🧪THE INTERNATIONAL SYSTEM OF UNITS (SI Units) – Complete, Simple Guide for Students

🎯 Goal: Understand how we measure everything in science using standard units — the SI units.

🧠WHAT IS A UNIT? (Introduction)

Imagine you’re baking a cake 🍰 and the recipe says “2 cups of flour.”
What’s a “cup”? It’s a unit — a way to measure flour.

✅ In science, we measure things like length, mass, time, temperature, etc. using standard units so that everyone around the world can understand and compare results easily.

🔹 Why Do We Measure?

Every time we measure something — a distance, weight, time, temperature — we’re comparing it to a standard. That standard is called a unit.

🧠 Definition:
A measurement = a number + a unit
Example: 5 metres, 2 kilograms, 60 seconds

We don’t need separate units for every physical quantity. Why?

👉 Because most physical quantities are connected (e.g., speed = distance ÷ time), we can build many from just a few basic units.

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📏 Key Concepts:

Term	Meaning
Measurement	Comparing a quantity with a standard unit
Unit	A fixed, agreed way to measure a physical quantity
Physical Quantity	Anything we can measure — like mass, length, time, etc.
Base/Fundamental Units	Basic units used to define all others
Derived Units	Made by combining base units (e.g., speed = metre/second)
System of Units	Complete set of base + derived units

🧠 Think of it like LEGO®:
Base units = LEGO bricks
Derived units = anything cool you build with them!

🧱 Types of Units

Type of Unit	Meaning	Examples
Fundamental (Base) Units	The basic units used to measure base physical quantities	metre, second, kilogram
Derived Units	Units formed by combining base units	m/s, newton, joule
System of Units	A complete set of base and derived units	SI system, CGS, MKS

🔁 We only need 7 base units to describe the whole physical world!

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🌍 1.2 WHY INTERNATIONAL UNITS? (The SI System)

Long ago, different countries used different units. That was a mess!
Imagine one country using centimetres, another using feet — chaos! 🌀

So scientists agreed on a universal language of measurement:
🌐 The SI System (Système Internationale d’Unités)

🌍 THE INTERNATIONAL SYSTEM OF UNITS (SI)

Earlier, different countries used different systems — it caused confusion:

System	Length	Mass	Time
CGS	centimetre	gram	second
FPS (British)	foot	pound	second
MKS	metre	kilogram	second

To solve this, scientists adopted a universal system:

🏛️ Système Internationale d’Unités (SI) — French for International System of Units

Now, we all use the SI System — it’s neat, logical, and based on decimals (easy for calculations!). It was adopted officially in 1971 and revised in 2018 for better accuracy.

✅ Introduced by: BIPM (International Bureau of Weights and Measures)
✅ Adopted globally in: 1971
✅ Revised in: 2018 for higher precision using universal constants like the speed of light, Planck constant, etc.

✨ SI is used in science, engineering, industry, and even daily life worldwide.

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📊 The 7 SI Base Units (As per 2018 Definitions)

Physical Quantity	Name	Symbol	Defined Using
Length	metre	m	Distance light travels in vacuum in 1/299,792,458 seconds
Mass	kilogram	kg	Defined using the Planck constant: 6.62607015×10⁻³⁴ Js
Time	second	s	Time taken for 9,192,631,770 cycles of microwave radiation in caesium-133
Electric Current	ampere	A	Defined using charge of electron 1.602176634×10⁻¹⁹ C
Thermodynamic Temperature	kelvin	K	Using Boltzmann constant 1.380649×10⁻²³ J/K
Amount of Substance	mole	mol	Contains exactly 6.02214076×10²³ entities (Avogadro number)
Luminous Intensity	candela	cd	Defined using luminous efficacy of 540 THz light as 683 lm/W

🧠 Memory Tip:
“My King Took A Long Mango Salad Coldly”
(Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela)

📝 Note: These definitions are based on fixed physical constants for precision.
❗ You don’t need to memorize the constants — just know they give us very accurate units.

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📐 Extra SI Units — Angles (Bonus Units)

Two more units are part of the SI system, used to measure angles, but are dimensionless.

Quantity	Unit	Symbol	Formula
Plane Angle	radian	rad	θ = ds / r
Solid Angle	steradian	sr	Ω = dA / r²

🧠 Visual:

• Radian: Angle made by an arc = arc length ÷ radius
• Steradian: 3D version = surface area ÷ radius²

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🧪 Derived SI Units (Built from Base Units)

Quantity	Unit	Formula	Special Name
Speed	m/s	m/s	—
Acceleration	m/s²	m/s²	—
Force	kg·m/s²	—	newton (N)
Energy	kg·m²/s²	—	joule (J)
Power	kg·m²/s³	—	watt (W)
Pressure	N/m²	—	pascal (Pa)
Frequency	s⁻¹	—	hertz (Hz)
Electric Charge	A·s	—	coulomb (C)

🧠 Many of these are listed in Appendix A 6.2 & A 6.3 with special names and symbols.

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🧠 Important Notes on Mole and Candela

• Mole: Must specify the entity: atoms, molecules, ions, etc.
• Candela: Focuses on how bright a light is in a specific direction using a reference light frequency (540 THz)

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⚖️ Other Units Still in Use (Not SI)

Even though SI is the standard, some non-SI units are still used for convenience:

• 📚 Useful Extras You Should Know:
• ✅ Some non-SI units (like minute, hour, litre) are still used — find these in Table 1.2.
• ⚠️ When using mole, always specify the entities (atoms, molecules, etc.).
• 📏 SI uses decimal prefixes like kilo (1000×), milli (1/1000), etc. — See Appendix A2.
• 📐 Follow standard symbols & rules — See Appendices A7 & A8 for guidance.

Quantity	Common Units
Time	minute, hour, day
Volume	litre (L)
Mass	tonne
Energy	calorie
Pressure	bar

See Table 1.2 in your book for more such examples.

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📏 SI Prefixes (For Big and Small Quantities)

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

📌 Complete list in Appendix A2

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✍️ General Rules to Remember (from Appendices A7 & A8)

• Always use correct symbols, not full names in equations (e.g., use kg, not kilogram)
• No plural for units (write 5 kg, not 5 kgs)
• Leave a space between number and unit (25 m, not 25m)
• Never mix units from different systems

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🚀 Final Thought

The SI system makes science clear, connected, and universal. Whether you’re in Delhi or Denmark, when you say “1 kilogram,” everyone knows exactly what you mean! 🌍

🌟 Final Recap: Why SI Units Matter

✅ Simple to use (decimal-based)
✅ Universal and accepted worldwide
✅ Based on physical constants — super accurate
✅ Helps scientists, engineers, and even daily life