SEC G3 Combined Science Physics component K326/K327
CP1: Physical Quantities, Units and Measurement
Use SI units, instruments, data, graphs, and uncertainty within the Combined Science component scope.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
A measurement is useful only when its quantity, numerical value, unit, scale, instrument range, and precision are sensible for the object being studied.
Quantities, SI units, and prefixes
Write every measured quantity as a magnitude followed by a unit. The required base quantities are mass in kilograms, length in metres, time in seconds, electric current in amperes, temperature in kelvin, and amount of substance in moles.
A prefix rescales a unit by a power of ten: nano n is , micro µ is , milli m is , centi c is , deci d is , kilo k is , mega M is , giga G is , and tera T is . Convert before calculation and preserve symbol capitalisation.
Scale and instrument decisions
Estimate the size of the object first. An atom, cell, person, building, and Earth occupy very different orders of magnitude, so an estimate can expose a unit conversion that is wrong by thousands or millions.
Useful scale anchors are an atom at about m, a typical cell at about m, a person at about m, a building at about to m, and Earth's diameter at about m. These are order-of-magnitude estimates, not exact dimensions.
Choose an instrument whose range contains the expected value and whose smallest division can resolve the required change. Explain both range and precision rather than naming an instrument without a reason.
Scalars and vectors
A scalar is fully described by magnitude. A vector also needs direction. Distance and speed are scalars, while displacement, velocity, force, and acceleration are vectors.
The unit does not determine the classification. Speed and velocity can share units, but velocity remains a vector because its direction is part of the quantity.
Physics key quantities, symbols, units, and required definitions
The official list accepts the symbols and units below. A starred quantity carries a definition that learners should be able to state and apply.
| Quantity | Accepted symbols | Accepted units | Definition where required |
|---|---|---|---|
| length | l, h, ... | km, m, cm, mm | |
| area | A | , | |
| volume | V | , | |
| weight* | W | N | gravitational force acting on a body |
| mass | m, M | kg, g, mg | |
| time | t | h, min, s, ms | |
| period* | T | s | time for one complete oscillation or cycle |
| density* | , | mass per unit volume | |
| speed* | u, v | km/h, m/s, cm/s | distance travelled per unit time |
| acceleration* | a | rate of change of velocity | |
| acceleration of free fall | g | , | |
| force* | F, f | N | a push or pull that can change motion or shape |
| moment of force* | - | N m | turning effect equal to force times perpendicular distance from the pivot |
| work done* | W, E | J | energy transferred when a force moves its point of application through a distance |
| energy | E | J, kW h | |
| power* | P | W | rate of energy transfer or work done |
| pressure* | p, P | , | normal force per unit area |
| temperature | , | °C, K | |
| frequency* | f | Hz | number of complete cycles passing a point per unit time |
| wavelength* | m, cm | shortest distance between points at the same phase | |
| focal length | f | m, cm | |
| angle of incidence | i | degree (°) | |
| angle of reflection or refraction | r | degree (°) | |
| potential difference* | V | V, mV | work done per unit charge across a component |
| current* | I | A, mA | rate of flow of charge |
| charge | q, Q | C, A s | |
| e.m.f.* | E | V | energy supplied by a source per unit charge |
| resistance | R | Ω | |
| proton number | Z | - | |
| nucleon number | A | - | |
| half-life | s | ||
| activity of a radioactive source | A | Bq |
Formulae and relationships
| Relationship | Use |
|---|---|
| Convert millimetres to metres. | |
| Convert micrometres to metres. |
Worked examples
Example 1: A sheet is about thick. Express this in metres and choose a suitable instrument.
- Replace milli by .
- Calculate .
- Choose a micrometer because its range and resolution suit a thin sheet.
Answer: The thickness is ; use a micrometer.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Pair every measured value with its unit and convert prefixes before substituting.
- Choose an instrument by range and precision, then check whether the reading is physically sensible.
- Classify a quantity from whether direction is required, not from the unit alone.
Official outcome coverage
K326 CP1: 6 mapped outcomes, references CP1(a), CP1(b), CP1(c), CP1(d), CP1(e), CP1(f). Check the official K326 syllabus.
K327 CP1: 6 mapped outcomes, references CP1(a), CP1(b), CP1(c), CP1(d), CP1(e), CP1(f). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Measure length with a tape, rule, digital caliper or digital micrometer; time with a digital stopwatch; volume with a measuring cylinder; mass with an electronic balance; weight with a spring balance; temperature with a laboratory thermometer; and current or voltage with the correct meter. Select a range that contains the expected value, record to the instrument precision, repeat readings where useful, and identify one limitation that affects the conclusion.
Exam traps and retrieval check
Avoid these traps
- Giving a value without its unit.
- Converting a prefix in the wrong direction.
- Calling velocity a scalar because no direction was printed beside one value.
Check from memory
What completes a numerical measurement?
A unit for the named physical quantity.
Why estimate an order of magnitude?
It helps reject physically unreasonable values and conversion errors.
What extra information does a vector require?
Direction.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Measurement, SI quantities and units, prefixes, scale, instrument selection, and scalar-vector classification.