Axis titles & column headings
Physical Quantities & Units - OCR A-Level Physics
Key Convention
Axis title and column heading format
In a physics graph or data table, the quantity and its unit are separated by a forward slash ($/$). Any prefix or power of ten goes after the slash and before the unit. The label on the axis or column then represents a pure number, so the numerical entries underneath can be written without unit symbols.
In a physics graph or data table, the quantity and its unit are separated by a forward slash ($/$). Any prefix or power of ten goes after the slash and before the unit. The label on the axis or column then represents a pure number, so the numerical entries underneath can be written without unit symbols.
$$\text{quantity symbol} \;/\; (\text{prefix or power of ten})(\text{unit})$$
Diagram pending
A small line graph with the $x$-axis labelled "$t \;/\; \times 10^{-3} \text{ s}$" and the $y$-axis labelled "$v \;/\; \text{m s}^{-1}$". A best-fit straight line through five data points labelled with numbers only (no units), illustrating that the axis title carries the unit so the numbers do not.
Will be replaced with a styled SVG axis label in stream 2.
- Velocity in $\text{m s}^{-1}$: write the axis as "$v \;/\; \text{m s}^{-1}$". The numbers underneath are pure (for example $1.5$, $3.0$, $4.5$).
- Distance in millimetres: "$s \;/\; \text{mm}$". A value of $25$ in the column means $25 \text{ mm}$, not $25 \text{ m}$.
- Time in milliseconds (using a power of ten instead of a prefix): "$t \;/\; \times 10^{-3} \text{ s}$". A value of $4$ in the column means $4 \times 10^{-3} \text{ s} = 4 \text{ ms}$.
- Temperature in degrees Celsius: "$\theta \;/\; ^\circ\text{C}$". Numbers in the column are pure.
- Energy in joules: "$E \;/\; \text{J}$".
- Useful when plotting graphs because the axis label and the cell value combine to give a quantity in its full units.
Common Mistake
HIGH
Wrong: Writing each data point with its unit ("$25 \text{ mm}$", "$30 \text{ mm}$") inside the column, or writing "$s \, (\text{mm})$" instead of "$s \;/\; \text{mm}$" on the axis.
Right: Move the unit to the heading after a forward slash: "$s \;/\; \text{mm}$". The numbers underneath are then pure. The forward-slash convention is the only format OCR's mark scheme accepts on plotted graphs.
Right: Move the unit to the heading after a forward slash: "$s \;/\; \text{mm}$". The numbers underneath are then pure. The forward-slash convention is the only format OCR's mark scheme accepts on plotted graphs.
Examiner Tips and Tricks
- "Quantity / unit" reads as "quantity divided by unit". A current $I = 0.20 \text{ A}$ gives $I / \text{A} = 0.20$, which is a pure number, exactly what gets plotted.
- When the numbers are inconveniently large or small, use a power-of-ten label: "$t \;/\; \times 10^{-3} \text{ s}$" or "$E \;/\; \times 10^{6} \text{ J}$". This keeps the plotted numbers between about $0.1$ and $1000$.
- Apply the same rule to table column headings: "$\theta \;/\; ^\circ\text{C}$" at the top, raw numbers in the cells underneath.