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Units and Measurements Class 11 – Gurugyanam.com

Units and Measurements is one of the fundamental chapters in Class 11 Physics. It lays the groundwork for understanding how physical quantities are measured and how units play a vital role in expressing those quantities. Let’s explore this important topic in detail.

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➤ What is Measurement?

Measurement is the process of comparing a physical quantity with a standard unit (reference). When we say that the length of a rod is 5 meters, we mean the rod is five times as long as the standard unit called a meter.

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➤ What are Units?

Units are standard quantities used to measure physical quantities. There are two main types of units:

1. Fundamental Units
2. Derived Units

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➤ Fundamental Units

These are basic units that are independently defined and form the foundation for all other units. In the SI system, there are 7 fundamental units:

Physical Quantity	SI Unit	Symbol
Length	Meter	m
Mass	Kilogram	kg
Time	Second	s
Electric Current	Ampere	A
Temperature	Kelvin	K
Amount of Substance	Mole	mol
Luminous Intensity	Candela	cd

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➤ Derived Units

Derived units are combinations of fundamental units. Examples:

• Velocity = Distance / Time → meter/second (m/s)
• Force = Mass × Acceleration → Newton (N) = kg·m/s²
• Energy = Work → Joule (J) = N·m

Some derived units have special names like Newton, Joule, Watt, etc.

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➤ International System of Units (SI System)

To ensure global consistency in measurements, scientists use the International System of Units, abbreviated as SI (from the French “Système International d’Unités”).
The SI system was standardized by BIPM (International Bureau of Weights and Measures) in 1971 and revised in 2018.

It is now the globally accepted standard for scientific, technical, industrial, and commercial purposes.

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➤ SI Prefixes

To represent very large or very small quantities, prefixes are added to SI units:

Prefix	Symbol	Factor
kilo	k	10³
mega	M	10⁶
milli	m	10⁻³
micro	µ	10⁻⁶
nano	n	10⁻⁹

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➤ Scientific Notation

Scientific notation is used to write extremely large or small numbers in a compact form:
Examples:

• 5,000,000 = 5 × 10⁶
• 0.000045 = 4.5 × 10⁻⁵

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➤ Dimensional Analysis

Dimensions express the nature of physical quantities.
Examples:

• Dimension of Length = [L]
• Velocity = Distance / Time = [L][T]⁻¹

Dimensional analysis helps verify the correctness of equations and derive relationships between physical quantities.

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➤ Summary – Units and Measurements

1. Physics is a quantitative science based on measurement of physical quantities.
2. All measurements are based on fundamental and derived units.
3. The SI system is the globally accepted system for all types of measurements.
4. Every unit has a standard symbol like meter (m), second (s), Newton (N), etc.
5. Prefixes, scientific notation, and dimensional analysis are important tools in expressing measurements clearly.
6. In calculations, only significant figures should be retained based on measurement accuracy.

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