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Key quantities, symbols and units

Syllabus
9702–2028–2029
Topic
—
Level
A2

Learning objectives

—Q.1-Base quantities, symbols and SI unitsUse the syllabus conventions for base quantities and common physical quantities, including usual symbols and units for mechanics, waves, electricity, fields, thermal physics, quantum/nuclear physics, medical physics and astronomy.—Q.2-Quantity notation and unit interpretation in answersInterpret and write physical quantities with appropriate symbols, prefixes, derived units and dimensions, and distinguish quantities that use similar symbols in different contexts.

Build every derived unit from the equation that defines the quantity

SI base quantities used in Physics: length metre (m), mass kilogram (kg), time second (s), electric current ampere (A), thermodynamic temperature kelvin (K), amount mole (mol), luminous intensity candela (cd).

velocity:ms−1acceleration:ms−2force:N=kgms−2energy:J=Nm=kgm2s−2power:W=Js−1velocity: m s⁻¹ acceleration: m s⁻² force: N=kg m s⁻² energy: J=N m=kg m² s⁻² power: W=J s⁻¹

charge:C=Aspotentialdifference:V=JC−1resistance:Ω=VA−1capacitance:F=CV−1magneticflux:Wb=Vsfluxdensity:T=Wbm−2charge: C=A s potential difference: V=J C⁻¹ resistance: Ω=V A⁻¹ capacitance: F=C V⁻¹ magnetic flux: Wb=V s flux density: T=Wb m⁻²

Other common coherent units follow their definitions: pressure Pa=N m⁻², frequency Hz=s⁻¹, field strength N C⁻¹ or V m⁻¹, activity Bq=s⁻¹, dose Gy=J kg⁻¹. Angular quantities use rad where clarity needs it.

When unsure, start from the defining equation, replace every quantity by its unit, then cancel and collect powers. For the ohm, R=V/I, so Ω=V A⁻¹.

A named derived unit is not a base unit. Quantity symbols are italic algebraic labels such as F or V; unit symbols such as N or V are upright and case-sensitive.

Audit every final answer for quantity, scale, unit algebra and context

A complete measured quantity has numerical value × prefix scale × unit, with a named physical meaning. Write 3.71 N kg⁻¹, not just 3.71 or 'force'.

1km=103m;1ms=10−3s;1μF=10−6F(1cm)2=10−4m2;(1cm)3=10−6m31 km=10³ m; 1 ms=10⁻³ s; 1 μF=10⁻⁶ F (1 cm)²=10⁻⁴ m²; (1 cm)³=10⁻⁶ m³

Use a space or centred dot for multiplication and negative powers or one solidus for division: m s⁻², N m, W m⁻². Preserve every denominator factor when rearranging or converting.

Read symbols from their local definition: V may denote potential difference or volume; I may denote current or intensity; E may denote energy, electric field or Young modulus. Subscripts distinguish related quantities, for example V₀ and Vout.

Unit symbols are case-sensitive, have no plural and normally no full stop: 5 kg, 20 N, 3.0 MHz. Leave a space between value and unit; keep °C for temperature differences/context but convert to K where an absolute thermodynamic temperature is required.

Before submitting, ask: does the unit match the requested quantity, are all prefixes converted, do exponents apply to units, and is the magnitude physically plausible? Equivalent units such as m s⁻² and N kg⁻¹ can describe different but dimensionally related meanings.