Singapore-Cambridge A-Level Physics (H2)

## Base quantities and SI units A physical quantity is a number multiplied by a unit, so a statement such as a length of $2.5\,\mathrm{m}$ means 2.5 times the metre. Every unit used in physics is built from a small set of **SI base units**, each attached to a base quantity. | Base quantity | Unit | Symbol | |---|---|---| | mass | kilogram | kg | | length | metre | m | | time | second | s | | electric current | ampere | A | | thermodynamic temperature | kelvin | K | | amount of substance | mole | mol | All other quantities are derived, and their units are products or quotients of the base units. A unit that has been given its own name, such as the newton, is shorthand for such a combination. | 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 | $\mathrm{A^2\,s^4\,kg^{-1}\,m^{-2}}$ | | magnetic flux density | tesla, T | $\mathrm{kg\,s^{-2}\,A^{-1}}$ | | frequency | hertz, Hz | $\mathrm{s^{-1}}$ | Each entry follows from a defining equation. Force is mass times acceleration, so $\mathrm{N} = \mathrm{kg\,m\,s^{-2}}$. Energy is force times distance, so…