Convection
Explain moving warm fluids, predict a current, and use practical evidence without memorising “heat rises”.
All this lesson is Core (also needed for Extended). This is Cambridge IGCSE, separate from Egyptian Baccalaureate / Thanaweya Amma and not Year 3.
Core 2.3.2.1–2 · Density skill from 1.4 · PDF pp.194–199 / printed pp.186–191.
A fluid carries energy as it moves
A fluid is a liquid or a gas: its particles can move past one another so the material can flow. Convection transfers thermal energy by bulk movement of a fluid. An ordinary solid does not flow in this model.
Worked example
In a saucepan, energy conducts through the stationary metal base. Water circulating inside carries energy by convection. More than one transfer mechanism can operate in one situation.
Think first: Is every transfer through water convection?
No. Water also conducts; convection specifically involves bulk movement.
Move the heater; rethink the current
Predict the circulation
Idealised front view of a beaker of water initially at 20 °C. No fan, pump or stirring. Predict before revealing the flow. This diagram shows direction, not speed or a temperature simulation.
Unscored activity. Changing the heating position clears your prediction. Nothing here is saved beyond this page session.
Worked answer: how can a low heater warm a room?
A strong explanation has a linked cause-and-effect chain. Each point below is worth one suggested mark in this original four-mark example.
- The heater transfers energy to nearby air, which expands. [1]
- The same mass occupies a greater volume, so its density falls. [1]
- It is less dense than surrounding cooler air, so it rises carrying thermal energy. [1]
- Relatively cooler, denser air sinks and returns towards the heater, producing circulation around the room. [1]
“Hot air rises” alone leaves out the density reason and return flow. “Particles expand” is also wrong: their spacing changes, not their individual size.
Worked density calculation and comparison
Before: ρ = m/V = 1.20/1000 = 0.0012 g/cm³. After: ρ = 1.20/1200 = 0.0010 g/cm³. The mass is unchanged; density decreases by 0.0002 g/cm³. Compared with the cooler surrounding air at 0.0012 g/cm³, this expanded parcel is less dense. A sealed rigid container would not allow this volume increase, so do not apply this parcel model to every heating situation.
Temperature evidence: top versus bottom heating
| Time / min | Top heating: upper | Top heating: lower | Bottom heating: upper | Bottom heating: lower |
|---|---|---|---|---|
| 0 | 20 | 20 | 20 | 20 |
| 2 | 31 | 21 | 25 | 28 |
| 4 | 40 | 22 | 31 | 34 |
After 4 min, top heating gives an 18 °C difference between sensors; bottom heating gives a 3 °C difference. This supports greater mixing with bottom heating under the stated matched conditions. The sensors do not measure every location, and temperature alone does not reveal a unique flow path.
Make the comparison trustworthy
Match water volume, starting temperature, beaker geometry, heater power, heating duration and sensor heights. Repeat; do not stir one beaker but not the other. Combine temperature records with a suitable tracer and an unheated control. Use teacher-approved heating and dye, eye protection and heatproof support; hot water and heaters can burn. Never check temperature with a finger.
A smokebox is a gas version of the idea
Candle below the right chimney → warmed air expands and becomes less dense → air exits upwards on the right → cooler replacement air enters down the left and crosses the box. Smoke reveals the path; it does not create the thermal current. This is a classroom explanation, not a home activity: open flames and smoke require a teacher’s risk assessment, ventilation and suitable apparatus.
Practise, then explain
Four multiple-choice questions, three numerical responses and seven written tasks. All are Core, including earlier density skills applied to convection. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
Which description identifies convection?
Optional hint
Distinguish energy transfer by flow from energy transfer through a stationary solid.
Source clarifications and safe limits
The PDF remains unchanged. Figures 27.1–27.3 show a heated-water loop, a smokebox and room circulation. Our original diagram makes the heater location and complete return path explicit. It is an idealised model, not a prediction of flow speed or exact dye trajectories.
Top heating limits whole-depth natural circulation; it does not prevent all energy transfer or all dye motion. Dye can also spread by diffusion. A top-cooled fan-free cabinet is a useful natural-convection model, but it does not establish that modern bottom-freezer refrigerators are ineffective: fans and designed air passages matter.
The usual expansion explanation applies to ordinary room air and water above 4 °C in these examples. Water near freezing has unusual density behaviour, so avoid claiming that every liquid always becomes less dense whenever warmed. “Heat rises” should be replaced by an explanation of moving fluid and relative density.
Source alignment: supplied Physics-0625 Volume A, PDF pp.194–199 / printed pp.186–191, Figures 27.1–27.3; official Cambridge 2026–2028 syllabus, section 2.3.2 (Core). Original lesson, illustrative data and practice. Original source-download access is unchanged.