Topic 1: Working as a Physicist — Edexcel A-Level Physics
Test yourself on Topic 1: Working as a Physicist with PEARSON EDEXCEL A-Level practice questions.
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Topic 1: Working as a Physicist explained
A base quantity is one of the small set from which all others are built: mass, length, time, electric current, thermodynamic temperature, amount of substance and luminous intensity.
Read the full explanation
Each has a base SI unit (kg, m, s, A, K, mol, cd). A derived quantity is formed by combining base quantities, so its unit is a product or quotient of base units. Speed is length ÷ time, giving m s⁻¹; force is mass × acceleration, giving kg m s⁻², named the newton (N). Recognise a derived unit by resolving it into base units, and check an equation is homogeneous by comparing base-unit dimensions on each side.
Your focus
- State the seven SI base quantities and their units.
- Classify a given quantity as base or derived and justify the choice.
- Resolve a named derived unit into its base units and use this to check an equation.
Topic 1: Working as a Physicist exam tips
Marking Points
- Identifies the seven SI base quantities and their units: mass (kg), length (m), time (s), electric current (A), thermodynamic temperature (K), amount of substance (mol), luminous intensity (cd).
- Explains that a derived quantity is formed by combining base quantities, so its unit is a product or quotient of base units.
- Resolves a named derived unit into base units, e.g. the newton is kg m s⁻² and the joule is kg m² s⁻².
- Uses base-unit dimensions to check the homogeneity of an equation or to test whether a stated unit is consistent with a quantity.
Examiner Tips
- 💡Learn the seven base units as a fixed list and test any unfamiliar unit against it.
- 💡When a unit looks derived, split it into base units before deciding whether it is base or derived.
- 💡Use dimensional checking on both sides of an equation to confirm consistency quickly.
Common Mistakes
- Treating the newton or joule as a base unit: correct by resolving it into kg m s⁻² or kg m² s⁻² respectively.
- Confusing the unit of a quantity with the quantity itself, e.g. calling the kilogram a force: correct by linking each unit to its base quantity.
- Writing derived units with baseline exponents such as m s-2: correct to raised notation, m s⁻².