IGCSE Physics / Practical and maths skills
Plotting a graph and drawing a line of best fit revision notes
Edexcel International GCSE Physics (4PH1), chapter 29: Practical and maths skills, specification point M4C, M4A, M2H. Free lesson.
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A well-plotted graph uses suitable axes and a clear scale, accurate points, and one line of best fit. These choices help you show the pattern in results and decide whether two quantities are directly proportional.
Choose and label the axes
Put the variable you changed along the bottom, and the variable you measured up the side. Label each axis with the quantity and its unit, so the reader can tell what the graph shows. For example, write force / N on an axis that shows force in newtons.
Choose a clear scale and plot points
A suitable scale uses more than half the grid in each direction. This spreads out the points, making them easier to plot and read accurately. Use simple steps between the grid lines. Use steps of 1, 2 or 5 per square, not steps of 3. Plot each reading as a small, neat cross, accurate to within half a small square.
Draw one line of best fit
A line of best fit shows the overall trend in the readings. Real measurements have small errors, so the line does not need to pass through every point. Choose a ruled straight line or one smooth curve, with the points balanced on either side. An anomaly is a result that does not fit the pattern. Ignore it when drawing the line. Do not join each point to the next. That copies the small errors instead of showing the trend.
Describe the pattern and proportionality
Describe what happens to the measured quantity as the changed quantity increases. For spring results, extension increases steadily as force increases. This describes a positive relationship, but it does not by itself show that the quantities are directly proportional. Directly proportional means that doubling one quantity doubles the other. Its graph is a straight line through the origin, where both quantities are zero. A straight line that does not pass through the origin is linear, but not directly proportional. If both quantities increase but the graph does not meet both conditions,…
Check whether a scale is suitable
If the time axis is labelled 0, 3, 6, 9, 12, with one number per large square, a value such as 7 is difficult to plot and read accurately. Choose a scale that makes intermediate values easier to place. Use steps of 1, 2 or 5 per square, and make the plotted results cover more than half the grid.