Physical quantity
Physical Quantities & Units - OCR A-Level Physics
Key Definition
Physical quantity
A property of a material or system that can be measured. Every physical quantity has two parts: a numerical magnitude and a unit. You cannot write a quantity without a unit; "$5$" on its own is not a physical quantity but "$5 \text{ m}$" is.
A property of a material or system that can be measured. Every physical quantity has two parts: a numerical magnitude and a unit. You cannot write a quantity without a unit; "$5$" on its own is not a physical quantity but "$5 \text{ m}$" is.
- Magnitude tells you "how much"; the unit tells you "of what". Example: $m = 70 \text{ kg}$ has magnitude $70$ and unit $\text{kg}$.
- All physical quantities split into two families: base quantitiesThe seven fundamental physical quantities defined by the SI system: mass, length, time, current, temperature, amount of substance, luminous intensity. (defined directly by international agreement) and derived quantitiesPhysical quantities defined in terms of base quantities through equations. Example: speed = distance / time, so its unit is $\text{m s}^{-1}$. (built from base quantities through equations).
- Every measurement in physics can be traced back to combinations of base units.
SI base units
The seven fundamental units from which every other unit is built. They are defined by international agreement and are independent of each other. OCR examines you on the first six (luminous intensity is not assessed at A-level).
Diagram pending
A 2-column table titled "The seven SI base units". Left column: physical quantity (mass, length, time, electric current, temperature, amount of substance, luminous intensity). Right column: unit and symbol (kilogram kg, metre m, second s, ampere A, kelvin K, mole mol, candela cd). The bottom row (candela) is shaded grey with the note "not assessed at OCR A-level".
Will be replaced with a styled SVG table in stream 2.
- Mass: kilogram ($\text{kg}$)
- Length: metre ($\text{m}$)
- Time: second ($\text{s}$)
- Electric current: ampereThe SI unit of current. One ampere is a flow of one coulomb of charge per second. ($\text{A}$)
- TemperatureA measure of the average kinetic energy of the particles in a substance. An SI base quantity measured in kelvin.: kelvin ($\text{K}$)
- Amount of substanceA measure of the number of entities (atoms, molecules, ions) in a sample. SI unit: mole.: mole ($\text{mol}$)
- Luminous intensity: candela ($\text{cd}$) — listed for completeness, not examined at OCR A-level.
Common Mistake
MEDIUM
Wrong: Writing the base unit of mass as gram ($\text{g}$) and then forgetting to convert to $\text{kg}$ inside $F = ma$, giving a force $1000\times$ too small.
Right: The SI base unit of mass is the kilogram, not the gram. Convert every mass to $\text{kg}$ before substituting into a physics equation. The kilogram is the only base unit that already contains a prefix in its name.
Right: The SI base unit of mass is the kilogram, not the gram. Convert every mass to $\text{kg}$ before substituting into a physics equation. The kilogram is the only base unit that already contains a prefix in its name.
Examiner Tips and Tricks
- Learn the six examinable base quantities and units off by heart: a one-mark "state" question appears most years.
- If asked to "name three base quantities", do not list joule, newton or volt: these are derived units.
- The kilogram is the only SI base unit with a prefix (kilo) built into its name. When converting, remember that the base unit is $\text{kg}$, not $\text{g}$.