Physical quantities — OCR A-Level Physics
Test yourself on Physical quantities with OCR A-Level practice questions.
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Physical quantities explained
Every physical quantity is expressed as a numerical value multiplied by a unit.
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For example, a length might be 2.5 m, where 2.5 is the numerical value and m is the unit. Without a unit, a numerical value is meaningless in physics: stating that a speed is 5 tells you nothing unless you know whether it is 5 m s⁻¹, 5 km h⁻¹ or 5 cm s⁻¹. You must be able to identify the numerical value and unit in any given quantity, choose appropriate SI units, and convert between prefixes such as kilo (10³), milli (10⁻³) and micro (10⁻⁶). The seven SI base units include the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd).
(b) making estimates of physical quantities listed in this specification.
Estimating means choosing a sensible order-of-magnitude value for a quantity named in the specification, then justifying it with familiar anchors. For example, a person's mass is about 70 kg, so a car's mass is about 1000 kg and a grain of rice about 10⁻⁵ kg. Lengths run from atomic scales (about 10⁻¹⁰ m) to Earth's radius (about 10⁶ m); times from atomic periods (about 10⁻¹⁵ s) to the age of the universe (about 10¹⁷ s). Work in S.I. base units, keep one significant figure, and check the power of ten rather than chasing precision.
Your focus
- State that every physical quantity comprises a numerical value and a unit.
- Identify the numerical value and unit in a given physical quantity and convert between common SI prefixes.
- Distinguish between SI base units and derived units and express derived units in terms of base units.
Show all 6 objectives
- Estimate the order of magnitude of a named physical quantity in S.I. base units.
- Justify an estimate by reference to a familiar physical anchor.
- Select the most reasonable value from a set of options and reject implausible powers of ten.
Physical quantities exam tips
Marking Points
- States that a physical quantity consists of a numerical value and a unit, and that both are required for the quantity to be meaningful.
- Identifies the numerical value and the unit in a given expression, such as 9.81 m s⁻² having numerical value 9.81 and unit m s⁻².
- Converts between SI prefixes, for example 1 km = 10³ m, 1 mm = 10⁻³ m, and 1 μs = 10⁻⁶ s.
- Recognises the seven SI base units and distinguishes them from derived units such as the newton (N) or joule (J).
- Recognises that an estimate is an order-of-magnitude value expressed in S.I. base units, not a precise measurement.
- Selects a familiar physical anchor (for example a person's mass of about 70 kg) and scales it to the target quantity.
- States the estimate with a sensible power of ten and a plausible unit, for example the mass of a car as about 10³ kg.
- Checks the reasonableness of the value against everyday experience or a known reference before committing to the answer.
Examiner Tips
- 💡Always write the unit alongside your numerical answer, even in multiple-choice questions where units may be part of the options.
- 💡Check prefix conversions carefully: write out the power of ten before substituting into an equation.
- 💡Learn the seven SI base units and be ready to express derived units in terms of them.
- 💡Convert every estimate into S.I. base units before comparing options in a multiple-choice question.
- 💡Use anchors you already know, such as a 100 m sprint time of about 10 s or a room height of about 3 m.
- 💡Eliminate options whose power of ten is clearly impossible, then choose between the remaining plausible values.
Common Mistakes
- Writing a numerical answer without a unit: the correction is always to attach the correct SI unit to every physical quantity.
- Confusing prefixes, for example treating 1 mm as 10⁻⁶ m instead of 10⁻³ m; the correction is to learn the prefix powers of ten systematically.
- Assuming all units are SI base units: derived units such as the newton and joule are combinations of base units and are equally valid in calculations.
- Treating an estimate as a precise measurement and quoting many significant figures; the correction is to give one significant figure with the correct power of ten.
- Confusing mass with weight when estimating; the correction is to give mass in kg and, if weight is wanted, multiply by g ≈ 9.81 N kg⁻¹.
- Choosing a power of ten that is wildly wrong, such as a person's mass as 10⁵ kg; the correction is to compare with a familiar anchor before answering.