Edexcel GCSE Physics

## SI units Every physical quantity is measured in a unit, and physics uses the International System (SI) so that a value means the same thing everywhere. Six base units are needed at GCSE, and every other unit in the course is built from them. | Quantity | Unit | Symbol | |---|---|---| | length | metre | m | | mass | kilogram | kg | | time | second | s | | electric current | ampere | A | | temperature | kelvin | K | | amount of substance | mole | mol | Some combinations of base units are used so often that they have their own name. These are the derived units the course uses. | Quantity | Unit | Symbol | In base units or other units | |---|---|---|---| | energy | joule | J | $\mathrm{N\,m}$ | | electric charge | coulomb | C | $\mathrm{A\,s}$ | | potential difference | volt | V | $\mathrm{J/C}$ | | resistance | ohm | $\Omega$ | $\mathrm{V/A}$ | | force | newton | N | $\mathrm{kg\,m/s^2}$ | | pressure | pascal | Pa | $\mathrm{N/m^2}$ | | frequency | hertz | Hz | one wave per second, $\mathrm{s^{-1}}$ | | magnetic flux density | tesla | T | $\mathrm{N/(A\,m)}$ | | power | watt | W | $\mathrm{J/s}$ | Temperature differences are often given in degrees Celsius, and a change of 1 °C is the same size as a change of 1 K. Units such as $\mathrm{m/s}$ and $\mathrm{kg/m^3}$ are compound units, made by dividing one unit by another, and the way they are written tells you how the quantity is built: speed is distance per unit time, and density is mass per unit volume. A calculation gives…