Cambridge IGCSE Physics Notes 1.8: Pressure
Cambridge IGCSE Physics pressure notes on force per area, liquid pressure, atmospheric pressure, depth, density, and pressure differences.
Q: What does Cambridge IGCSE Physics Notes 1.8: Pressure cover?
A: It covers pressure = force / area for Core candidates and liquid pressure = density x gravitational field strength x depth for Supplement candidates, with applications to pressure differences.
Pressure describes how concentrated a force is over area, while liquid pressure increases with depth and liquid density.
The ideas that organise this section
- For the same force, a smaller contact area produces greater pressure.
- At the same depth, a denser liquid produces greater pressure.
- A pressure difference can produce a resultant force and fluid movement.
Core route
Core candidates should be ready to describe, calculate, interpret, and apply the following:
- Calculate pressure from normal force and area.
- Describe qualitatively how liquid pressure varies with depth and density.
- Explain familiar effects of atmospheric pressure.
Supplement route
Extended candidates study all Core content and add the following depth:
- Calculate the pressure due to a liquid column.
- Apply pressure differences to manometers, barometers, or related contexts when data are supplied.
Formula route
- Relationship: pressure = force / area.
- Relationship: liquid pressure = density x gravitational field strength x depth.
Write the relationship before substituting. Convert units first, keep extra figures during working, and round only the final answer to sensible precision.
Build a connected model
In Pressure, a strong answer connects the named quantity or model to observable evidence. Begin by defining the physical quantity in words or with its relationship. Identify which values are scalars and which require a direction. Represent the situation with a labelled diagram, graph, field pattern, ray or circuit when that makes the relationship visible. The representation is part of the reasoning: its labels, arrows and scale should agree with the written explanation.
For a calculation, write the governing relationship before substitution, convert prefixes and time units, keep unrounded values during working and attach the correct final unit. For a qualitative question, use a cause-link-consequence chain. Name what changes, state the physical mechanism that links the change to the system, and then give the measurable result. If two cases are compared, hold unrelated variables constant and use the same physical principle for both.
Check the answer against limiting cases. A zero input, a doubled input or a reversed direction often exposes an incorrect proportionality or sign. Also check whether the value is plausible for the apparatus or context. This final check is especially important when a calculator gives a precise-looking result from an incorrect unit conversion.
Worked reasoning example
A sharp knife cuts more easily because the same force acts over a smaller area, producing greater pressure at the edge.
For explanation questions, state the physical principle, apply it to the named system, then give the consequence. A formula or keyword by itself is not a complete explanation.
Extend the example by identifying the evidence that would distinguish the correct model from a tempting alternative. State what would be measured or observed, which variable must remain controlled, and how the conclusion follows from the result. This turns a numerical or descriptive answer into a testable physical argument. When the question asks for an explanation rather than a calculation, preserve the same chain but express it as principle, application and consequence.
A reliable answer method
- Identify the quantity, law, graph feature, or physical model being tested.
- Mark a positive direction or label the diagram when direction matters.
- Write the equation or principle before inserting data.
- Keep units consistent and show the main substitution.
- Check whether the magnitude, direction, trend, and unit are physically reasonable.
Common mistakes
- Using total surface area when only the contact area matters.
- Confusing depth below the surface with total container height.
- Claiming liquid pressure depends on container shape.
Assessment guidance
Questions on Pressure can mix recall, calculation, graph or diagram interpretation and unfamiliar application. Read the command word first. A definition needs the precise physical meaning; a description reports a pattern; an explanation supplies the mechanism; and an evaluation weighs evidence or method quality. Show equations and substitutions so method marks remain visible. Label every diagram and state directions where relevant. In multi-step problems, carry forward unrounded values and make the final answer's unit and significant figures consistent with the data. Never rely on a memorised keyword when the question asks how or why a result occurs.
Retrieval practice
Without notes, rebuild the Core and Supplement lists for Pressure. Define every named quantity, reproduce the principal relationship or representation and explain one everyday application. Then solve one direct calculation and one unfamiliar context, checking units and limiting cases. Finish by writing two misconception corrections and one practical measurement that could test the topic's main relationship.
Theory and practical stay separate
Use the mechanics practical post for contact-area comparisons and liquid-column measurements, including safe handling and repeat strategy.
Official source
Cambridge International, Physics 0625 syllabus for examinations in 2026, 2027 and 2028. Cambridge states that Physics 0972 is graded from 9 to 1 but otherwise follows the same subject content as 0625.
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