Edexcel A-Level Physics

## Base and derived quantities Every physical quantity is a number multiplied by a unit, and the whole of physics can be written in terms of a small set of **base quantities**, each measured in an **SI base unit**. A base quantity is not defined in terms of any other quantity. The ones used in A-level physics are in the table. | Base quantity | Symbol | SI base unit | Unit symbol | |---|---|---|---| | Mass | $m$ | kilogram | kg | | Length | $l$ | metre | m | | Time | $t$ | second | s | | Electric current | $I$ | ampere | A | | Thermodynamic temperature | $T$ | kelvin | K | | Amount of substance | $n$ | mole | mol | A **derived quantity** is defined by an equation that links it to base quantities, and its unit is the matching combination of base units. Speed is length divided by time, so its unit is $\mathrm{m\,s^{-1}}$. Many derived units have their own names, and it is worth being able to unpack each into base units because that is how equations are checked. | Quantity | Named unit | In base units | |---|---|---| | Force | newton, N | $\mathrm{kg\,m\,s^{-2}}$ | | Energy, work | joule, J | $\mathrm{kg\,m^2\,s^{-2}}$ | | Power | watt, W | $\mathrm{kg\,m^2\,s^{-3}}$ | | Pressure | pascal, Pa | $\mathrm{kg\,m^{-1}\,s^{-2}}$ | | Charge | coulomb, C | $\mathrm{A\,s}$ | | Potential difference | volt, V | $\mathrm{kg\,m^2\,s^{-3}\,A^{-1}}$ | | Resistance | ohm, $\Omega$ | $\mathrm{kg\,m^2\,s^{-3}\,A^{-2}}$ | | Capacitance | farad, F | $\mathrm{kg^{-1}\,m^{-2}\,s^4\,A^2}$ | |…