SI UNITS

The S.I. Units (International System of Units) are the globally accepted way of measuring physical quantities.
Think of them as the “official language of measurement” used in science, engineering, medicine, and exams — including WAEC and JAMB.

If you understand SI units, calculations become cleaner and mistakes reduce massively.

Let’s break everything down properly.

1️⃣ What Are Physical Quantities?

A physical quantity is anything that can be measured.
Examples: length, mass, time, temperature, current, force, pressure, etc.

Physical quantities are divided into two types:

(a) Base (Fundamental) Quantities

These are the most basic quantities — they cannot be defined in terms of other quantities.

(b) Derived Quantities

These are formed by combining base quantities through multiplication/division.
Example:
Speed = distance ÷ time
Force = mass × acceleration

2️⃣ The Seven Base Quantities & Their SI Units (VERY IMPORTANT)

WAEC & JAMB always ask this table.
You must know it.

Base Quantity	SI Unit	Symbol of Unit
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Electric current	ampere	A
Amount of substance	mole	mol
Luminous intensity	candela	cd

Memorize this like your life depends on it.

3️⃣ Derived Quantities and Their Units

These come from combinations of base units.

Derived Quantity	SI Unit	Unit Symbol	Unit Expression
Area	square metre	m²	m × m
Volume	cubic metre	m³	m × m × m
Density	kg per m³	kg/m³	kg ÷ m³
Speed/Velocity	m per second	m/s	m ÷ s
Acceleration	m per s²	m/s²	(m/s) ÷ s
Force	newton	N	kg·m/s²
Pressure	pascal	Pa	N/m² or kg/(m·s²)
Energy/Work	joule	J	N·m or kg·m²/s²
Power	watt	W	J/s or kg·m²/s³
Charge	coulomb	C	A·s
Voltage	volt	V	W/A
Frequency	hertz	Hz	1/s

These are the ones most repeated in WAEC/JAMB.

4️⃣ Prefixes Used in SI Units (VERY IMPORTANT)

These help in expressing very large or small values without stress.

Prefix	Symbol	Value
kilo	k	10³
mega	M	10⁶
giga	G	10⁹
milli	m	10⁻³
micro	µ	10⁻⁶
nano	n	10⁻⁹
centi	c	10⁻²

Examples:

2 km = 2 × 10³ m
500 mA = 500 × 10⁻³ A
4 µs = 4 × 10⁻⁶ s

5️⃣ Why SI Units Are Important (Exam Points)

They make measurements standard worldwide
They reduce confusion
They allow scientists to communicate clearly
They help avoid calculation errors
They are used in all scientific exams, including WAEC, NECO, JAMB

6️⃣ Common Mistakes Students Make

Writing “seconds” as sec instead of s
Writing “kgf” instead of N for force
Using “°C” when the question expects K
Forgetting unit conversion
Mixing up mass (kg) with weight (N)

7️⃣ Conversion to SI Units Examples

Example 1: Convert 5 km to m

1 km = 1000 m
5 km = 5000 m

Example 2: Convert 250 g to kg

1 kg = 1000 g
250 g = 0.25 kg

Example 3: Convert 150 cm³ to m³

1 cm³ = 1 × 10⁻⁶ m³
150 cm³ = 150 × 10⁻⁶ m³ = 1.5 × 10⁻⁴ m³

8️⃣ Likely WAEC/JAMB Questions

List the seven base quantities and their units.
What is the SI unit of pressure?
Convert 3.5 km/h to m/s.
State two reasons why SI units are preferred.
Distinguish between mass and weight.
What is the SI unit of energy?