SI Units: The Building Blocks of Physics
1️⃣ Base Units – The 7 Pillars
The International System of Units (SI) starts with seven base units. Think of them as the LEGO bricks that build everything else.
- 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) 💡
2️⃣ Derived Units – Mixing the Bricks
Derived units are combinations of base units. Below is a handy table that shows how to express each derived unit as a product or quotient of base units.
| Derived Unit | Symbol | Expression in Base Units |
|---|
| Force | N | \$\,\frac{kg \cdot m}{s^2}\,\$ |
| Energy / Work | J | \$\,\frac{kg \cdot m^2}{s^2}\,\$ |
| Power | W | \$\,\frac{kg \cdot m^2}{s^3}\,\$ |
| Electric Charge | C | \$\,A \cdot s\,\$ |
| Electric Potential (Voltage) | V | \$\,\frac{kg \cdot m^2}{A \cdot s^3}\,\$ |
| Electrical Resistance | Ω | \$\,\frac{kg \cdot m^2}{A^2 \cdot s^3}\,\$ |
| Magnetic Flux Density (Magnetic Field) | T | \$\,\frac{kg}{A \cdot s^2}\,\$ |
| Inductance | H | \$\,\frac{kg \cdot m^2}{A^2 \cdot s^2}\,\$ |
| Pressure | Pa | \$\,\frac{kg}{m \cdot s^2}\,\$ |
3️⃣ Using Derived Units in the A‑Level Syllabus
Below are key quantities you’ll encounter, with their SI units and a quick analogy to help remember.
- Force (N) – The push or pull on an object. Analogy: A 1 kg mass on Earth feels a force of 9.8 N (≈ 1 kg × 9.8 m s⁻²).
- Work/Energy (J) – The amount of effort to move something. Analogy: Lifting a 1 kg weight 1 m gives 1 J of work.
- Power (W) – Work done per unit time. Analogy: A 60 W light bulb uses 60 J of energy each second.
- Electric Charge (C) – Quantity of electrons. Analogy: One coulomb equals about 6.24 × 10¹⁸ electrons.
- Voltage (V) – Potential difference. Analogy: A 12 V battery can push 12 J of energy per coulomb of charge.
- Resistance (Ω) – Opposition to current. Analogy: A 1 Ω resistor allows 1 A of current for every volt applied.
- Magnetic Field (T) – Strength of magnetism. Analogy: Earth’s magnetic field is about 50 µT.
- Inductance (H) – Ability to store magnetic energy. Analogy: A 1 H coil stores 0.5 J of magnetic energy when 1 A flows.
- Pressure (Pa) – Force per unit area. Analogy: Atmospheric pressure at sea level ≈ 101 kPa.
4️⃣ Quick Reference Cheat Sheet
Use this as a handy reminder when you’re solving problems.
| Quantity | Symbol | SI Unit | Base Units |
|---|
| Force | N | kg m s⁻² | \$\,\frac{kg \cdot m}{s^2}\,\$ |
| Energy | J | kg m² s⁻² | \$\,\frac{kg \cdot m^2}{s^2}\,\$ |
| Power | W | kg m² s⁻³ | \$\,\frac{kg \cdot m^2}{s^3}\,\$ |
| Resistance | Ω | kg m² s⁻³ A⁻² | \$\,\frac{kg \cdot m^2}{A^2 \cdot s^3}\,\$ |
5️⃣ Tips for Remembering Units
- Think “MASS × LENGTH / TIME²” for force – like a 1 kg weight on Earth (≈ 9.8 N).
- Energy is just force times distance: J = N × m.
- Power is energy per time: W = J / s.
- Voltage is energy per charge: V = J / C.
- Resistance is voltage per current: Ω = V / A.
Keep this guide handy, and you’ll never get lost in the unit jungle again! 🌱🚀