CCEA A-Level Physics

## Magnitude and unit A **physical quantity** is something that can be measured, and every one is written as a number multiplied by a unit. "5" on its own says nothing, whereas 5 m, 5 s and 5 kg are three different statements. The number is the **magnitude** and the unit says what was being counted. A quantity keeps the same meaning whichever unit it is written in, so a length of 2.5 m is also 250 cm or 0.0025 km, and only the number changes with the unit. ## Base units All measurements in physics can be built from a small set of independent **base units** (the SI system). Six of them are needed in this course. | Quantity | Base unit | Symbol | |---|---|---| | mass | kilogram | kg | | length | metre | m | | time | second | s | | electric current | ampere | A | | temperature | kelvin | K | | amount of substance | mole | mol | Every other unit is a **derived unit**, meaning a combination of base units found from the equation that defines the quantity. The newton, for example, comes from $F = ma$, which has units of mass times acceleration: $$\mathrm{N} = \mathrm{kg} \times \mathrm{m\,s^{-2}} = \mathrm{kg\,m\,s^{-2}}$$ Other derived units follow in the same way. | Quantity | Defining equation | Unit in base units | |---|---|---| | velocity | $v = s/t$ | $\mathrm{m\,s^{-1}}$ | | acceleration | $a = \Delta v/t$ | $\mathrm{m\,s^{-2}}$ | | force (newton, N) | $F = ma$ | $\mathrm{kg\,m\,s^{-2}}$ | | energy (joule, J) | $W = Fs$ | $\mathrm{kg\,m^2\,s^{-2}}$ | | power (watt, W) | $P…