Physical Quantities & the SI Base Units
Physical Quantities & the SI Base Units
- A measurement must say what was measured, how large it is and which unit was used.
- This lesson starts with that complete statement, then introduces the five SI base quantities required by 9702.
A number and a unit
- A is a measurable property. Its value contains a numerical magnitude (the number that gives its size) and a unit.
- For example, 5.0 m is a length and 5.0 s is a time. The same number describes two different physical quantities because the units are different.
- In a numerical answer, write the unit unless the answer line already supplies it. A number with the wrong unit describes the wrong quantity even when the calculation is correct.
A physical quantity has a numerical magnitude and a unit. Check both before moving to the next line of a calculation.
The SI base units
- Scientists need one common measurement system. SI means the International System of Units. It uses a small set of starting quantities whose units are called .
- Cambridge 9702 requires you to recall these five SI base units:
| Base quantity | Base unit (symbol) |
|---|---|
| mass | kilogram (kg) |
| length | metre (m) |
| time | second (s) |
| electric current | ampere (A) |
| thermodynamic temperature | kelvin (K) |
- Units such as the newton, joule, pascal, volt and coulomb are combinations of base units. They are derived units.
Your turn9702/11/M/J/24 Q1 · 1 mark
Which is an SI base unit: ampere, coulomb, degree Celsius or gram?
Show worked answer
ampere. Coulomb is derived from ampere and second; the SI base units for temperature and mass are kelvin and kilogram.
Your turn9702/22/M/J/24 Q1(a) · 1 mark
Which is not an SI base quantity: charge, current, length or time?
Show worked answer
charge. Electric current is the base quantity; charge is derived because 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.