This topic teaches you to reason about an experiment from the information given. You work out what was changed, what was measured, what the graph shows and how far the evidence can be trusted. It supports almost every other topic in IGCSE Physics, because any topic can be tested through data and explanation. For the current specification, check the Cambridge IGCSE Physics page for code 0625.
What do you need to know first?
You should be comfortable with basic equations such as speed = distance ÷ time and V = I × R, with rearranging formulae, and with reading values off a scale. Knowing what a straight-line graph y = mx + c means is especially useful.
A worked example to orient you
Invented data. A student hangs loads on a different spring and measures its extension.
| Load F / N | 1.0 | 2.0 | 3.0 | 4.0 |
|---|---|---|---|---|
| Extension x / cm | 5.0 | 10.1 | 14.9 | 20.0 |
Variables: the independent variable is the load, the dependent variable is the extension, and the same spring and ruler are controlled.
Axes: with F = kx, plot F on the y-axis against x on the x-axis so the gradient is k.
Gradient: the line passes through the origin and (20.0 cm, 4.0 N). Gradient = 4.0 N ÷ 20.0 cm = 0.20 N/cm = 20 N/m (0.20 N ÷ 0.01 m).
Limitation: the readings scatter slightly (10.1 instead of 10.0, 14.9 instead of 15.0), which suggests random reading error.
Claim with evidence: The extension is directly proportional to the load. Doubling the load from 2.0 N to 4.0 N doubles the extension from 10.1 cm to 20.0 cm, about a factor of 2.
That single example touches all five lessons.
Which order should you study the lessons in?
- Identify independent, dependent and controlled variables: every later step depends on knowing what was changed and what was measured.
- Choose graph axes from a model: turns an equation into a straight line so the data can test it.
- Interpret gradient with units: extracts a physical quantity from the line and states its unit.
- Explain systematic and random limitations from supplied evidence: judges how far the gradient and readings can be trusted.
- Develop a concise claim-evidence-reasoning answer: brings the evidence together in a short written answer.
What are the common traps?
- Swapping the independent and dependent variables because “time” sounds like the thing you change.
- Plotting an inverse relationship directly and drawing a line through a curve.
- Quoting a gradient without its unit, or without converting cm to m.
- Writing “human error” instead of a specific cause and effect.
- Stating a conclusion with no number from the data to support it.
How should you use the practice set?
Try the mixed practice set after the lessons, not before. Write full answers with units on paper, open the working and mark the step where you first differed. The routing table at the end of the set sends each type of mistake back to the right lesson.
Some students can do each lesson separately but lose the thread when a full question chains them together. In online one-to-one Physics tuition, your assigned teacher can walk through that whole chain with you on new experiments.