International System of Units (SI Units) – Base & Derived Units Explained with Examples (Class 9 Physics 2025)

International System of Units (SI Units) – Base & Derived Units Explained with Examples (Class 9 Physics 2025)

⚙️ 1.3 International System of Units (SI Units) 🌍 Introduction

In 1961, the International Committee on Weights and Measures 👨‍🔬 recommended a standardized system consisting of seven base units, known as the International System of Units (SI) 📐.

This system is now used worldwide 🌎, helping scientists share and compare their measurements, observations, and results easily across different countries and languages! 🤝

📏 The Seven Base Units

These are the fundamental building blocks 🧱 of all measurements in physics:

No. Physical Quantity 📊 Unit 📏 Symbol 🔤 1️⃣ Length metre m 2️⃣ Mass kilogram kg 3️⃣ Time second s 4️⃣ Temperature kelvin K 5️⃣ Electric Current ampere A 6️⃣ Intensity of Light candela cd 7️⃣ Amount of Substance mole mol

🌟 Real-World Examples:

• Length (metre) 📏: Height of a door ≈ 2 m
• Mass (kilogram) ⚖️: A bag of sugar = 1 kg
• Time (second) ⏱️: Blink of an eye ≈ 0.3 s
• Temperature (kelvin) 🌡️: Room temperature ≈ 293 K (20°C)
• Electric Current (ampere) ⚡: Phone charger ≈ 1-2 A
• Light Intensity (candela) 💡: A standard candle ≈ 1 cd
• Amount of Substance (mole) 🧪: 1 mole of water = 18 grams

⚡ Derived Units

Base units cannot be derived from one another—they are independent 🔓.

However, many physical quantities like speed 🏃, area 📐, volume 🧊, force 💪, and pressure 🌬️ can be expressed using combinations of these base units.

Such quantities use derived units ⚙️.

📊 Common Derived Units

Sr. Physical Quantity 🔬 Unit Name Symbol 🔤 1️⃣ Area square metre m² 2️⃣ Volume cubic metre m³ 3️⃣ Speed metre per second m/s 4️⃣ Force newton N 5️⃣ Pressure pascal Pa 6️⃣ Electric Charge coulomb C 7️⃣ Plane Angle radian rad

🧮 Examples of Derived Units 1️⃣ Area 📐

Formula:

Area = Length × Breadth = metre × metre = m²

Real-World Example: 🏠
A small room measuring 4 m × 3 m has an area of 12 m²

2️⃣ Speed 🏎️

Formula:

Speed = Distance/Time = metre/second = m/s

Real-World Example: 🚗
A car traveling 100 meters in 5 seconds has a speed of 20 m/s

3️⃣ Volume 🧊

Formula:

Volume = Length × Breadth × Height = m × m × m = m³

Real-World Example: 📦
A cube-shaped box with sides of 2 m has a volume of 8 m³

4️⃣ Force 💪

Formula:

Force = Mass × Acceleration = kg × m/s² = N (newton)

Real-World Example: 🍎
The force of gravity on a 1 kg apple is approximately 10 N

5️⃣ Pressure 🌬️

Formula:

Pressure = Force/Area = N/m² = Pa (pascal)

Real-World Example: 🎈
Atmospheric pressure at sea level ≈ 101,325 Pa (or 101.3 kPa)

🔢 Prefixes and Multiples of SI Units

The SI System is a decimal system 🔟, meaning it uses powers of 10.

To express very large 🐘 or very small 🐜 numbers conveniently, prefixes are used.

📊 Common SI Prefixes Table:

Prefix Symbol Power of 10 Example giga G 10⁹ 1 GB = 1,000,000,000 bytes 💾 mega M 10⁶ 1 MHz = 1,000,000 Hz 📻 kilo k 10³ 1 km = 1,000 m 🚶 centi c 10⁻² 1 cm = 0.01 m 📏 milli m 10⁻³ 1 mm = 0.001 m 🔬 micro μ 10⁻⁶ 1 μs = 0.000001 s ⏱️ nano n 10⁻⁹ 1 nm = 0.000000001 m 🧬

🧭 Real-World Examples Using Prefixes 1️⃣ Millimetres to Metres 📏 5000 mm = 5000/1000 m = 5 m

Example: The width of a large TV screen 📺 ≈ 1500 mm = 1.5 m

2️⃣ Centimetres to Metres 📐 50000 cm = 50000/100 m = 500 m

Example: Height of a 5-story building 🏢 ≈ 1500 cm = 15 m

3️⃣ Grams to Kilograms ⚖️ 3000 g = 3000/1000 kg = 3 kg

Example: A large bag of rice 🍚 = 5000 g = 5 kg

4️⃣ Microseconds to Seconds and Milliseconds ⏱️ 2000 μs = 2000 × 10⁻⁶ s = 2 × 10⁻³ s = 2 ms

Example: Computer processor speed 💻 operates in nanoseconds (ns) and microseconds (μs)

5️⃣ Kilometres to Metres 🚗 5 km = 5 × 1000 m = 5000 m

Example: Distance from home to school 🏫 ≈ 2 km = 2000 m

6️⃣ Megabytes to Bytes 💾 5 MB = 5 × 10⁶ bytes = 5,000,000 bytes

Example: A high-quality photo 📸 ≈ 3-5 MB

⚡ Quick Quiz Solutions Question 1: Write the unit of charge in terms of base unit ampere and second ⚡

Answer:
Electric charge is measured in coulombs (C).

Formula:

Charge = Current × Time C = A × s

Therefore: 1 coulomb (C) = 1 ampere-second (A·s) ✅

Real-World Example: 🔋
A phone battery rated at 3000 mAh (milliampere-hours) passing 3 A for 1 hour transfers 10,800 coulombs of charge!

Question 2: Express the unit of pressure "pascal" in some other units 🌬️

Answer:
Pressure is measured in pascals (Pa).

Formula:

Pressure = Force/Area Pa = N/m²

Since Force (N) = kg·m/s², we can write:

Pa = (kg·m/s²)/m² = kg/(m·s²)

Therefore: 1 pascal (Pa) = 1 N/m² = 1 kg/(m·s²) ✅

Real-World Example: 🏀
A basketball inflated to 8 psi has a pressure of approximately 55,158 Pa or 55.2 kPa

Another example: Tire pressure 🚗 ≈ 200-250 kPa (or 30-35 psi)

🎯 Summary

✅ SI System = 7 base units + derived units
✅ Base units are independent 🔓
✅ Derived units are combinations of base units ⚙️
✅ Prefixes make large/small numbers easier to handle 🔢
✅ Used worldwide for scientific consistency 🌍

Remember: Understanding SI units is like learning a universal language 🗣️ that all scientists speak! 🧑‍🔬👩‍🔬