Revise SI units for Cambridge 9702 (1.2)

A physical quantity is any characteristic of matter or energy that can be measured. Whenever you measure something, whether it's the length of a table or the temperature of a liquid, your answer must always include two parts: a numerical value (how much) and a unit (what kind of measurement). For example, saying '5' isn't helpful, but '5 metres' (5 m) tells you exactly what has been measured and its magnitude.

The SI system is built upon a foundation of seven fundamental Base Quantities, each with its own independent Base Unit. These units cannot be broken down further into simpler units and form the core from which all other units are derived. You must memorise these for your exams.

While base units are fundamental, most quantities we encounter in physics use derived units. These are units created by combining two or more base units through multiplication or division. For example, to find speed, you divide distance (length) by time, so its derived unit is metres per second (m s⁻¹).

Understanding derived units is crucial for dimensional analysis. This is the process of checking if an equation is physically plausible by comparing the units on both sides. An equation is said to be dimensionally homogeneous (or consistent) if the base units on both sides of the equation are identical. This is a powerful tool to spot potential errors in your formulae!

Physical quantities can range from incredibly tiny to astronomically huge. To make these numbers easier to write and understand, we use SI prefixes. These prefixes attach to the start of a unit and indicate a specific power of 10 multiplier.

While the Joule (J) is the SI derived unit for energy, you'll often encounter other energy units, especially in specific contexts like atomic physics or domestic electricity. Knowing how to convert them to Joules is essential for consistency in calculations.

1 electronvolt (eV) = 1.60 × 10⁻¹⁹ J 1 kilowatt-hour (kWh) = 3.6 × 10⁶ J