Topic 1 – Key concepts of physics — Edexcel GCSE Physics
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Topic 1 – Key concepts of physics explained
This criterion asks you to know the SI base and derived units for the quantities listed in Appendix 3 of the specification, and to apply them when writing or checking an answer.
Read the full explanation
Learn each quantity with its unit symbol: length in metres (m), mass in kilograms (kg), time in seconds (s), electric current in amperes (A), temperature in kelvin (K), amount of substance in moles (mol), force in newtons (N), energy in joules (J), power in watts (W), potential difference in volts (V), resistance in ohms (Ω), charge in coulombs (C), frequency in hertz (Hz), pressure in pascals (Pa) and density in kilograms per cubic metre (kg/m³). Check the unit against the quantity asked for, not against the number given.
Your focus
- State the SI unit for each physical quantity listed in Appendix 3.
- Select the correct SI unit for a named quantity in a multiple-choice question.
- Apply the correct SI unit when recording a calculated or measured value.
Topic 1 – Key concepts of physics flashcards
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SI base units
Topic 1 – Key concepts of physics exam tips
Marking Points
- Selects the SI unit that matches the physical quantity named in the question, such as newtons for force.
- Uses the correct unit symbol and capitalisation, for example N for newtons and K for kelvin.
- Rejects a non-SI or wrong-quantity unit offered as a distractor, such as choosing watts when the quantity is energy.
- Applies the unit consistently when the answer requires a value with a unit, such as 12 N rather than 12.
Examiner Tips
- 💡Read which quantity the question names before choosing a unit, because distractors often belong to a different quantity.
- 💡Learn the unit symbols with their correct capital or lower-case letters, since marks can depend on exact notation.
- 💡When a calculation gives a value, write the unit straight after the number to secure the unit mark.
- 💡Check derived units such as N, J, W, V, Ω, C, Hz and Pa against their quantity rather than guessing from the symbol.
Common Mistakes
- Confuses energy and power units; keep joules for energy and watts for power.
- Writes a lower-case k for the kelvin unit; use the capital K.
- Gives a numerical answer with no unit; always attach the SI unit that matches the quantity.
- Treats the newton as a base unit; recognise it as the SI unit of force derived from kg·m/s².