Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall and use the SI unit for physical quantities, as listed in Appendix 3
Full specification wording
Topic 1 – Key concepts of physics: 1.1 Recall and use the SI unit for physical quantities, as listed in Appendix 3
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.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 1 of 4
Study notes
From the checked teaching notes for this specification point.
The idea
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. 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.
What earns the marks
- 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.
Common mistakes and how to put them right
Confuses energy and power units.
Put it right: Keep joules for energy and watts for power.
Writes a lower-case k for the kelvin unit.
Put it right: Use the capital K.
Gives a numerical answer with no unit.
Put it right: Attach the SI unit that matches the quantity.
Treats the newton as a base unit.
Put it right: Recognise it as the SI unit of force derived from kg·m/s².
Exam 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.
Try the lesson
The start of the Topic 1 – Key concepts of physics: 1.1 Recall and use the SI unit for physical quantities, as listed in Appendix 3 lesson, made from checked teaching notes for this specification point.
Have a go first
A student measures the mass of a metal block on a balance. Which SI unit should be recorded with the mass?
Watch it explained
What is the one idea the video explains? Say it in a sentence.
Read the described scene instead
This lesson is about SI units in GCSE Physics. You need to recall the unit that matches each physical quantity listed in Appendix 3, and use the correct symbol when you write an answer. The key habit is simple: check the quantity named in the question, not the number given. SI units include base units and derived units. The base units here are metres for length, kilograms for mass, seconds for time, amperes for current, kelvin for temperature, and moles for amount of substance. Derived units include newtons for force, joules for energy, watts for power, volts for potential difference, ohms for resistance, coulombs for charge, hertz for frequency, pascals for pressure, and kilograms per cubic metre for density. Write each symbol exactly as the specification gives it. Use lower-case m for metres, kg for kilograms and s for seconds. Use capital A for amperes, capital K for kelvin, and mol for moles. For derived units, use capital N for newtons, J for joules, W for watts, V for volts, C for coulombs and Hz for hertz. The ohm symbol is Ω, and the pascal symbol is Pa. Pause to think about this example. A question asks for a force, and a calculation gives twelve. The quantity is force, so the SI unit is the newton, symbol N. Write twelve newtons as 12 N, not just 12. If an option offers watts, reject it for force, because watts are the unit of power, not force. Watch for common confusions. Keep joules for energy and watts for power. Do not write a lower-case k for kelvin; use capital K. Never give a numerical answer with no unit. And remember that the newton is not a base unit: it is the SI unit of force, derived from kilogram metre per second squared. Before you finish, check two things. First, does the unit match the quantity named in the question? For example, newtons for force and joules for energy. Second, is the symbol written correctly, including capital or lower-case letters? Use the specification list, write the unit after the number, and check derived units against their quantity rather than guessing from the symbol.
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