton	N	kg·m/s²
Work	joule	J	N·m = kg·m²/s²
Pressure	pascal	Pa	N/m² = kg/m·s²

🔹 Prefixes in SI System

To handle very large or small numbers, prefixes are used for powers of 10.

Prefix	Symbol	Multiplier
kilo	k	10³
centi	c	10⁻²
milli	m	10⁻³
micro	μ	10⁻⁶
nano	n	10⁻⁹

✅ Summary Points:

• Measurement = Numerical Value + Unit

• Fundamental units form the base of all measurements.

• SI system is the globally accepted standard.

• SI has 7 base units and two supplementary (dimensionless) units.

• Derived units are formed from base units; some have special names.

• SI system uses a decimal-based prefix system for scaling.

• Standard symbols and formats ensure global consistency in scientific communication.

Reading Comprehension Worksheet-1 for Class 11 CBSE Physics – Chapter 1: Units and Measurement (Sections 1.1 and 1.2):

📘 Physics Reading Comprehension Worksheet – 1

Chapter 1: Units and Measurement (Section 1.1 & 1.2)
Class: 11 | Subject: Physics | CBSE

📖 Read the following passage carefully:

Measurement of a physical quantity involves comparing it with a chosen standard known as a unit. Every measurement includes a numerical value and a unit. While there are many physical quantities, only a few fundamental quantities are needed to define them all. The units for these are called base units, and those formed from combinations of base units are derived units. The complete collection is referred to as a system of units.

Historically, different countries used systems like CGS (centimetre-gram-second), FPS (foot-pound-second), and MKS (metre-kilogram-second). However, to maintain consistency, the SI system (Système Internationale d’Unités) was adopted globally. It has seven base units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).

The SI system also includes two supplementary units: radian (for plane angles) and steradian (for solid angles), both of which are dimensionless. Many derived units, like newton (for force), joule (for work), and pascal (for pressure), are formed by combining base units and are given special names. The SI system uses decimal prefixes like kilo, centi, and milli for easier conversions. Proper guidelines exist for writing symbols and using units in scientific work.

📝 Section A: Multiple Choice Questions (1 mark each)

1. What does every measurement consist of?
   a) Unit only
   b) Numerical value only
   c) Both numerical value and unit
   d) Only symbols

2. Which of the following is a base unit in SI?
   a) Newton
   b) Joule
   c) Kilogram
   d) Pascal

3. The SI unit of plane angle is:
   a) Degree
   b) Radian
   c) Steradian
   d) Arcminute

4. Which system of units uses foot, pound, and second as its base units?
   a) MKS
   b) SI
   c) CGS
   d) FPS

5. What type of quantity is steradian?
   a) Scalar with dimension
   b) Vector
   c) Dimensionless quantity
   d) None of these

📝 Section B: Short Answer Questions (2–3 marks each)

6. Define fundamental and derived units with one example each.

7. Why was the SI system adopted internationally? Mention any two benefits.

8. List any three base quantities in SI units and mention their corresponding units and symbols.

9. What is the significance of using prefixes like milli-, centi-, and kilo- in the SI system?

📝 Section C: Long Answer Question (5 marks)

10. Explain the evolution of measurement systems from CGS, FPS, and MKS to the SI system.
    Include in your answer:

• Why different systems existed

• The challenges they posed

• How SI overcame these challenges

• Importance of standardization

Answer Key for Reading Comprehension Worksheet-1 (Class 11 CBSE Physics – Chapter 1: Units and Measurement, Sections 1.1 & 1.2):

✅ Answer Key – Worksheet-1

📘 Chapter 1: Units and Measurement – Sections 1.1 & 1.2

📝 Section A: Multiple Choice Questions

1. c) Both numerical value and unit

2. c) Kilogram

3. b) Radian

4. d) FPS

5. c) Dimensionless quantity

📝 Section B: Short Answer Questions

Fundamental units are the basic units that are independent and cannot be derived from other units.
Example: Metre (m) for length.
Derived units are those that are formed by combining two or more base units.
Example: Newton (N) for force = kg·m/s².

The SI system was adopted internationally because:

• It brings uniformity and standardization in measurements across countries.

• It is decimal-based, which makes conversions easier and calculations more convenient.

Physical Quantity	SI Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s

SI prefixes like milli-, centi-, and kilo- are used to express quantities in powers of ten.

• They simplify conversions (e.g., 1 kilometre = 1000 metres).

• They help in representing very large or very small values in compact, readable form.

📝 Section C: Long Answer Question

Evolution of Measurement Systems:

• Initially, different countries used different systems like CGS, FPS, and MKS, each having their own base units (e.g., foot vs metre).

• This created confusion in communication, especially in scientific research and global trade.

• To resolve this, the SI system (Système Internationale d’Unités) was introduced by the BIPM in 1971.

• SI system has seven base units, uses decimal prefixes, and is easy to use and globally accepted.

• It promotes international consistency in technical, commercial, industrial, and academic work.

✍️ Extra Activity (Example Table)

Physical Quantity	SI Unit	Symbol	Real-world Example
Length	metre	m	Measuring distance between two poles
Mass	kilogram	kg	Mass of a watermelon
Time	second	s	Duration of a 100m race
Electric current	ampere	A	Current in a household wire
Thermodynamic temperature	kelvin	K	Measuring temperature in labs
Amount of substance	mole	mol	Number of particles in chemistry
Luminous intensity	candela	cd	Brightness of an LED bulb