AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Use of mass, length, time, amount of substance, temperature, electric…
Full specification wording
Use of SI units and their prefixes: Use of mass, length, time, amount of substance, temperature, electric current and their associated SI units.
Mass is measured in kilograms (kg), length in metres (m), time in seconds (s), amount of substance in moles (mol), thermodynamic temperature in kelvin (K) and electric current in amperes (A).
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Mass is measured in kilograms (kg), length in metres (m), time in seconds (s), amount of substance in moles (mol), thermodynamic temperature in kelvin (K) and electric current in amperes (A). When a quantity is calculated, its unit follows from the defining equation; for example charge Q = I t gives coulombs, C = A s, and energy E = m c Δθ gives joules, J = kg m² s⁻². Always convert prefixed values to base units before substituting, such as 25 cm to 0.25 m or 2.5 kJ to 2500 J, and give the final unit with the correct symbol and spacing.
What earns the marks
- Select the correct base quantity and unit for a given physical context, such as using kelvin for thermodynamic temperature.
- Convert prefixed values to base units before substitution, for example 50 mA to 0.050 A and 20 °C to 293 K where absolute temperature is required.
- Derive the unit of a calculated quantity from its defining equation, for example C = A s from Q = I t.
- Present answers with the correct unit symbol, including negative or positive indices where needed, such as m s⁻¹ or kg m⁻³.
- Check that both sides of an equation have the same base units when validating a relationship.
Common mistakes and how to put them right
Substituting a value in centimetres or milliamperes directly into an equation.
Put it right: Convert to metres or amperes first so the result is in coherent SI units.
Using degrees Celsius in the ideal gas equation instead of thermodynamic temperature in kelvin.
Put it right: Add 273 to convert.
Writing the unit of amount of substance as grams or as a number without a unit.
Put it right: The correct unit is the mole, mol.
Leaving a calculated answer without a unit or with an inconsistent unit.
Put it right: Attach the unit derived from the equation.
Exam tips
- Write down the equation, rearrange it, then substitute values with units so errors in unit handling are visible.
- Convert all data to base units at the start of a calculation and state the conversion briefly.
- Give final answers to an appropriate number of significant figures and include the unit; a missing unit can lose credit.
- Use standard unit symbols and spacing, for example N m⁻¹ rather than N/m, to avoid ambiguity.
Try the lesson
The start of the Use of SI units and their prefixes: Use of mass, length, time, amount of substance, temperature, electric current and their associated SI units. lesson, made from checked teaching notes for this specification point.
Have a go first
Which SI unit is used to measure electric current?
Listen
Mass is measured in kilograms (kg), length in metres (m), time in seconds (s), amount of substance in moles (mol), thermodynamic temperature in kelvin (K) and electric current in amperes (A). When a quantity is calculated, its unit follows from the defining equation; for example charge Q = I t gives coulombs, C = A s, and energy E = m c Δθ gives joules, J = kg m² s⁻². Always convert prefixed values to base units before substituting, such as 25 cm to 0.25 m or 2.5 kJ to 2500 J, and give the final unit with the correct symbol and spacing. A strong answer earns marks for these points. Select the correct base quantity and unit for a given physical context, such as using kelvin for thermodynamic temperature. Convert prefixed values to base units before substitution, for example 50 mA to 0.050 A and 20 °C to 293 K where absolute temperature is required. Derive the unit of a calculated quantity from its defining equation, for example C = A s from Q = I t. Present answers with the correct unit symbol, including negative or positive indices where needed, such as m s⁻¹ or kg m⁻³. Exam tip: Write down the equation, rearrange it, then substitute values with units so errors in unit handling are visible.
Key words
Say what each word means before you turn the card.
SI base quantities
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Unit conversion and prefixes
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Coherent SI units in calculation
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Dimensional consistency
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See every definition
- SI base quantities
- Mass, length, time, amount of substance, thermodynamic temperature and electric current, each with its associated SI unit.
- Unit conversion and prefixes
- Prefixed values are converted to base units before substituting, such as 25 cm to 0.25 m or 2.5 kJ to 2500 J.
- Coherent SI units in calculation
- When a quantity is calculated, its unit follows from the defining equation, for example C = A s from Q = I t.
- Dimensional consistency
- Both sides of an equation have the same base units when validating a relationship.
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