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What causes gas pressure, how does it depend on temperature and volume, and how does pressure change with depth in a liquid?

Gas pressure as the result of particle collisions, the effect of temperature and volume on gas pressure, pressure in liquids increasing with depth, the pressure due to a column of liquid, and pressure on a surface.

A focused answer to OCR Gateway GCSE Physics A topic P1 on pressure, covering gas pressure as particle collisions, the effect of temperature and volume on gas pressure, why pressure in a liquid increases with depth, the pressure due to a column of liquid, and pressure on a surface.

Generated by Claude Opus 4.89 min answer

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  1. What this topic is asking
  2. Gas pressure and the particle model
  3. Pressure and volume
  4. Pressure in liquids
  5. Pressure on a surface
  6. Try this

What this topic is asking

OCR wants you to explain gas pressure using the particle model, describe how temperature and volume change gas pressure, explain why pressure in a liquid increases with depth, and use the equation for the pressure due to a column of liquid. This is topic P1.3 of the OCR Gateway Physics A (J249) specification.

Gas pressure and the particle model

This collision picture explains how gas pressure responds to changes:

  • Temperature (at constant volume). Heating the gas gives the particles more kinetic energy, so they move faster, hit the walls more often and with more force each time. The pressure therefore increases with temperature.
  • Volume (at constant temperature). Squeezing a fixed mass of gas into a smaller volume means the same number of particles hit a smaller wall area more frequently, so the pressure increases. Letting it expand reduces the pressure. This is the inverse relationship between pressure and volume.

So pressure rises if you heat a gas in a sealed rigid container, or if you compress it at constant temperature.

Pressure and volume

A worked feel for this: pushing in a sealed syringe with your finger over the end is hard, because compressing the trapped gas raises its pressure until it pushes back on your hand. Releasing the plunger lets the gas expand and the pressure fall back.

Pressure in liquids

This is why a dam is built thicker at the bottom than at the top, why your ears hurt as you dive deeper, and why a submarine needs a strong hull. Note the pressure depends only on the depth and the density of the liquid, not on the shape or total amount of liquid, so a tall thin tube and a wide tank give the same pressure at the same depth.

Pressure on a surface

For a solid pushing on a surface, or for the force exerted by a fluid on an area, pressure is the force per unit area:

p=FAp = \frac{F}{A}

where pp is the pressure in pascals (Pa\text{Pa}), FF is the force in newtons and AA is the area in square metres. A large force spread over a small area gives a high pressure, which is why a sharp knife or a drawing pin (small area) cuts or pierces easily, while snowshoes (large area) reduce the pressure so you do not sink.

Try this

Q1. Calculate the pressure when a force of 40 N40\,\text{N} acts on an area of 0.20 m20.20\,\text{m}^2. [2 marks]

  • Cue. p=FA=400.20=200 Pap = \dfrac{F}{A} = \dfrac{40}{0.20} = 200\,\text{Pa}.

Q2. Explain why the pressure of a fixed mass of gas increases when it is squeezed into a smaller volume at constant temperature. [2 marks]

  • Cue. The same number of particles hit a smaller wall area more often, so the force per unit area, and therefore the pressure, increases.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20183 marksExplain, in terms of the particles, why the pressure of a fixed mass of gas in a sealed rigid container increases when the gas is heated.
Show worked answer →

A P1 Explain question worth three marks. Heating the gas gives the particles more kinetic energy, so they move faster (1 mark). The faster particles collide with the walls of the container more often and with more force each time (1 mark). Since pressure is caused by these collisions and the volume is fixed, more frequent and harder collisions mean a greater force on each unit area of the walls, so the pressure increases (1 mark). Markers reward faster particles, more frequent and harder collisions, and the link to increased pressure at constant volume. A common error is to say the particles get bigger or that there are more of them.

OCR 20214 marksA diver descends in seawater of density 1030 kg/m31030\,\text{kg/m}^3 to a depth of 20 m20\,\text{m}. Using g=10 N/kgg = 10\,\text{N/kg}, calculate the pressure due to the seawater at that depth, and explain why a submarine must be strongly built.
Show worked answer →

A P1 Calculate question using the given equation p=ρghp = \rho g h. Substitute: p=1030×10×20p = 1030 \times 10 \times 20 (1 mark for selecting the equation and substituting). This gives p=206 000 Pap = 206\,000\,\text{Pa}, about 2.1×105 Pa2.1 \times 10^{5}\,\text{Pa} (2 marks for the calculation and units). The explanation: pressure in a liquid increases with depth because there is a greater weight of water above pushing down, so at 20 m20\,\text{m} the water exerts a large pressure on the submarine's hull from all sides, and it must be strongly built to withstand this without being crushed (1 mark). Markers reward the substitution, the answer in pascals, and the link from increasing depth to greater pressure.

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