IGCSE Physics Convection Questions and Worked Solutions (2.3.2) | 0625

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IGCSE Physics 0625 · Topic 2.3.2

IGCSE Physics Convection Questions and Worked Solutions

50 topic-wise practice questions on Convection (Unit 2: Thermal Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions40 Core0 Extended10 Challenge118 marksTextbook §2.3 · pp. 115–126
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Convection

Convection is the transfer of thermal energy by the movement of a fluid. Heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks, forming a convection current.

Key formulas

density = mass ÷ volumeheated fluid: same mass, larger volume → lower densityCore

Prerequisite knowledge

  • Density (1.4)
  • Thermal expansion (2.2.1)

Exam tips & common mistakes

  • Use the chain: heated → expands → less dense → rises
  • Never say 'heat rises' — say 'warm air (or fluid) rises'
  • Convection cannot happen in a vacuum or a solid

Key facts from the syllabus (2.3.2)

  • Convection only occurs in liquids and gases (fluids)
  • Heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks to replace it
  • Experiment: a potassium permanganate crystal at the bottom of a heated beaker shows the current
  • Applications: room heaters placed low, freezer compartments at the top, hot water tanks heated at the bottom, sea and land breezes
  • Convection is prevented by trapping air in small pockets (insulation, double glazing gaps)

Textbook: §2.3 Transfer of thermal energy, pp. 115–126

1
Core[1 mark]

State in which states of matter convection can occur.

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Method: Fluids.

Liquids and gases
2
Core[3 marks]

Explain how convection currents form in a liquid heated from below.

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Method: Heated fluid expands, becomes less dense, rises; cooler denser fluid sinks.

The heated liquid expands, becomes less dense and rises; cooler, denser liquid sinks to replace it, forming a current
3
Core[3 marks]

Describe an experiment to show convection currents in water.

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Method: Potassium permanganate crystal.

Place a crystal of potassium permanganate at the bottom of a beaker of water and heat gently beneath it; purple streams rise above the heat then sink at the sides
4
Core[2 marks]

Explain why a heater in a room is placed near the floor.

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Method: Warm air rises, sets up current.

Air heated near the floor rises and cooler air sinks to replace it, so convection warms the whole room
5
Core[2 marks]

Explain why the freezer compartment is at the top of an old-style refrigerator.

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Method: Cold air sinks.

Air cooled at the top becomes denser and sinks, setting up a convection current that cools the whole fridge
6
Core[1 mark]

Explain why convection cannot occur in a solid.

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Method: Particles cannot move freely.

Particles are fixed in position and cannot flow
7
Core[2 marks]

Explain why a hot air balloon rises.

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Method: Hot air less dense.

Heated air inside expands and is less dense than the surrounding air
8
Core[2 marks]

Describe what happens to the density of air when it is heated.

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Method: It decreases.

Density decreases because it expands (same mass, larger volume)
9
Core[2 marks]

Explain how sea breezes form during the day.

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Method: Land heats faster; air above rises.

Land heats faster; air above it rises and cooler air from the sea moves in to replace it
10
Core[2 marks]

Explain why smoke rises above a fire.

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Method: Hot gases less dense.

Hot gases and smoke are less dense than surrounding air, so they rise in a convection current
11
Core[2 marks]

State which of air, water and wood can transfer energy by convection.

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Method: Fluids only.

Air and water only
12
Core[2 marks]

Explain why an air conditioner is mounted high on a wall.

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Method: Cool air sinks.

Cool air is denser and sinks, setting up a convection current through the room
13
Core[3 marks]

A kettle element is at the bottom. Explain how all the water is heated.

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Method: Convection.

Water near the element is heated, expands, becomes less dense and rises; cooler water sinks to the element; the convection current heats all the water
14
Core[2 marks]

Explain why a vacuum flask reduces energy transfer by convection.

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Method: No medium.

A vacuum has no particles, so no convection currents can form
15
Core[2 marks]

Explain why trapping air in small pockets (e.g. foam) prevents convection.

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Method: Air cannot circulate.

The air cannot move around freely, so convection currents cannot form
16
Core[2 marks]

A beaker of water is heated at one side. Describe the direction of the convection current.

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Method: Rises at the hot side, sinks at the cold side.

Water rises above the heated side, flows across the top and sinks at the cooler side
17
Core[2 marks]

Explain why warm water in a lake stays near the surface in summer.

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Method: Warm water less dense.

Warm water is less dense, so it floats above the colder, denser water
18
Core[2 marks]

Explain why a chimney carries smoke out of a house.

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Method: Hot gases rise.

Hot gases are less dense and rise up the chimney by convection
19
Core[2 marks]

Explain why convection is not effective at heating a liquid from the top.

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Method: Hot liquid stays at the top.

Hot, less dense liquid stays at the top, so no convection current forms to carry energy downwards
20
Core[1 mark]

Name the method of energy transfer that involves the movement of the heated fluid itself.

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Method: Convection.

Convection
21
Core[2 marks]

State why convection can occur in liquids and gases but not solids.

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Method: Particles can move.

In fluids the particles are free to move, so heated fluid can flow
22
Core[3 marks]

Describe the convection current in a room with a heater on one wall.

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Method: Rises, spreads, sinks.

Air above the heater warms, expands and rises; it spreads across the ceiling, cools, becomes denser and sinks on the far side, then flows back to the heater
23
Core[2 marks]

Explain why hot water is drawn off from the top of a hot water tank.

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Method: Hot water rises.

The hottest water is least dense and collects at the top
24
Core[2 marks]

Explain why the heating element in a hot water tank is near the bottom.

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Method: Convection.

So the heated water rises and convection currents heat all the water
25
Core[2 marks]

Explain land breezes at night.

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Method: Sea warmer than land.

The land cools faster; air above the warmer sea rises and cooler air flows from the land to replace it
26
Core[2 marks]

Explain why the top of a room is warmer than the floor.

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Method: Warm air rises.

Warm air is less dense and rises to the top
27
Core[2 marks]

Explain why a gliding bird can rise without flapping its wings.

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Method: Rising warm air.

It rides on rising warm air currents (thermals)
28
Core[2 marks]

State what happens to the volume of a fixed mass of air when it is heated at constant pressure.

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Method: Increases.

It increases
29
Core[3 marks]

Describe an experiment to show convection in air.

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Method: Chimney box with smoke.

Use a box with two chimneys and a candle under one; hold smoking paper over the other chimney; the smoke is drawn down and out of the chimney above the candle
30
Core[2 marks]

Explain why a fridge is cooled from the top.

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Method: Cool air sinks.

Cool air is denser and sinks, setting up a convection current throughout the fridge
31
Core[2 marks]

Explain why an ice cube placed at the top of a drink cools it more effectively than one held at the bottom.

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Method: Cold water sinks.

Water cooled at the top becomes denser and sinks, setting up a convection current
32
Core[2 marks]

Explain why the space between double-glazed panes is narrow.

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Method: Prevents convection.

A narrow gap stops convection currents forming in the air
33
Core[2 marks]

State the name of the circular flow of fluid caused by heating.

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Method: Convection current.

Convection current
34
Core[2 marks]

Explain why a candle flame points upwards.

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Method: Hot gases rise.

Hot gases are less dense and rise, drawing in cooler air from below
35
Core[2 marks]

Explain why warm water rises in cold water even though both are liquid water.

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Method: Density.

The warm water has expanded and is less dense
36
Core[2 marks]

Explain why convection does not occur in space where there is no gravity.

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Method: No buoyancy.

Without gravity, less dense fluid does not rise, so no convection currents form
37
Core[2 marks]

A pan of soup is heated from below. Describe the motion of the soup.

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Method: Convection current.

Soup at the bottom heats, rises; cooler soup sinks to replace it, forming a circulating current
38
Core[2 marks]

Explain why fibreglass insulation in lofts reduces convection.

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Method: Traps air.

It traps air in small pockets so it cannot circulate
39
Core[2 marks]

Explain why a room heater placed near the ceiling is less effective than one near the floor.

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Method: Warm air stays at top.

Warm air stays near the ceiling because it is less dense, so no convection current circulates warm air down to the floor
40
Core[2 marks]

Name the process that transfers energy from the hot core of a star to its surface in some layers by moving material.

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Method: Convection.

Convection

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Explain in detail how a convection current forms in a room heated by a radiator, referring to density changes.

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Method: Expansion and density.

Air near the radiator gains energy, expands and becomes less dense, so it rises; cooler, denser air sinks and moves towards the radiator to be heated, creating a circulating current
42
ChallengeCore[4 marks]

Explain why an air conditioner is fitted high on a wall but a heater is placed low.

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Method: Cold sinks, warm rises.

Cold air from a high unit sinks through the room; warm air from a low heater rises — both create convection currents that circulate through the whole room
43
ChallengeCore[4 marks]

Describe an experiment with a crystal of potassium permanganate to show convection, and explain the pattern seen.

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Method: Coloured path.

Place a crystal at the bottom edge of a beaker of water and heat gently below it; purple streams rise above the heat, spread across the top, and sink on the far side — showing the convection current
44
ChallengeCore[3 marks]

Explain why sea breezes blow from sea to land during the day.

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Method: Land heats faster.

Land heats up faster; air above it warms, becomes less dense and rises; cooler air from over the sea moves in to replace it
45
ChallengeCore[3 marks]

Explain why the freezer box in older fridges is at the top.

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Method: Cold air sinks.

Cold air from the freezer sinks, setting up a convection current that cools the whole fridge
46
ChallengeCore[4 marks]

A hot water tank has its heater at the bottom and the hot outlet at the top. Explain why this design is used and what would happen with the heater at the top.

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Method: Convection.

Heated water rises, so convection heats all the water and hottest water collects at the outlet; with the heater at the top, only the top layer would heat
47
ChallengeCore[3 marks]

Explain why convection cannot occur in a vacuum or in a solid.

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Method: Need fluid particles.

Convection needs particles that can flow; a vacuum has none and solid particles are fixed
48
ChallengeCore[4 marks]

Explain why a glider pilot looks for areas above ploughed fields or car parks to gain height.

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Method: Thermals.

Dark surfaces heat quickly, warming the air above, which rises as thermals that lift the glider
49
ChallengeCore[3 marks]

Explain why double glazing gaps are narrow, using convection.

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Method: Prevent currents.

A narrow gap prevents convection currents forming in the air, so energy is transferred mainly by slow conduction
50
ChallengeCore[4 marks]

Explain why the air at the ceiling of a heated room is warmer than at the floor, and suggest how ceiling fans can save energy in winter.

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Method: Warm air rises.

Warm, less dense air collects at the ceiling; a fan pushes it back down so the occupied space stays warm with less heating
All IGCSE Physics topics← 2.3.1 Conduction2.3.3 Radiation →

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