OCR GCSE Physics A: Gas pressure, compression and the atmosphere (J249) – Revision Notes

Condensed recall for the final weeks. For full explanations use the study guide. These notes follow the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), statements P1.3a to P1.3g. P1.3a and P1.3b are in both tiers. P1.3c to P1.3g appear in bold type: Higher Tier only.

Also useful: the practice questions, the Matter revision notes, the OCR GCSE Physics hub, the printable checklist and the free diagnostics.

Key ideas at a glance

Statement	Tier	One-line summary
P1.3a	Both	Gas pressure comes from particle collisions with the walls. Faster particles mean more frequent, harder collisions. Closed systems only.
P1.3b	Both	Constant volume: temperature up, pressure up (qualitative only).
P1.3c	Higher Tier only	Pressure changes compress or expand gases. The pressure acts as a net force normal to a surface.
P1.3d	Higher Tier only	Constant temperature: bigger volume, lower pressure. pressure × volume = constant.
P1.3e	Higher Tier only	Work done on a gas can raise its temperature (bicycle pump).
P1.3f	Higher Tier only	Atmosphere: a layer of air held by gravity; simple model uses uniform density.
P1.3g	Higher Tier only	Atmospheric pressure falls with height: less air above, and thinner air.

P1.3a and P1.3b: Temperature, motion and pressure (both tiers)

Definition. Gas pressure is the force per unit area on a surface caused by particles colliding with it.

Method in steps for any “explain the pressure change” answer:

1. Say what happens to the particles’ speed (faster if hotter, slower if cooler).
2. Say the collisions with the walls are more frequent or less frequent.
3. Say each collision is harder or gentler.
4. Link to the total force on the walls, then to pressure.
5. State that the volume and number of particles are fixed (closed system).

Must-know distinctions

• Temperature reflects the average kinetic energy of the particles, not their size.
• Pressure comes from collisions with the walls, not between particles.
• At constant volume the pressure and temperature change in the same direction.
• The specification wants the relationship qualitatively only: no proportionality equation is needed.

Practicals named in the specification: heated and frozen balloon, manometers on heated or cooled gas, the exploding can, film canister rockets.

P1.3c (Higher Tier only): Compression and force direction

• Gases compress easily because the particles are far apart. Liquids and solids hardly compress.
• Pressure acts at right angles to a surface. Sideways parts of the particle bounces cancel, leaving a normal force.
• Net force = (difference in pressure on the two sides) × area. Use pressure (Pa) = force normal to surface (N) ÷ area (m²).
• Collapsing can: inside pressure falls after steam condenses, so the greater outside pressure pushes the can inwards.

Worked reminder. A flat panel of area 0.25 m² has 8.0 × 10⁴ Pa more pressure on one side than the other. Net force = 8.0 × 10⁴ × 0.25 = 2.0 × 10⁴ N, pushing from the high-pressure side towards the low-pressure side and at right angles to the panel.

P1.3d (Higher Tier only): Volume and pressure

Explanation. Larger volume at constant temperature means the particles travel further between wall collisions. The collisions happen less often. Same speed, so the same force per collision. Total force per unit area falls. Pressure falls.

Equation. With a fixed mass of gas at one temperature:

pressure (Pa) × volume (m³) = constant, so p₁V₁ = p₂V₂

Quantity	Rearrangement
New pressure	p₂ = p₁V₁ ÷ V₂
New volume	V₂ = p₁V₁ ÷ p₂

Method in steps

1. Write p₁, V₁, p₂, V₂ and mark the unknown.
2. Check that the temperature and amount of gas are constant.
3. Match the units on both sides (both in Pa or both in kPa; both volumes the same unit).
4. Substitute into p₁V₁ = p₂V₂ and rearrange.
5. Give the answer with a unit, and check the direction: volume up means pressure down.

Worked reminder. A gas at 120 kPa and 0.50 m³ is compressed to 0.20 m³ at constant temperature. p₂ = 120 × 0.50 ÷ 0.20 = 300 kPa.

Second reminder. A gas at 2.0 × 10⁵ Pa and 0.0030 m³ is released into a larger volume at 1.0 × 10⁵ Pa. V₂ = 2.0 × 10⁵ × 0.0030 ÷ 1.0 × 10⁵ = 0.0060 m³.

P1.3e (Higher Tier only): Work done on a gas

• Doing work on a gas transfers energy to it. Bicycle pump example: the piston moves in, particles rebound from it faster.
• Average kinetic energy rises, so the temperature rises.
• Work done = force × the distance moved along the line of the force.
• pV = constant needs constant temperature. A quick compression warms the gas, so the pressure rises by more than pV = constant suggests.
• In practice, friction also warms a pump. State the compression as the main cause.

P1.3f and P1.3g (Higher Tier only): The atmosphere

Simple model (P1.3f). A layer of air held around the Earth by gravity. Assume uniform density. Knowledge of layers is not expected. Atmospheric pressure is the force per unit area from the weight of air above, and it acts in all directions.

Why pressure falls with height (P1.3g).

1. Less air above, so less weight pressing.
2. Air thins with height, because the air below is squashed by the air above. Fewer particles per cubic metre means fewer collisions.

Worked reminder. With pressure 1.0 × 10⁵ Pa and g = 10 N/kg, one square metre has 1.0 × 10⁵ N of air above it. Mass = 1.0 × 10⁵ ÷ 10 = 1.0 × 10⁴ kg.

Distinction. P1.3b changes the temperature at constant volume. P1.3d changes the volume at constant temperature. Name which quantity is fixed before you start an answer.

Quick self-test

1. A sealed rigid tin of gas is cooled. What happens to the pressure, and why? (both tiers)
2. Name two ways the collisions change when a gas in a closed container is heated. (both tiers)
3. A student says that gas pressure is caused by particles hitting each other. What is wrong with that? (both tiers)
4. Why is it easy to compress a syringe of air but not a syringe of water? (Higher Tier only)
5. In which direction does gas pressure produce a force on a wall? (Higher Tier only)
6. A gas at 90 kPa has volume 0.40 m³. It is expanded to 0.60 m³ at constant temperature. Find the new pressure. (Higher Tier only)
7. Calculate the force from a pressure difference of 5.0 × 10⁴ Pa on a surface of area 0.30 m². (Higher Tier only)
8. Why does a hand pump warm up when it is used quickly to inflate a ball? (Higher Tier only)
9. In the simple model of the atmosphere, what is assumed about density? (Higher Tier only)
10. Give two reasons why atmospheric pressure is lower on a mountain top than at sea level. (Higher Tier only)

Answers

1. The pressure falls. The particles slow down, so they hit the walls less often and less hard.
2. The collisions are more frequent, and each collision is harder.
3. Pressure is caused by collisions with the walls of the container, not by collisions between particles.
4. A gas is mostly empty space, so its particles can be pushed closer. The particles in water are already touching.
5. At right angles (normal) to the surface.
6. p₂ = 90 × 0.40 ÷ 0.60 = 60 kPa.
7. F = 5.0 × 10⁴ × 0.30 = 1.5 × 10⁴ N.
8. Work is done on the air by the piston. Particles bounce off the moving piston faster, so their average kinetic energy and the temperature rise. Friction also adds a little.
9. The density is assumed to be the same all the way up.
10. There is less air above, so less weight. The air is also thinner, with fewer particles and collisions per unit area.

Where marks are usually lost

• Writing “particles expand” or “particles get hot” instead of “particles move faster”.
• Missing one half of the pair: more frequent collisions and harder collisions.
• Forgetting to link the force from collisions to pressure at the end of the answer.
• Explaining a volume change without saying the temperature is constant.
• Mixing kPa and Pa, or cm³ and m³, in the same equation.
• Rearranging p₁V₁ = p₂V₂ the wrong way up, so that the pressure rises when the volume rises.
• Leaving out the unit or giving the force with no direction.
• Saying “suction” for a collapsed can, when the outside pressure is the cause.
• Saying there is no air at height instead of less air above.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), first assessment 2018, topic P1.3 Pressure, statements P1.3a to P1.3g.