40% Get Big Discount For a Limited time

1.3: International System of Units (SI Units)

9th Physics Book PTCB (2025)

100%
Chapter 1: Physical Quantities and Measurements

1.3: International System of Units (SI Units)

16 views Chapter 1: Physical Quantities and Measurements

⚙️ 1.3 International System of Units (SI Units)

🌍 Introduction

In 1961, the International Committee on Weights and Measures 👨‍🔬 introduced a globally accepted system of measurement called the International System of Units (SI Units) 📏.

Today, SI Units are used all over the world 🌎, making scientific communication easy, accurate, and universal.
It ensures same measurements, no matter which country, language, or lab you belong to! 🤝

📏 The Seven SI Base Units

These are the fundamental building blocks 🧱 of all measurements.
Every other unit (speed, force, area, volume) is derived from these.

✅ SI Base Units Table

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 Luminous Intensity candela cd
7️⃣ Amount of Substance mole mol

🌟 Real-World Examples of Base Units

  • Length (m) 📏 → Height of a door ≈ 2 m

  • Mass (kg) ⚖️ → Packet of sugar = 1 kg

  • Time (s) ⏱️ → Blink of an eye ≈ 0.3 s

  • Temperature (K) 🌡️ → Room temp ≈ 293 K (20°C)

  • Electric current (A) ⚡ → Mobile charger ≈ 1–2 A

  • Light intensity (cd) 💡 → Candle flame ≈ 1 cd

  • Amount of substance (mol) 🧪 → 1 mole of water = 18 g

⚡ Derived Units

Base units are independent, but most physical quantities we use daily (speed, force, pressure) are made by combining base units.

These are called derived units ⚙️.

📊 Common Derived Units

No. 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 = m × m = m²

Real Example:
Room size 4 m × 3 m → 12 m²

2️⃣ Speed 🏎️

Formula:
Speed = Distance / Time = m/s

Real Example:
A car covers 100 m in 5 s → Speed = 20 m/s

3️⃣ Volume 🧊

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

Real Example:
Cube of 2 m side → 2³ = 8 m³

4️⃣ Force 💪

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

Real Example:
Gravity on 1 kg apple → ≈ 10 N

5️⃣ Pressure 🌬️

Formula:
Pressure = Force / Area = N/m² = Pa

Real Example:
Pressure at sea level → 101,325 Pa

🔢 Prefixes & Multiples of SI Units

The SI system is decimal-based 🔟 (powers of 10).
To express very large 🐘 or very small 🐜 quantities easily, we use prefixes.

📊 Common SI Prefixes

Prefix Symbol Power of 10 Example
giga G 10⁹ 1 GB = 1 billion bytes 💾
mega M 10⁶ 1 MHz = 1,000,000 Hz 📻
kilo k 10³ 1 km = 1000 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 = 1×10⁻⁹ m 🧬

🧭 Real-World Prefix Examples

1️⃣ mm → m
5000 mm = 5 m

2️⃣ cm → m
1500 cm = 15 m

3️⃣ g → kg
3000 g = 3 kg

4️⃣ μs → ms
2000 μs = 2 ms

5️⃣ km → m
5 km = 5000 m

6️⃣ MB → bytes
5 MB = 5,000,000 bytes

⚡ Quick Quiz – With Answers

❓ Question 1

Write the unit of electric charge in terms of base units (ampere & second).

✅ Answer:

Charge = Current × Time
C = A × s
1 C = 1 A·s

❓ Question 2

Express pascal (Pa) using base units.

✅ Answer:

Pressure = Force / Area = N/m²
Since 1 N = 1 kg·m/s²,

➡️ 1 Pa = 1 kg / (m·s²)

🎯 Summary

✔️ SI = Universal system of 7 base units
✔️ Derived units = Made from base units
✔️ Prefixes = Make numbers easy
✔️ Used worldwide for accuracy & communication
✔️ Essential for science, engineering & daily life

Up to Top