NCERT Class 11 Physics Physics Part-I: Chapter 1 — 1 INTRODUCTION
This chapter, "Units and Measurement," introduces fundamental concepts in Physics. It explains that measuring any physical quantity requires comparison with a standard unit. The result of a measurement is expressed as a numerical value along with its unit. The chapter details the evolution of measurement systems, from CGS, FPS, and MKS to the internationally accepted Système International d' Unités (SI). It outlines the seven SI base units: metre, kilogram, second, ampere, kelvin, mole, and candela, along with their definitions and symbols. Additionally, it introduces two supplementary units: radian for plane angle and steradian for solid angle. The chapter emphasizes the importance of a consistent system of units for scientific and commercial work, highlighting the convenience of the SI system due to its decimal nature. This foundational knowledge is crucial for understanding subsequent physics concepts.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 1 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 5 minutes |
| Word count | 995 |
Learning outcomes
- Understand the concept of units and their necessity in physical measurements.
- Differentiate between fundamental/base units and derived units.
- Identify and recall the seven SI base quantities and their corresponding units.
- Recognize the historical development of measurement systems and the adoption of SI units.
- Understand the definitions of radian and steradian for plane and solid angles.
Vocabulary
| Word | Meaning |
|---|---|
| Unit | A basic, arbitrarily chosen, internationally accepted reference standard for measurement. |
| Physical quantity | A property of a phenomenon, body, or substance that can be quantified by measurement. |
| Base unit | A unit for a fundamental or base quantity. |
| Derived unit | A unit obtained by combining base units for derived quantities. |
| System of units | A complete set of base units and derived units. |
| SI | Système International d' Unités (International System of Units). |
| Radian | The SI unit for plane angle. |
| Steradian | The SI unit for solid angle. |
| Planck constant | A fundamental physical constant denoted by 'h', used in the definition of the kilogram. |
| Caesium frequency | The hyperfine transition frequency of the caesium-133 atom, used to define the second. |
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Practice questions
- What is a unit in the context of physical measurement? Answer: A unit is a basic, arbitrarily chosen, internationally accepted reference standard used for comparison in measuring a physical quantity.
- Name the three older systems of units mentioned in the chapter. Answer: The three older systems mentioned are CGS, FPS (British), and MKS.
- What does 'SI' stand for, and why is it important? Answer: SI stands for Système International d' Unités (International System of Units). It is important because it is the internationally accepted system for scientific, technical, industrial, and commercial work.
- List the seven SI base quantities and their units. Answer: Length (metre, m), Mass (kilogram, kg), Time (second, s), Electric current (ampere, A), Thermodynamic temperature (kelvin, K), Amount of substance (mole, mol), Luminous intensity (candela, cd).
- What are the SI units for plane angle and solid angle? Answer: The SI unit for plane angle is radian (rad), and for solid angle is steradian (sr).
Practice MCQs
Q1. The result of a measurement is expressed by:
Explanation: The chapter states that the result of a measurement is expressed by a number (numerical measure) and a unit.
Q2. Which of the following was NOT one of the older systems of units mentioned?
Explanation: The SI system is the internationally accepted system, developed later than CGS, FPS, and MKS.
Q3. The SI unit of mass is:
Explanation: The chapter explicitly lists kilogram (kg) as the SI unit for mass.
Q4. The SI unit for thermodynamic temperature is:
Explanation: Table 1.1 in the chapter defines the kelvin (K) as the SI unit of thermodynamic temperature.
Q5. Radian is the SI unit for:
Explanation: The text mentions that the unit for plane angle is radian (rad).
Q6. The SI unit for luminous intensity is:
Explanation: Table 1.1 lists candela (cd) as the SI unit for luminous intensity.
Frequently asked questions
What is the primary purpose of a unit in physics?
A unit serves as a standard reference for comparison, allowing us to quantify and express the magnitude of a physical quantity.
Why was the SI system developed?
The SI system was developed to provide a single, internationally accepted system of units for consistent measurement across scientific, technical, industrial, and commercial fields.
How many base units are there in the SI system?
There are seven base units in the SI system: metre, kilogram, second, ampere, kelvin, mole, and candela.
What is the difference between a base unit and a derived unit?
Base units are fundamental units for base quantities, while derived units are formed by combining base units for quantities that are related to base quantities.
Are radian and steradian considered base units in SI?
No, radian (for plane angle) and steradian (for solid angle) are considered supplementary units in the SI system, not base units.
What is the significance of defining SI units using fundamental constants?
Defining SI units using fundamental constants ensures their stability, universality, and reproducibility, making measurements independent of physical artifacts.
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NCERT Class 11 Physics — Physics Part-I — Chapter 1 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.