Welcome! Scuba divers learn one golden rule: never hold your breath on the way up. Today you’ll find out why, using the physics of pressure. Then you’ll write a dive safety briefing. (Separate Physics only.)
🔑 Key words for today
- Pressure – how hard something pushes on each square metre. Measured in pascals (Pa).
- Particles – the tiny bits a gas is made of. They move around quickly.
- Collide – bump into something.
- Volume – how much space something takes up.
- Depth – how far below the surface you are.
- Expand – get bigger.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Pressure in gases and liquids – Deep Sea Diver.
- ⭐ Core = everyone does this (about 40 minutes). 🚀 Stretch = extra challenge (Higher tier) if you finish.
- Write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Copy the blue 📝 boxes into your book – they are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. explain gas pressure using particles
2. explain why pressure increases with depth
3. use pV = constant
4. (🚀) use p = hρg.
⭐ Step 1: Starter (5 minutes)
At the bottom of a deep swimming pool, your ears hurt. A diver’s bubble gets bigger as it rises. Can you explain either of these?
Show answer
The deeper you go, the more water is above you pushing down, so the pressure is higher. As a bubble rises, the pressure on it gets lower, so the bubble can expand.
⭐ Step 2: Read and write (8 minutes)
Copy these three questions. Then read the text and answer them.
- What causes the pressure of a gas?
- What happens to a gas’s volume if the pressure on it goes down?
- Why does pressure increase as you go deeper in water?
📖 Read: Pressure under the sea
Gas pressure. Gas particles move around very fast. They keep hitting the walls of their container. Each hit pushes on the wall. All these pushes together make the pressure of the gas.
Squeezing a gas. If you squash a gas into a smaller space, the particles hit the walls more often. So the pressure goes up. It works the other way too: if the pressure goes down, the gas expands. If the pressure halves, the volume doubles. We write this as pV = constant.
Pressure in water. The deeper you go, the more water there is above you. Its weight pushes down on you. So the pressure increases with depth. Every 10 m of seawater adds about the same pressure again as the whole atmosphere.
Show answers
(a) Gas particles hitting (colliding with) the walls of the container.
(b) The volume gets bigger (it expands).
(c) There is more water above you pushing down.
📝 Copy into your book
Gas pressure is caused by particles hitting the walls.
For a gas at the same temperature: p × V = constant, so p₁ × V₁ = p₂ × V₂
Pressure in a liquid increases with depth.
⭐ Step 3: Using pV = constant (10 minutes)
👀 Watch me first
A balloon holds 4.0 litres of air at 100 kPa. It is squeezed until the pressure is 200 kPa. What is its new volume?
1. Equation: p₁ × V₁ = p₂ × V₂
2. Numbers in: 100 × 4.0 = 200 × V₂
3. Work it out: 400 = 200 × V₂, so V₂ = 400 ÷ 200
4. Answer: V₂ = 2.0 litres – double the pressure, half the volume.
⭐ Now you try: at 20 m deep, the pressure is 300 kPa. A diver breathes out a 2.0 cm³ bubble. What is its volume at the surface, where the pressure is 100 kPa?
Show answer
300 × 2.0 = 100 × V₂ → 600 = 100 × V₂ → V₂ = 6.0 cm³ – three times bigger!
⭐ Step 4: Main task – Dive Safety Briefing (12 minutes)
You’re a dive instructor. Explain the physics behind these safety rules.
- ⭐ “Never hold your breath while swimming up.” Use your bubble answer.
- ⭐ “Don’t leave your air tank in the sun.” Think about what heat does to the particles.
- 🚀 “Your air tank runs out faster on deep dives.” Think about the pressure of each breath.
✏️ Sentence starters
1. As you swim up, the pressure gets ______, so the air in your lungs ______. If you hold your breath, ______.
2. Heating the tank makes the particles move ______. They hit the walls ______, so the pressure ______.
Show a model briefing
1. As you swim up, the pressure gets lower, so the air in your lungs expands – from 20 m it would try to become three times bigger. If you hold your breath, it can’t escape and could damage your lungs. Breathe out steadily.
2. Heating the tank makes the particles move faster. They hit the walls harder and more often, so the pressure goes up. The tank can’t get bigger, so the pressure could become dangerous.
3. (🚀) At 20 m the pressure is about three times higher. Each breath is squashed to that pressure, so it holds about three times as much air. The tank empties about three times faster.
Step 5: Exam practice (10 minutes)
⭐ Q1 (3 marks) Explain, using particles, why the pressure of a gas in a sealed container goes up when it is heated.
✏️ Start with: “The particles move…” then “They hit the walls…”
Show model answer
The particles gain energy and move faster (1). They hit the walls more often (1) and with more force (1), so the pressure increases.
⭐ Q2 (3 marks) A gas has a volume of 0.60 m³ at 150 kPa. It is squashed at the same temperature until the pressure is 450 kPa. Calculate the new volume.
Show model answer
p₁V₁ = p₂V₂ (1)
150 × 0.60 = 450 × V₂ (1)
V₂ = 0.20 m³ (1)
🚀 Q3 (3 marks – Higher) Calculate the pressure caused by the water at the bottom of a 3.0 m deep pool. Use p = h × ρ × g, with density of water = 1000 kg/m³ and g = 9.8 N/kg.
Show model answer
p = h × ρ × g (1)
p = 3.0 × 1000 × 9.8 (1)
p = 29 400 Pa (1)
✅ Core score out of 6 (Q1–Q2). Stretch: out of 9.
⭐ Step 6: Exit ticket (5 minutes)
- Gas pressure is caused by…
- A bubble gets bigger as it rises because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes. Want more? Try the Particle model questions or Pressure in a liquid.