Eduqas A-Level Physics

## SI base units Every physical quantity is measured in a unit that can be built from a small set of base units. Six of them are needed for this course. | Quantity | Base unit | Symbol | |---|---|---| | Mass | kilogram | kg | | Length | metre | m | | Time | second | s | | Electric current | ampere | A | | Amount of substance | mole | mol | | Thermodynamic temperature | kelvin | K | Every other unit is a derived unit, a product or quotient of base units. The newton is the force that gives a mass of 1 kg an acceleration of $1\,\mathrm{m\,s^{-2}}$, so $1\,\mathrm{N} = 1\,\mathrm{kg\,m\,s^{-2}}$. The joule is a newton metre, so $1\,\mathrm{J} = 1\,\mathrm{kg\,m^2\,s^{-2}}$, and the watt is a joule per second, so $1\,\mathrm{W} = 1\,\mathrm{kg\,m^2\,s^{-3}}$. The coulomb is an ampere second, the volt is a joule per coulomb, which gives $1\,\mathrm{V} = 1\,\mathrm{kg\,m^2\,s^{-3}\,A^{-1}}$, and the ohm is a volt per ampere, so $1\,\Omega = 1\,\mathrm{kg\,m^2\,s^{-3}\,A^{-2}}$. ### Prefixes A prefix multiplies the unit by a power of ten. The ones used most are tera (T, $10^{12}$), giga (G, $10^9$), mega (M, $10^6$), kilo (k, $10^3$), centi (c, $10^{-2}$), milli (m, $10^{-3}$), micro ($\mu$, $10^{-6}$), nano (n, $10^{-9}$), pico (p, $10^{-12}$) and femto (f, $10^{-15}$). The prefix belongs to the whole unit that follows it, so $1\,\mathrm{cm^3} = (10^{-2}\,\mathrm{m})^3 = 10^{-6}\,\mathrm{m^3}$, not $10^{-2}\,\mathrm{m^3}$. This is the commonest source of a wrong power of ten when…