Particle motion in gases; Pressure in gases (physics only)

⚛ Physics Only

SPEC 8463 • TOPIC 3.05

Particle Motion in Gases

Why does a gas push on its container? The particle model gives the answer — but the mark-scheme phrasing is unforgiving. Master the words "rate" and "force per collision" and you'll bank the Level-3 marks others lose every year.

The Big Idea

Pump up a bike tyre and the rubber feels harder. Heat a sealed jar and it can burst. Both involve gas pressure, and both have one explanation:

Pressure comes from collisions

Gas particles fly around in random directions at huge speeds, constantly colliding with the walls of the container. Each collision exerts a tiny force on the wall. The total force per unit area, averaged over countless collisions per second, is what we measure as pressure.

Change the temperature, change the volume — both alter how many collisions happen per second and how hard each one is. That's the entire mechanism, and the rest of this lesson is about saying it in the exact words AQA wants.

Two Facts About Gas Particles

Whenever AQA shows you a balloon or sealed container of gas, they'll often ask you to identify how the particles move. The mark scheme always wants two of the same answers:

Fact 1

Particles move in random directions.

No preferred direction, no predictable path.

Fact 2

Particles move with a range of speeds.

Not all the same speed — that's why "temperature" is an average.

Not credited

"Particles all move in the same direction" · "Particles all move at the same speed" · "Particles move in circular paths" · "Particles vibrate about fixed positions" (that's solids, not gases).

Mark-Scheme Definition

This single sentence is worth knowing word-for-word. It contains all three elements the mark scheme wants: force per area, walls of the container, and caused by collisions.

Gas Pressure

The force per unit area exerted on the walls of a container due to gas particles colliding with those walls.

p = F / A

Two implications

From p = F/A, pressure rises if (1) the total force on the walls increases, or (2) the area decreases. The particle model tells us how each of those happens at microscopic level — coming up in the next few slides.

How Pressure Arises — 3-Step Chain

Any "explain how a gas exerts pressure" question expects all three statements. Miss one and you cap at Level 2.

1

Particles move randomly in all directions at a range of speeds.

2

They collide with the walls of the container.

3

Each collision exerts a small force on the wall (perpendicular to it). The total force per unit area from all the collisions per second is the pressure .

Why "perpendicular" matters

The spec specifically says pressure produces a net force at right angles to the wall. Random sideways motion cancels out across countless collisions — only the perpendicular component survives, giving a steady outward push on every part of the container surface.

Temperature = Mean Kinetic Energy

The single most important link in this whole topic. Get the wording wrong and you lose marks before you've even started explaining pressure.

Temperature ↔ KE

The temperature of a gas is a measure of the average (mean) kinetic energy of its molecules. Higher temperature → higher average KE → particles move faster on average.

✓ Credited phrasing

"Mean kinetic energy of the particles increases"

"Particles move at greater speeds on average"

"Average KE of particles increases"

Risky

"The KE of the particles increases" (no mean/average)

"All particles move faster" (some still move slower)

"Particles vibrate more" — only for solids; rejected for gases

Heating a Gas at Constant V

Same sealed container, same number of particles, two different temperatures. What changes?

What changes

Same V, same number of particles. But the particles are faster on average. That changes two things — both contribute to higher pressure.

The Two-Part Level-3 Answer

Every year, examiner reports flag the same issue: students give one of these two effects and stop. Both are required for full marks on a "why does T increase P at constant V" question.

Effect 1: Rate of collisionsEffect 2: Force per collision

Faster particles cross the container quicker → they hit the walls more often per second .Each collision is harder because particles arrive faster → greater force per collision .

Mark-scheme phrase: "more collisions per second" / "rate of collisions increases" / "more frequently"Mark-scheme phrase: "greater force in each collision" / "greater force per collision because particles move faster" / "greater rate of change of momentum" (advanced alternative)

Result

Greater total force on the walls. Area unchanged (constant V). So pressure (force per area) rises. Chain: higher T → higher mean KE → both effects → higher P.

PEE — Worked Example

A sealed metal can contains air at room temperature. The can is heated in a flame. The volume of the can does not change.

Explain, in terms of the particle model, why the pressure inside the can rises. 4 marks

P

Point: Heating raises the mean kinetic energy of the air particles, so they move faster on average.

E

Evidence (rate): The faster particles collide with the walls of the can more often per second.

E

Evidence (force): Each collision exerts a greater force on the wall because the particles are moving faster.

E

Explain: The total force on the walls increases. The area of the walls is unchanged (constant V). Pressure = force ÷ area, so the pressure rises.

Pressure Acts on the Container Walls

One spec line that's easy to forget: the pressure of a gas produces a net force at right angles to the wall of the container (or any surface inside the gas).

Why "net"

Individual collisions go in random directions, so any individual sideways impulse is cancelled by another collision somewhere else. What's left is a steady, perpendicular force outward on the wall. This is the "push" we feel when we touch a pressurised tyre or balloon from outside.

Direction of the force

The force from gas pressure on a flat wall always points outwards, perpendicular to the wall. On a curved wall (like a balloon), it points outwards along the local normal at every point — which is why balloons inflate into spheres.

Uniform throughout

Gas pressure has the same value everywhere inside a connected container. Every patch of wall experiences the same force per unit area, regardless of orientation — which is why balloons inflate evenly, tyres bulge equally on all sides, and a gas can't tell which wall it's pushing on.

Compressing a Gas (Qualitative)

What about changing the volume instead of the temperature? Same number of particles, same average speed (temperature unchanged), but a smaller container.

Particle reasoning

Same particles, same mean KE (T unchanged), but less distance to travel before hitting a wall. So more collisions per second on the same area of wall → pressure rises. Force per collision is unchanged here, because the particles haven't sped up.

Important assumption

The "T constant" assumption only holds if compression is slow. If you compress a gas quickly (e.g. pushing down hard on a bike pump), you do work on the gas, transferring energy to its particles. The gas heats up, adding an extra pressure rise on top. Full quantitative treatment in Lesson 3.06 (HT).

For HT students

In Lesson 3.06 you'll meet the equation pV = constant (at constant T, for a fixed mass of gas). It quantifies what this diagram shows: halve the volume, double the pressure.

Examiner Alert

Phrases that lose marks every year

"More collisions" — without per second / rate / more frequently. Time-reference is essential.

"Particles vibrate more" — explicit reject. Gases don't vibrate; they move freely.

"Greater force per m²" alone — this is just restating "higher pressure". Need the mechanism (faster particles, harder collisions).

"Volume decreased" — restating the question doesn't earn marks. Link it to particle behaviour.

"The kinetic energy increased" — needs the word mean or average.

The fix

Every gas-pressure explanation needs three checks before you commit to it: (1) does it name mean KE? (2) does it say collisions happen per second? (3) does it mention force per collision? Three boxes ticked = Level 3.

Weak vs Strong Answer

Question: a sealed gas canister is heated. Explain why its pressure rises.

Weak — 1 mark

"The particles move faster and have more collisions with the walls, so the pressure goes up."

Strong — 4 marks

"Heating raises the mean kinetic energy of the particles, so they move faster on average. The particles collide with the walls more often per second, AND each collision exerts a greater force because the particles are moving faster. Volume is constant, so pressure (force ÷ area) rises."

What changed

The weak answer says "faster" and "more collisions" but never anchors them: no mean, no per second, no force per collision. The strong answer hits all three — and ties it back to p = F/A.

Common Errors Table

Students Often WriteCorrect VersionWhy It Matters

"More collisions""More collisions per second " / "rate of collisions"Time-reference required; explicit reject otherwise

"Particles vibrate more""Particles move with greater (mean) speed"Gases move freely; vibrate is solid-only

"All particles move at the same speed""Particles move at a range of speeds; T is the mean KE"Distribution matters

"The kinetic energy increased""The mean / average kinetic energy increased"Loses A1 without the qualifier

"Greater force per m² causes greater pressure"Add the mechanism — faster particles cause more collisions per second AND greater force per collisionRestates definition without explaining

"Pressure pushes the gas outward"Pressure is the force per unit area on the container walls , due to collisionsWrong direction of conceptualisation

Practice Question

A football is inflated with air. The volume of the football stays the same. The football is then left out on a cold winter morning, and its temperature falls.

Explain, in terms of the particle model, why the pressure inside the football falls. 4 marks

P

Point: Lowering the temperature decreases the mean kinetic energy of the air particles, so they move more slowly on average.

E

Evidence (rate): The slower particles collide with the inner walls of the football less often per second.

E

Evidence (force): Each collision exerts a smaller force on the wall because the particles arrive with less momentum.

E

Explain: The total force on the inner walls falls. Volume is unchanged (so area unchanged). Pressure = force ÷ area, so the pressure falls.

What's Next — HT Students Only

This lesson covered the qualitative particle model. The follow-up, Lesson 3.06 (Higher Tier, physics only), makes it quantitative.

You can now (everyone)

Define gas pressure in particle-model terms

Explain how T affects P at constant V

Explain qualitatively how V affects P at constant T

Use mark-scheme language: mean KE · per second · force per collision

Coming in 3.06 (HT)

pV = constant (on equation sheet)

Calculations: find p₂ or V₂ when one is changed

Work done on a gas → internal energy → temperature rises (bike-pump effect)

Lesson Summary

The Core Mechanism

Gas particles move randomly in all directions at a range of speeds .

They collide with the container walls. Each collision exerts a small perpendicular force.

Pressure = force per unit area = the total wall force, divided by the wall area.

Temperature is a measure of the mean (average) kinetic energy of the particles.

The Two-Part Level-3 Explanation

When asked why T → P at constant V, every answer must contain BOTH:

Rate : more collisions per second / more frequently .

Force : greater force per collision because particles move faster.

The Three Mark-Scheme Checks

Before submitting any gas-pressure answer, scan it for: (1) "mean" or "average" in front of KE — (2) "per second" or "rate" in the collision phrase — (3) "force per collision" as a distinct effect. Three ticks = Level 3.