Irish Leaving Certificate Physics

## Fundamental and derived units Every physical quantity is a number multiplied by a unit, and a number with no unit tells you nothing. Physics builds its units from a small set of fundamental (base) units in the International System, known as SI. The ones used in this course are the metre (m) for length, the kilogram (kg) for mass, the second (s) for time, the ampere (A) for electric current and the kelvin (K) for temperature. The mole and the candela complete the set of seven but rarely appear here. All other units are derived: they are combinations of the fundamental ones, found by following the definition of the quantity. Speed is distance over time, so its unit is $\mathrm{m\,s^{-1}}$. Acceleration is change of speed over time, so its unit is $\mathrm{m\,s^{-2}}$. Force is mass times acceleration, so one newton is $1\,\mathrm{kg\,m\,s^{-2}}$. Several derived units have their own names because the combination is used so often. | Quantity | Unit | Symbol | 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 | $\Omega$ | $\mathrm{kg\,m^{2}\,s^{-3}\,A^{-2}}$ | | Frequency | hertz | Hz | $\mathrm{s^{-1}}$ | Temperature is usually measured…