Gas Pressure and Particle Collisions
Explain gas pressure using wall collisions and compare temperature or volume changes with the other conditions held fixed.
On this page
1. Definition
Gas pressure is the force exerted by gas particles on a surface per unit area. Particles repeatedly strike the container walls; each collision changes the particle’s momentum and exerts a force on the wall.
2. Key Ideas
Pressure depends on the rate of momentum transfer per unit area. To predict a change, state what changes and what stays fixed.
Keep the gas volume fixed and raise its temperature, then hold temperature fixed and compress it to compare wall collisions and pressure.
A gas at 300 kelvin in a volume of 1000 cubic centimetres. The pressure is about 0 kilopascals.
- State
- —
- Average close neighbours
- —
- Pressure
- — kPa
- Volume
- 1000 cm³
- pV / T
- — kPa cm³ K⁻¹
- Mixed
- — %
Try this
0 of 4 doneCool the substance until its particles are fixed in a regular pattern. (not done yet)
In a solid the particles only vibrate about fixed positions: strong attractions hold them in a lattice.
Heat it until the particles spread out through the whole container. (not done yet)
In a gas the particles move fast enough to escape their attractions, so they are far apart and fill the container.
Halve the volume of the gas at the same temperature and wait for the pressure to settle. (not done yet)
The pressure doubles: in half the volume, particles hit each part of the wall twice as often.
Remove the partition and wait until the two gases have mixed. (not done yet)
Random motion alone mixes the gases. Nothing pushes them; they spread from where they are concentrated.
Your readings
| # | V / cm³ | p / kPa | 1/V / cm⁻³ | T / K | Remove |
|---|---|---|---|---|---|
| No readings yet. Set up a measurement, then record it. | |||||
| Change | What stays fixed? | Particle explanation | Pressure |
|---|---|---|---|
| Temperature rises | Amount of gas and volume | Particles move faster on average; wall collisions are more frequent and transfer more momentum on average | Increases |
| Volume decreases | Amount of gas and temperature | Average speed is unchanged; the collision rate per unit area increases | Increases |
| More gas is added | Volume and temperature | More particles strike each unit area of wall per second | Increases |
3. Detailed Explanations
Explain the force on a wall
A particle approaching a wall has momentum towards it. After bouncing back, its momentum points away. The wall exerts a force on the particle during the collision, and the particle exerts an equal and opposite force on the wall.
Many impacts over a short time produce an average outward force on the wall. Dividing this force by the wall area gives the gas pressure. Pressure acts on all the walls, not just the bottom of the container.
Raise the temperature at fixed volume
Consider a fixed amount of gas in a sealed, rigid container. A temperature rise increases average random kinetic energy. Particles move faster on average, hit a given wall more often and transfer more momentum in each impact on average. The increased rate of momentum transfer produces a larger force per unit area.
The fixed-volume condition matters. If the gas can expand, temperature alone is insufficient to predict its pressure change.
Compress the gas at fixed temperature
Now reduce the volume of a fixed amount of gas while keeping its temperature constant. Average kinetic energy and speed are unchanged. The same particles occupy less space, so wall collisions are more frequent per unit area and pressure increases.
Do not say that each particle becomes larger or faster in this comparison. Rapid compression, without time for energy transfer to the surroundings, can instead raise the temperature. That is a different set of conditions.
4. Common Mistakes
- “Gas pressure comes from the weight of the gas.” Wall impacts explain the pressure on the sides and top as well as the bottom.
- “A hotter gas always has higher pressure.” State the fixed amount and fixed volume.
- “Constant-temperature compression makes particles faster.” Their average speed remains unchanged; the collision rate per unit area changes.
5. Exam Tips
Use a causal chain: condition → particle motion → wall collisions → force per unit area. Include “on average” when comparing particle speeds or individual collision effects.
6. Worked Examples
Examiner practice 1
Gas pressure and temperature (constant volume)
Examination question
A sealed rigid can containing gas is warmed so that its temperature rises. State and explain the pressure change using the particle model. [4 marks]
Build the causal chain to wall force
View solution step by step
Link heating to particle motion
2 marksMethod
State that average kinetic energy and average particle speed increase.
Reason
Temperature tracks the particles’ average kinetic energy.
Working
Heating → faster random particle motion.
Link collisions to pressure
2 marksMethod
State that wall collisions become more frequent and have greater momentum change, so pressure rises.
Reason
The rigid volume is fixed; the increased collision force per unit area raises pressure.
Working
Greater rate of momentum change at the walls → greater force per unit area.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark kinetic energy, speed, collision effect and pressure conclusion.
Challenge 2
Gas pressure and volume (constant temperature)
Changed-condition transfer
A fixed mass of gas is compressed while its temperature remains constant. State and explain the pressure change using particles.
Hold particle speed constant and change spacing
Hints
Hint 1: respect constant temperature
Average kinetic energy and average speed do not increase.
Hint 2: focus on travel distance
In less space, particles reach the walls more often.
View solution step by step
Apply the fixed-temperature condition
Method
Keep average particle kinetic energy and speed unchanged.
Reason
The temperature is explicitly constant.Working
Average particle speed: unchanged.Use the smaller volume
Method
State that particles hit each unit area of wall more frequently, increasing pressure.
Reason
Particles travel shorter distances between wall collisions in the compressed gas.
Working
Smaller volume → more wall collisions per second per unit area → higher pressure.
7. Mind Stretchers
A gas expands while its temperature rises. Can you conclude that its pressure rises?
Show Answer
No. The temperature rise tends to increase pressure, while expansion tends to decrease it for the same amount of gas. You need more information about both changes. The fixed-volume explanation cannot be applied unchanged.
8. Practise and check
Use the Kinetic Particle Model topic check to practise linking particle motion to pressure.
Syllabus and review details
- SEC G3 Physics 2027 · 2027
Content Structure, PDF page 9; Subject Content, PDF pages 10–28