(d) plotting and interpreting suitable graphs from experimental results,…

The idea

Plotting and interpreting suitable graphs is essential for analysing experimental results. A graph has the independent variable on the x-axis and dependent variable on the y-axis, with axes labelled with quantities and units. The scale must allow data to occupy at least half of each axis, with points plotted accurately. A line or curve of best fit shows the trend. Interpreting involves identifying the shape, determining gradients and intercepts, and relating these to physical quantities.

For example: A graph of force against extension for a spring yields a straight line; the gradient gives the spring constant $k$.

What earns the marks

• Identify the independent variable (x-axis) and dependent variable (y-axis) from the experimental context.
• Label axes with the correct quantity and unit, e.g. 'Time / s' or 'Current / A'.
• Choose a scale that uses at least half of the available axis length and allows all data points to be plotted.
• Plot points accurately and draw a line or curve of best fit.
• Interpret the graph by determining the gradient and intercept, and relate these to the physical relationship.

Common mistakes and how to put them right

• Misunderstanding: the independent variable should be on the y-axis.

  Put it right: The independent variable is conventionally plotted on the x-axis.

• Misunderstanding: axes need only be labelled with the quantity name, not the unit.

  Put it right: Axes must be labelled with both quantity and unit, e.g. 'Force / N'.

• Misunderstanding: the scale should start at zero always.

  Put it right: The scale should be chosen to make best use of the graph paper; it need not start at zero if the data range does not include zero.

• Misunderstanding: a line of best fit must pass through every data point.

  Put it right: A line of best fit should be a smooth line or curve that shows the overall trend, not necessarily passing through all points.

Exam tips

• Check that the axes are labelled with the correct quantity and unit before considering the shape of the graph.
• Ensure the scale is linear and easy to read; avoid awkward scales such as 3 units per major division.
• When interpreting, consider what the gradient and intercept represent in terms of the physical quantities.
• Look for whether the graph should pass through the origin based on the theoretical relationship.