SI units
Cambridge A-Level Physics (9702) · Unit 1: Physical quantities and units · 8 flashcards
SI units is topic 1.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 1 — Physical quantities and units , alongside Physical quantities, Errors and uncertainties and Scalars and vectors. In one line: The SI base units are: mass (kilogram, kg), length (meter, m), time (second, s), current (ampere, A), and temperature (kelvin, K). These units form the foundation for all other physical measurements.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 3 definitions, 1 key concept and 4 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The SI base units for mass, length, time, current, and temperature
The SI base units are: mass (kilogram, kg), length (meter, m), time (second, s), current (ampere, A), and temperature (kelvin, K). These units form the foundation for all other physical measurements.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall these SI base quantities with their units: length (m), mass (kg), time (s), electric current (A) and temperature (K).
- Express a derived unit in terms of SI base units by multiplying or dividing them, and apply the appropriate derived units to the quantities in this syllabus.
- Check whether a physical equation is homogeneous by comparing the SI base units on each side.
- Recall the following prefixes and their symbols, and use them to show decimal multiples and submultiples of both base units and derived units: tera (T), giga (G), mega (M), kilo (k), deci (d), centi (c), milli (m), micro (μ), nano (n), pico (p).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in SI units
- › Express every unit in the term (e.g., N = kg m s⁻²) then combine them; moment of force SI base units are kg m² s⁻².
- › Check that the units used in a calculation are consistent, particularly for prefixes like k, μ, or M, before finalizing your answer.
- › Check homogeneity using the SI base units (kg, m, s, A, K); convert mass to kg first.
- › Memorize SI prefixes: Giga (G) is 10 to the power of 9. Use standard form throughout your calculations.
What are the SI base units for mass, length, time, current, and temperature?
The SI base units are: mass (kilogram, kg), length (meter, m), time (second, s), current (ampere, A), and temperature (kelvin, K). These units form the foundation for all other physical measurements.
Express the derived unit for force (Newton, N) in terms of SI base units.
Force (N) is defined as mass × acceleration (F=ma). Therefore, 1 N = 1 kg × m/s² = kg m s⁻².
How can you use SI base units to check the homogeneity of a physical equation?
An equation is homogeneous if the units on both sides are the same. Express all terms in SI base units and simplify; if the units match, the equation is likely correct (but this doesn't guarantee correctness).
What is the meaning of the prefix 'micro' (μ) and what power of 10 does it represent?
The prefix 'micro' (μ) represents 10⁻⁶.
Convert 500 picoseconds (ps) into seconds (s), expressing your answer in scientific notation.
Since pico (p) represents 10⁻¹², 500 ps = 500 × 10⁻¹² s = 5 × 10⁻¹⁰ s.
Express a frequency of 2.5 GHz in Hz. Show your working.
Giga (G) means 10⁹, so 2.5 GHz = 2.5 × 10⁹ Hz = 2,500,000,000 Hz.
The equation for potential energy is PE = mgh. Using SI base units, show that both sides of the equation have the same units.
PE: Energy is measured in Joules (J) and 1J = kg m² s⁻². mgh: kg × m/s² × m = kg m² s⁻². Both sides have the same base units.
What is the purpose of using prefixes like 'kilo' or 'milli' in SI units?
Prefixes are used to express very large or very small quantities in a more manageable and easily understandable form. They provide a convenient way to avoid writing many zeros.
Review the material
Read full revision notes on SI units — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 1 — Physical quantities and units
SI units sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this SI units deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this SI units deck
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