What are SI units and why are they important in Physics?

SI units, or the International System of Units, are the standard units of measurement used globally in science and engineering. They provide a consistent framework for measuring physical quantities, ensuring that scientists and students, including those studying Cambridge International A Levels, can communicate and compare results accurately. This system helps avoid confusion and errors that could arise from using different measurement systems.

How many base SI units are there and what do they represent?

There are seven base SI units, each representing a fundamental physical quantity. These are: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for the amount of substance, and candela (cd) for luminous intensity. Understanding these units is crucial for Cambridge International A Levels Physics students as they form the foundation for all derived units.

Can you explain the difference between base units and derived units in the SI system?

Base units are the fundamental units in the SI system from which all other units are derived. Derived units are combinations of base units that measure more complex physical quantities. For example, the unit of force, the newton (N), is a derived unit expressed as kg·m/s². This distinction is important for Cambridge International A Levels students to grasp the relationships between different physical quantities and units.

Why is the kilogram the only SI base unit with a prefix, and how is it defined?

The kilogram is unique among SI base units because it includes a prefix. Originally, it was defined as the mass of a specific platinum-iridium alloy cylinder. However, since 2019, it has been redefined based on the Planck constant, a fundamental constant of nature. This change ensures greater precision and stability in measurements, which is essential for advanced studies like Cambridge International A Levels Physics.

How do SI units facilitate international collaboration in scientific research?

SI units provide a universal language for scientists around the world, enabling seamless collaboration and data sharing. By using a standardized system, researchers can accurately compare results and build upon each other's work without the risk of misinterpretation due to unit discrepancies. This is particularly beneficial for students and professionals involved in international projects, such as those studying Cambridge International A Levels Physics.

Why is "Treating area conversion same as length" a common mistake in SI units?

Why it happens: Linear conversion habit. How to avoid it: Area scales as length SQUARED, volume as length CUBED. 1 cm² = 10^-4 m² (NOT 10^-2). Source: 9702 Examiner Reports 2022-2024

Why is "Confusing m (milli) with M (mega) or μ (micro) with m (milli)" a common mistake in SI units?

Why it happens: Similar letters. How to avoid it: Capital M = 10^6. Lowercase m = 10^-3. μ = 10^-6 (Greek mu). Source: 9702 Examiner Reports 2022-2024

Physical Quantities and UnitsCambridge International A Levels Physics (9702)

SI units

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SI Units Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Base units. SI prefixes. Unit conversions. The everyday toolkit for physics calculations.

What you’ll learn

Mapped to the Cambridge IGCSE 9702 syllabus (2025-2027).

1.2.1 — Use SI base and derived units.
1.2.2 — Use SI prefixes.
1.2.3 — Convert between unit systems.

SI base units

Seven units, all of physics.

Seven base units. Memorise:

Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Current	ampere	A
Temperature	kelvin	K
Amount	mole	mol
Luminous intensity	candela	cd

Cambridge tip. Quantitative answers must include the unit. Mark scheme penalises missing or wrong units.

Seven base units.
Always include unit in answer.
Wrong unit = wrong answer.

SI prefixes

Multipliers from pico to giga.

Commonly used prefixes:

Prefix	Symbol	Multiplier
giga	G	$10^9$
mega	M	$10^6$
kilo	k	$10^3$
(none)	—	$10^0$
centi	c	$10^{-2}$
milli	m	$10^{-3}$
micro	μ	$10^{-6}$
nano	n	$10^{-9}$
pico	p	$10^{-12}$

Watch for confusable symbols.

M (mega, $10^6$ ) vs m (milli, $10^{-3}$ ).
μ (micro, Greek mu) vs m (milli).

Examples.

$3.5$ MHz $= 3.5 \times 10^6$ Hz.
$0.45$ mm $= 4.5 \times 10^{-4}$ m.
$2.7$ μA $= 2.7 \times 10^{-6}$ A.

Cambridge tip. Convert to base SI before substituting in formulae.

Memorise prefix table.
Distinguish M vs m carefully.
Convert to base SI before substituting.

See the full worked example for si units →

Area and volume conversions

Square / cube the length factor.

Area scales as length SQUARED. If 1 cm = $10^{-2}$ m, then 1 cm² = $10^{-4}$ m².

Volume scales as length CUBED. 1 cm³ = $10^{-6}$ m³.

Density. g/cm³ → kg/m³: multiply by $1000$ .

$1.00$ g/cm³ × $\frac{10^{-3} \text{ kg}}{10^{-6} \text{ m}^3} = 1000$ kg/m³.

Example. Water density = $1.00$ g/cm³ = $1000$ kg/m³ = $10^3$ kg/m³.

Cambridge tip. Cube/square the conversion factor — don't just apply linear conversion.

Area: square the conversion.
Volume: cube the conversion.
g/cm³ → kg/m³ multiply by 1000.

See the full worked example for si units →

How it’s examined

SI units are tested in every Paper 1 and Paper 2. Most-tested: prefix conversion (3-5 marks), area/volume conversion (4-5 marks), density conversion (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/12 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

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Step-by-step worked examples — SI units

Step-by-step solutions to past-paper-style questions on si units, written exactly the way a tutor would explain them at the board.

1SI prefix conversion (4 marks)
Extended• Adapted from 9702/12 May/Jun 2024• prefixes
Question
Convert: (a) $3.5 \text{ MHz}$ to Hz, (b) $0.45 \text{ mm}$ to m, (c) $2.7 \text{ μA}$ to A. (4 marks)
Step-by-step solution
1. Step 1
  (a) M = $10^6$ . $3.5 \text{ MHz} = 3.5 \times 10^6$ Hz.
2. Step 2
  (b) m (milli) = $10^{-3}$ . $0.45 \text{ mm} = 4.5 \times 10^{-4}$ m.
3. Step 3
  (c) μ (micro) = $10^{-6}$ . $2.7 \text{ μA} = 2.7 \times 10^{-6}$ A.
Answer
(a) $3.5 \times 10^6$ Hz. (b) $4.5 \times 10^{-4}$ m. (c) $2.7 \times 10^{-6}$ A.
2Area and volume conversions (5 marks)
Extended• conversions
Question
Convert: (a) $250 \text{ cm}^2$ to $\text{m}^2$ , (b) $1.5 \text{ cm}^3$ to $\text{m}^3$ . (5 marks)
Step-by-step solution
1. Step 1
  1 cm = $10^{-2}$ m.
2. Step 2
  (a) Area scales as length squared. $1 \text{ cm}^2 = 10^{-4} \text{ m}^2$ . So $250 \text{ cm}^2 = 2.5 \times 10^{-2}$ m².
3. Step 3
  (b) Volume scales as length cubed. $1 \text{ cm}^3 = 10^{-6} \text{ m}^3$ . So $1.5 \text{ cm}^3 = 1.5 \times 10^{-6}$ m³.
Answer
(a) $2.5 \times 10^{-2}$ m². (b) $1.5 \times 10^{-6}$ m³.
3Density unit conversion (5 marks)
Extended• conversions
Question
Density of water is $1.00 \text{ g/cm}^3$ . Convert to kg/m³. (5 marks)
Step-by-step solution
1. Step 1
  $1 \text{ g} = 10^{-3}$ kg.
2. Step 2
  $1 \text{ cm}^3 = 10^{-6}$ m³.
3. Step 3
  Combine.
  $1.00 \frac{\text{g}}{\text{cm}^3} = 1.00 \times \frac{10^{-3} \text{ kg}}{10^{-6} \text{ m}^3} = 1000 \text{ kg/m}^3$
Answer
$1.00 \text{ g/cm}^3 = 1000 \text{ kg/m}^3$ .

Key Formulae — SI units

The formulae you need to memorise for si units on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

SI prefixes
$\text{T}=10^{12};\; \text{G}=10^{9};\; \text{M}=10^{6};\; \text{k}=10^{3};\; \text{c}=10^{-2};\; \text{m}=10^{-3};\; \mu=10^{-6};\; \text{n}=10^{-9};\; \text{p}=10^{-12}$
When to use
Unit prefixes — memorise. Used throughout physics.

Key Definitions and Keywords — SI units

Definitions to memorise and the exact keywords mark schemes credit for si units answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

SI (Système International)
Examiner keyword
Internationally agreed system of units based on 7 base quantities.
SI prefix
Examiner keyword
Multiplier applied to a unit (e.g. k = 1000, M = $10^6$ ).

Common Mistakes and Misconceptions — SI units

The traps other students keep falling into on si units questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Treating area conversion same as length
9702 Examiner Reports 2022-2024
Why it happens
Linear conversion habit.
How to avoid it
Area scales as length SQUARED, volume as length CUBED. 1 cm² = $10^{-4}$ m² (NOT $10^{-2}$ ).
✕Confusing m (milli) with M (mega) or μ (micro) with m (milli)
9702 Examiner Reports 2022-2024
Why it happens
Similar letters.
How to avoid it
Capital M = $10^6$ . Lowercase m = $10^{-3}$ . μ = $10^{-6}$ (Greek mu).

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Video lesson

Short walkthrough of the concepts students most often get stuck on.

SI units — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

SI units

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1SI prefix conversion (4 marks)

2Area and volume conversions (5 marks)

3Density unit conversion (5 marks)

SI prefixes

SI (Système International)

SI prefix

✕Treating area conversion same as length

✕Confusing m (milli) with M (mega) or μ (micro) with m (milli)

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

SI units — frequently asked questions