Particle motion and pressure in gases.pptx

AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Particle motion and pressure in gases

Particle model of matter

Lesson 5 of 6

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up

Particle model of matter

Lesson 5 of 6

Answer each one, then check.

1

What is pressure?

2

What is the unit of pressure?

3

How do gas particles move?

4

Convert 100 kPa to Pa.

5

Write 3 × 4 = ?

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up - Answers

Particle model of matter

Lesson 5 of 6

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

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Learning Objectives

Particle model of matter

Lesson 5 of 6

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.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Particle model of matter

Lesson 5 of 6

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.

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Gas in a Container

Particle model of matter

Lesson 5 of 6

Smaller volume means more frequent collisions.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Explaining Pressure

Particle model of matter

Lesson 5 of 6

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.

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What Affects Gas Pressure

Particle model of matter

Lesson 5 of 6

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.

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Using pV = constant

Particle model of matter

Lesson 5 of 6

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

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Heating at Constant Volume

Particle model of matter

Lesson 5 of 6

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.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Common Mistakes

Particle model of matter

Lesson 5 of 6

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.

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Key Terms

Particle model of matter

Lesson 5 of 6

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.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Particle model of matter

Lesson 5 of 6

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.

WHAT A GOOD ANSWER SHOWS

The pressure increases (particles hit the walls more often). p₂ = 100 × 60 ÷ 20 = 300 kPa.

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Can I...?

Particle model of matter

Lesson 5 of 6

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.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Summary & Exam Focus

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.

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