SI units and measurement · Year 12
Scientists everywhere use the same system of units, the SI system, so that results can be shared and checked. Measuring well means choosing the right instrument, reading it carefully, and writing the answer with a sensible unit and number of figures.
Key facts
- Base SI units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd).
- Derived units come from formulae: speed m/s, force N = kg m/s², energy J = N m, power W = J/s, pressure Pa = N/m².
- Prefixes: G = 10⁹, M = 10⁶, k = 10³, c = 10⁻², m = 10⁻³, µ = 10⁻⁶, n = 10⁻⁹.
- Standard form: a number between 1 and 10 × a power of 10, e.g. 0.00042 = 4.2 × 10^(−4).
- The resolution of an instrument is the smallest change it can show (1 mm on a ruler).
- Round answers to a sensible number of significant figures, usually the same as the least precise data.