Physical Quantities & SI Units: Notes for CBSE Class 11 & NEET |

Understanding the fundamental building blocks of Physics: The 7 Base SI Units.

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Units and Measurement

The Subatomic Scale: At the level of 10^-13 m, we enter the world of the atomic nucleus. This is the domain of Nucleons (Protons and Neutrons) and heavier elementary particles. These particles are bound together by the exchange of Gluons, the messengers of the strong nuclear force.

Physical Quantities

All quantities that can be measured are called physical quantities. In physics, we study these quantities and their inter-relationships (e.g., length, mass, force, work done).

Types of Physical Quantities

• Fundamental Quantity: Physical quantities which cannot be expressed in terms of any other physical quantities.
  Examples: Length, Mass, Time, Temperature.
• Derived Quantity: Physical quantities which are derived from fundamental quantities.
  Examples: Area, Density, Force.

Measurement

Measurement is the comparison of a physical quantity with a standard of the same physical quantity. A standard unit is essential for:

1. Accuracy
2. Convenience
3. Uniformity
4. Equal justice to all

Characteristics of a Standard Unit

A chosen unit should be: Consistent (Invariable), Available, Imperishable (Permanent), Convenient, and Reproducible.

Classification of Units

• Fundamental Unit: Used to measure fundamental quantities (e.g., Metre, Kilogram).
• Derived Unit: Used to measure derived quantities (e.g., Square metre for area, g/cm³ for density).

Systems of Units

System	Length	Mass	Time
FPS (British)	Foot (ft)	Pound (lb)	Second (s)
CGS (Gaussian)	Centimetre (cm)	Gram (g)	Second (s)
MKS	Metre (m)	Kilogram (kg)	Second (s)

International System of Units (SI)

The SI system is a modern modification of the MKS system. It is the most widely used system globally and consists of three categories:

• 7 Fundamental Quantities
• 2 Supplementary Quantities (Radian and Steradian)
• Derived Quantities

Physics: The Science of Measurement

Understanding the Subatomic Scale

When we talk about 10^-13 m to 10^-15 m, we are looking at the building blocks of matter:

• Nucleons: Protons and Neutrons (size approx. 0.8 x 10^-15 m).
• Gluons: The "glue" (massless particles) that hold quarks together within nucleons.
• Measurement Context: These are measured using Fermi (1 fm = 10^-15 m).

1. Physical Quantities

Anything that can be measured numerically and follows the laws of physics is a Physical Quantity.

Quantity	Type	Real-World Example / Relationship
Mass	Fundamental	Quantity of matter in an object (Scalar).
Length	Fundamental	Distance between two points.
Force	Derived	Mass × Acceleration (kg·m/s²).
Density	Derived	Mass / Volume (kg/m³).

Why do we need Standard Units?

Imagine buying cloth where the "meter" changed every day. Standards ensure:

• Invariability: The unit doesn't change with time or temperature.
• Reproducibility: A scientist in India and a scientist in Brazil get the same result.

2. Global Systems of Units

While the world has converged on SI Units, historical systems provide context for how we measure today:

System	Length	Mass	Time
FPS (British)	Foot (ft)	Pound (lb)	Second (s)
CGS (Gaussian)	Centimeter (cm)	Gram (g)	Second (s)
MKS (Standard)	Meter (m)	Kilogram (kg)	Second (s)

3. The SI System (Modern Standard)

The International System (SI) is the complete version of MKS. It includes:

• 7 Base Units: (Meter, Kilogram, Second, Ampere, Kelvin, Mole, Candela).
• 2 Supplementary Units: Radian (plane angle) and Steradian (solid angle).

Fundamental Quantities & SI Units

1. Comparison of Unit Systems

Physical Quantity	CGS (Gaussian)	MKS (Standard)	FPS (British)
Length	Centimetre (cm)	Metre (m)	Foot (ft)
Mass	Gram (g)	Kilogram (kg)	Pound (lb)
Time	Second (s)	Second (s)	Second (s)

2. The 7 Fundamental SI Units

The International System of Units (SI) is the modern form of the metric system. Below are the precise scientific definitions for the base units:

Length m

Unit: Metre

The distance traveled by light in vacuum in 1/299,792,458 of a second.

Mass kg

Unit: Kilogram

Defined by the mass of a platinum-iridium cylinder kept at the International Bureau of Weights and Measures.

Time s

Unit: Second

The duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of Cesium-133.

Thermodynamic Temp. K

Unit: Kelvin

The fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Electric Current A

Unit: Ampere

The constant current which produces a force of 2 × 10⁻⁷ N/m between two parallel conductors of infinite length.

Luminous Intensity cd

Unit: Candela

The intensity of a blackbody of surface area 1m² at the temperature of freezing platinum under standard pressure.

Amount of Substance mol

Unit: Mole

The amount of substance containing as many elementary entities as there are atoms in 0.012 kg of carbon-12.

3. Supplementary Quantities

1. Plane Angle (Radian - rad): Defined as θ = arc / radius.

2. Solid Angle (Steradian - sr): Defined as Ω = Area / (Radius)².

Practice Module: Units & Measurement

Target: CBSE Class 11 & NEET 2026

Section A: Very Short Answer (1 Mark)

Q1. Define a Light Year.

View Solution

It is the distance traveled by light in vacuum in one year. 1 ly = 9.46 × 10¹⁵ m.

Section B: Multiple Choice Questions NEET Pattern

Q2. Which of the following is NOT a fundamental SI unit?

• (A) Ampere
• (B) Candela
• (C) Newton
• (D) Kelvin

View Solution

Correct Option: (C). Newton is a derived unit (kg·m/s²), while Ampere, Candela, and Kelvin are fundamental units.

Section C: Assertion & Reason

Directions: (A) Both A and R are true and R is the correct explanation. (B) Both A and R are true, but R is not the correct explanation. (C) A is true, R is false. (D) A is false, R is true.

Assertion (A): Light year and year, both measure time.
Reason (R): Because both have "year" in their name.

View Solution

Correct Option: (D). Assertion is false because Light Year is a unit of Distance, not time. Reason is true as a statement of naming, but does not justify the physics.

Section D: Descriptive Questions (3 & 5 Marks)

Q3. Distinguish between Fundamental and Derived units with two examples each.

View Solution

Fundamental Units: Independent units like Metre (m) and Kilogram (kg).
Derived Units: Units expressed in terms of base units, like Velocity (m/s) or Force (N).

Q4. State the characteristics of a standard unit of measurement. (Long Answer)

View Solution

A standard unit should be:

1. Invariable: It should not change with time or physical conditions.
2. Available: It should be easily accessible.
3. Reproducible: It can be recreated anywhere in the world.
4. Indestructible: It should be permanent.

Physical Quantities & Measurement

Comprehensive Question Bank for Class 11 Physics

CBSE PATTERN NEET PREP

I. Very Short Answer Questions (1 Mark)

1. What is meant by a "Physical Quantity"?

View Solution

All quantities that can be measured, either directly or indirectly, and in terms of which the laws of physics can be described are called physical quantities. Examples: Length, Mass, Force.

2. Distinguish between Fundamental and Derived quantities.

View Solution

Fundamental: Quantities that cannot be expressed in terms of others (e.g., Time, Temperature).
Derived: Quantities derived from fundamental ones (e.g., Area = Length × Breadth).

II. Multiple Choice Questions (NEET Pattern)

3. Which of the following is the fundamental unit in the FPS system for Mass?

• A) Gram
• B) Kilogram
• C) Pound
• D) Slug

View Solution

Correct Answer: C (Pound). In the FPS (British Engineering) system, the base units are Foot (length), Pound (mass), and Second (time).

4. The International System of Units (SI) is a modification of which system?

• A) FPS
• B) CGS
• C) MKS
• D) Gaussian

View Solution

Correct Answer: C (MKS). SI is an improved and extended version of the MKS system, including seven base and two supplementary units.

III. Assertion & Reason

Instructions:
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false but R is true.

Assertion (A): Measurement is essentially a process of comparison.
Reason (R): A standard unit should be easily reproducible and invariable.

View Solution

Correct Answer: B. Both statements are factually correct. However, the reason (characteristics of a unit) does not explain the definition of measurement (the process of comparison).

IV. Long Answer Questions (5 Marks)

5. What are the essential characteristics of a standard unit? Why is SI preferred globally?

View Solution

Characteristics:

1. Invariability: It must not change with time or physical conditions (temperature, pressure).
2. Availability: It should be easily available for comparison.
3. Reproducibility: It should be possible to replicate the standard anywhere.
4. Permanency: It should not be perishable.

Why SI is Preferred: It is a coherent and rational system of units used worldwide, ensuring uniformity in scientific data exchange and international trade.

Quick Review: Unit Systems

System	Length	Mass	Time
CGS	cm	g	s
MKS	m	kg	s
FPS	ft	lb	s