AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Particle motion and pressure in gases
Particle model of matter · Lesson 5 of 6
Warm-up
Answer each one, then check.
1. What is pressure?
Force per unit area
2. What is the unit of pressure?
Pascal (Pa)
3. How do gas particles move?
Randomly and quickly
4. Convert 100 kPa to Pa.
100 000 Pa
5. Write 3 × 4 = ?
12
Learning Objectives
1. Explain how the motion of gas molecules is related to temperature and pressure.
2. Explain qualitatively the relation between temperature and pressure of a gas at constant volume.
3. Explain how changing the volume of a gas at constant temperature changes its pressure.
4. Use pV = constant.
Gas Pressure
Gas molecules are in constant random motion. Their collisions with the container walls cause the pressure, which acts at right angles to the wall.
For a fixed mass of gas at constant temperature, pressure × volume = constant, pV = constant. It is given on the equation sheet.
Gas in a Container
|
Smaller volume means more frequent collisions. |
A gas in a large cylinder at low pressure and the same gas compressed at higher pressure.
Explaining Pressure
Use this chain of reasoning.
|
1 Particles move randomly They collide with the walls of the container. |
2 Each collision exerts a tiny force At right angles to the wall. |
3 Many collisions Together they give a steady force on the wall, and pressure = force ÷ area. |
What Affects Gas Pressure
Learn the explanation for each.
|
Change |
Effect on pressure |
Explanation |
|---|---|---|
|
Temperature up (constant volume) |
Increases |
Particles move faster, so collide harder and more often. |
|
Volume down (constant temperature) |
Increases |
Particles are closer together so collide more often with the walls. |
|
Volume up (constant temperature) |
Decreases |
Particles collide less often with the walls. |
Using pV = constant
|
A gas at 100 000 Pa occupies 0.30 m³. It is compressed to 0.10 m³ at constant temperature. Calculate the new pressure. |
1. Write the relationship
p₁V₁ = p₂V₂
2. Substitute
100 000 × 0.30 = p₂ × 0.10
3. Rearrange
p₂ = (30 000)/0.10
4. Answer
p₂ = 300 000 Pa
Answer: 300 000 Pa
Heating at Constant Volume
|
Explain why the pressure in a sealed can increases when it is heated. |
1. Temperature rises
The average kinetic energy of the molecules increases
2. Faster molecules
They hit the walls harder and more often
3. Result
The force on the walls, and so the pressure, increases
Answer: Pressure increases because the faster molecules collide with the walls more often and with greater force.
Common Mistakes
Avoid these.
▸ Molecules expand. The molecules do not get bigger: they move faster.
▸ Collisions. Say 'more frequent and harder', not just 'more collisions'.
▸ Units. Pressure in pascals (Pa) and volume in m³.
▸ Constant. pV = constant only for a fixed mass at constant temperature.
Key Terms
|
Pressure Force per unit area. |
Pascal The unit of pressure; one newton per square metre. |
|
Random motion Movement in any direction with no pattern. |
Fixed mass A sealed amount of gas with no particles added or removed. |
|
Compress Squash into a smaller volume. |
Kinetic energy Energy of movement. |
Your Task: Squash the Gas
10 minutes
|
A syringe holds 60 cm³ of air at 100 kPa. The plunger is pushed until the volume is 20 cm³ at constant temperature. Predict and calculate the new pressure. 1. Use p₁V₁ = p₂V₂. 2. Explain with particles. |
A good answer shows: The pressure increases (particles hit the walls more often). p₂ = 100 × 60 ÷ 20 = 300 kPa.
Can I...?
☐ Explain gas pressure with particles.
☐ Say the force is at right angles to the wall.
☐ Explain the effect of temperature.
☐ Explain the effect of volume.
☐ Use pV = constant.
☐ Give units of pressure.
☐ Compress and expand gases.
☐ Give clear explanations.
Summary
✓ Gas pressure is caused by collisions with the walls.
✓ Higher temperature: faster molecules, more force, more often.
✓ Smaller volume at constant temperature: higher pressure.
✓ pV = constant.
|
EXAM FOCUS Explain, in terms of particles, why the pressure of a gas in a sealed container increases when the gas is heated. (3 marks) Say faster molecules hit the walls more often and harder. |