Aakash Repeater Courses
ApplyTake Aakash iACST and get instant scholarship on coaching programs.
Ever wondered how we measure time, length, or even the speed of light? That is what you will learn in Class 11 Physics Chapter 1: Units and Measurement. This chapter helps you to understand how everything in science is measured and compared in a proper, standard way. It is the base of all physics and really important for board exams, JEE, or NEET. These NCERT notes for class 11 is prepared by careers360 expert faculty as per the latest CBSE syllabus.
In these notes, you will find simple explanations of topics like SI units , significant figures, and how to use dimensional analysis to check formulas. With real-life examples, important formulas, and easy tips, these NCERT Notes will help you start your physics journey with confidence.
New: JEE Main/NEET 2027 - Physics Important Formulas for Class 10
JEE Main Scholarship Test Kit (Class 11): Narayana | Physics Wallah | Aakash | Unacademy
NEET Scholarship Test Kit (Class 11): Narayana | Physics Wallah | Aakash | ALLEN
Also Read
The NCERT Notes of CBSE Class 11 Physics Chapter 1 - Units and Measurements provide a brief and neat overview of important principles, equations and procedures connected to the physical quantities and measurements. The notes are perfect to study and revise the topics on CBSE, JEE and NEET.
Physical quantity (Q) = Magnitude × Unit = n × u
Volume = length ✕ breadth ✕ height = m ✕ m ✕ m = m³
Density = mass/volume = kgm⁻³
A unit system is a comprehensive set of units that includes both fundamental and derived units for various physical quantities. Typical systems include:
Name of Quantity |
Name of unit |
Symbol |
Length |
Meter |
m |
Mass |
Kilogram |
kg |
Time |
Second |
s |
Electric current |
Ampere |
A |
Thermodynamic temperature |
Kelvin |
K |
Amount of substance |
Mole |
mol |
Luminous intensity |
Candela |
cd |
Name of Quantity |
Name of unit |
Symbol |
Plane angle |
Radian |
rad |
Solid angle |
Steradian |
sr |
Practical units of length:
1 kilometer(km) = 10³m
1 centimeter(cm) = 10⁻²m
1 millimeter (mm) = 10⁻³m
1 micron(μ) = 10⁻⁶m
1 nanometer (nm) = 10⁻⁹m
1 Angstrom (Å) = 10⁻¹ºm
1 fermometer = 1 fermi (fm) = 10⁻¹⁵m
Astronomical unit (AU): 1AU is the average distance of the sun from the Earth, commonly used for measuring astronomical distances.
1 AU = 1.496 ✕ 10¹¹m ≅ 1.5 ✕ 10¹¹m
Light Year (ly): 1 light-year is the distance travelled by light in one year.
1 light-year = (3 ✕ 10⁸ms⁻¹) ✕ (365 ✕24✕60✕60)s = 9.46 ✕ 10¹⁵m
Parallactic second or parsec(pc): The parallactic second (pc): 1 parsec is the distance between the average radius of the Earth's orbit around the Sun and an angle of 1" (second of arc).
Practical units of mass:
1 metric ton = 10³kg
1 quintal = 10²kg
1 gram = 10⁻³kg
1miligram = 10⁻³g = 10⁻⁶kg
To measure the mass of an atom or molecule we use a unified atomic mass unit(u) defined as 1/12th mass of an atom of the carbon-12 isotope.
1u = 1.66 ✕ 10⁻²⁷kg
Practical units of time:
1 millisecond (ms) = 10⁻³s
1 microsecond (*s) = 10⁻⁶s
1 nanosecond (ns) = 10⁻⁹s
1 picosecond (ps) = 10⁻¹²s
Shake is the unit of time used in microscopic physics
1 shake = 10⁻⁸s
Significant figures in a physical quantity's measured value indicate the number of digits in which we have confidence. The greater the number of significant figures obtained in a measurement, the greater the measurement's accuracy. The opposite is also true.
When counting the number of significant figures in a given measured quantity, the following rules apply.
(1) Non-zero digits: A number's non-zero digits are all considered significant.
For example, 41.3 contains three significant figures, 147.6 contains four significant figures, and 12.123 contains five significant figures.
(2) Between non-zero digits, a zero: Significant is a zero between two non-zero digits.
For example, 5.03 contains three significant figures, 5.404 contains four significant figures, and 6.004 contains four significant figures.
(3) Leading zeros: Leading zeros (zeroes to the left of the first non-zero digit) are ignored.
For example, the number 0.583 has three significant figures, the number 0.045 has two significant figures, and the number 0.003 has one significant figure.
(4) Trailing zeros: Trailing zeros are considered significant (zeroes to the right of the last non-zero digit).
For example, 4.250 has four significant figures, 434.00 has five significant figures, and 243.000 has six significant figures.
(5) Exponential notation: The numerical portion of exponential notation indicates the number of significant figures.
For example, 1.32 10*10-2 has three significant figures, while 1.32*104 has three significant figures.
When rounding off measurements, the following rules are followed:
For example, 7.82 is rounded to 7.8, and 3.94 is rounded to 3.9.
For example, 6.87 is rounded to 6.9, and 12.78 is rounded to 12.8.
For example, 16.351 is rounded to 16.4, and 6.758 is rounded to 6.8.
For example, 3.250 becomes 3.2, and 12.650 becomes 12.6.
For example, 3.750 is rounded to 3.8, and 16.150 is rounded to 16.2.
All derived physical quantities can be expressed using the seven base quantities as discussed in the beginning of this chapter. Hence these quantities are also referred as seven dimensions of physical world. They are represented by using square brackets. Thus the seven dimensions of physical world are represented as follows.
[M] for mass
[L] for length
[T] for time
[A] for electric current
[K] for thermodynamic temperature
[cd] for luminous intensity
[mol] for amount of substance
The dependence of all other physical quantities on these base quantities can be expressed in terms of their dimensions.
For example, the speed
Therefore the dimension of speed can be given as
Thus, the dimensions of a physical quantity are the powers (or exponents) to which the base quantities are raised to represent that quantity.
The expression of a physical quantity in terms of its dimensions is called its dimensional formula. For example, the dimensional formula of force is
The table given in the formulae chart at the end of this chapter, gives the dimensional formulae for some physical quantities.
An equation which contains a physical quantity on one side and its dimensional formula on the other side, is called the dimensional equation of that quantity. Dimensional equations for a few physical quantities are given below.
Speed
Area
Force
The physical quantities having same derived units have same dimensions.
The dimensions of a physical quantity are calculated by raising the fundamental units to the powers that are needed to derive its derived units.
The length, mass, and time are denoted by [L], [M], and [T] respectively.
Uses of Dimensional Equations
To convert units of one system into units of another system: The product of a physical quantity's numerical value and its corresponding unit is a constant. Let the numerical value of a physical quantity p are n₁ and n₂ in two different systems and the corresponding units are u₁ and u₂, then
If the dimensions of the physical quantity are a in mass, b in length, and c in time, then its dimensional formula will be
Similarly, if the fundamental units in the second system are M₂, L₂, and T₂, then
According to eqn (i), we have
Using this formula we can convert the numerical value of a physical quantity from one system of units into the other system.
To check the correctness of an equation: All terms on both sides of a physical equation must have the same dimensions. This is known as the dimension homogeneity principle.
Limitations:
Yes, Physics Class 12 Chapter 1 notes in PDF format are essential for JEE preparation, offering a concise and targeted resource for effective revision and problem-solving strategies.
The main topics covered in Units and Measurements include introduction to measurement, units of measurement, , precision, significant figures, dimensional analysis, unit conversions, measurement of physical quantities, dimensional formulas and equations and dimensional analysis and its applications.
Fundamental units are the basic units of measurement for base physical quantities like length (meter), mass (kilogram), and time (second). They are independent and cannot be broken down further. Derived units are combinations of fundamental units used to express other physical quantities like speed (m/s), force (newton), and pressure (pascal).
Dimensional analysis helps verify the correctness of physical equations, convert units from one system to another, and derive relations between physical quantities. It ensures that both sides of an equation are dimensionally consistent, which is a quick way to check if an equation could be correct.
Take Aakash iACST and get instant scholarship on coaching programs.
This ebook serves as a valuable study guide for NEET 2025 exam.
This e-book offers NEET PYQ and serves as an indispensable NEET study material.
As per latest syllabus. Physics formulas, equations, & laws of class 11 & 12th chapters
As per latest syllabus. Chemistry formulas, equations, & laws of class 11 & 12th chapters
As per latest 2024 syllabus. Study 40% syllabus and score upto 100% marks in JEE