Physical Quantities & the SI Base Units
Physical Quantities & the SI Base Units
- Every measurement in physics is a number together with a unit.
- This page covers what a physical quantity is, and the small set of base units that every other unit is built from.
A number and a unit
- A is anything you can measure. It is always written as a numerical magnitude (the size) times a unit.
- “5.0” on its own means nothing. “5.0 m” is a length. “5.0 s” is a time. The unit gives the number its meaning.
- Always write the unit after every answer. Mark schemes give a mark for the correct unit, and an answer with a missing or wrong unit loses that mark.
A physical quantity = a number × a unit. An answer without a unit is not finished.
The SI base units
- The SI system chooses a few quantities as its starting point. Their units are the .
- There are seven base units in total. 9702 asks you to recall five of them:
| Base quantity | Base unit (symbol) |
|---|---|
| mass | kilogram (kg) |
| length | metre (m) |
| time | second (s) |
| electric current | ampere (A) |
| thermodynamic temperature | kelvin (K) |
- The other two base units are the mole (amount of substance) and the candela (luminous intensity). They exist, but 9702 does not ask you to recall them in this list.
- Every other unit (newton, joule, pascal, volt, coulomb) is built from base units. The next page shows how.
Your turn— tap to reveal the worked answer (9702/11/M/J/24 Q1)
Which unit is an SI base unit: ampere, coulomb, degree Celsius or gram? (9702/11/M/J/24 Q1)
Answer: the ampere. The coulomb is built from the ampere and the second. The base unit of temperature is the kelvin, and the base unit of mass is the kilogram.
Your turn— tap to reveal the worked answer (9702/22/M/J/24 Q1(a))
From this list, pick the quantity that is not an SI base quantity: charge, current, length, time. (9702/22/M/J/24 Q1(a))
Answer: charge. Current is the base quantity; charge is derived from it (charge = current × time).
Common mistake
The base unit of mass is the kilogram, not the gram. The base unit of temperature is the kelvin, not the degree Celsius. The coulomb and the newton look basic, but both are derived.