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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall the following SI base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K)
- express derived units as products or quotients of the SI base units and use the derived units for quantities listed in this syllabus as appropriate
- use SI base units to check the homogeneity of physical equations
- recall and use the following prefixes and their symbols to indicate decimal submultiples or multiples of both base and derived units: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T)
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
- › Use standard units like Farads (F) and check if the axis uses ln (I/μA) which makes the value unitless.
- › Ensure all column headings use the accepted scientific convention (quantity/unit) and that final constants include units derived from the graph axes.
- › 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.
- › Distinguish between physical quantities and their units; ensure your answer includes both a magnitude and a unit for physical quantities.
- › Always refer back to the seven SI base units defined in the syllabus; ensure mass is converted to kg for base unit consistency.
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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