O-Level and SEC G3 Physics K323
P5: Pressure
Relate force and area, liquid depth and density, and pressure differences to the physical setup.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Pressure links force to area, while fluid pressure also depends on depth and density. K323 includes hydraulic transmission, density, atmospheric pressure and manometer pressure differences.
Pressure on surfaces and hydraulics
Pressure is normal force per unit area. For the same force, a smaller contact area produces a larger pressure. Convert areas carefully because squaring a length conversion also squares its scale factor.
In a confined fluid, applied pressure is transmitted through the fluid. A hydraulic press uses a larger output piston area to produce a larger output force. The force increases, but energy conservation means the larger piston moves through a smaller distance in an ideal system.
Density and liquid-column pressure
Density is mass per unit volume. Liquid pressure at a depth depends on vertical depth, liquid density and gravitational field strength. It does not depend directly on vessel shape or total liquid volume.
Use pressure difference when comparing two points. At the same depth in the same connected stationary liquid, pressure is the same, provided other applied pressures are accounted for.
Atmospheric pressure and manometers
A barometer balances atmospheric pressure against the pressure from a vertical liquid column. A denser liquid needs a shorter column for the same pressure.
A U-tube manometer uses a height difference to measure pressure difference. Decide which side has higher pressure from the displaced levels, then use the vertical height difference, not the length along the tube.
Formulae and relationships
| Relationship | Use |
|---|---|
| Find pressure on a surface. | |
| Find density. | |
| Find pressure from a liquid column. | |
| Relate ideal hydraulic pistons. |
Worked examples
Example 1: Water of density fills a tank. Find the pressure due to the water at a depth of , taking .
- Use .
- Substitute .
- This is gauge pressure due to the water column, before adding atmospheric pressure if required.
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Separate pressure from force and identify the area over which the force acts.
- Explain liquid pressure through depth and density rather than container shape.
- Use pressure differences to explain barometers, manometers, and hydraulic effects.
Official outcome coverage
K323 P5: 7 mapped outcomes, references P5(a), P5(b), P5(c), P5(d), P5(e), P5(f), P5(g). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Investigate how pressure or force changes with area, depth, or liquid density while controlling the other relevant quantities.
Exam traps and retrieval check
Avoid these traps
- Using centimetres squared as though the numerical conversion were linear.
- Using the sloping length of liquid instead of vertical height.
- Claiming a hydraulic machine creates energy because its output force is larger.
Check from memory
What happens to pressure if area halves at constant force?
Pressure doubles.
Does a wider tank give greater pressure at the same depth?
No, not for the same liquid, gravity and applied surface pressure.
Why can a hydraulic press multiply force?
The same transmitted pressure acts over a larger output area.
Pure versus Combined scope
Combined Physics revisits part of this core under CP3 Force and Pressure, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.