2.3.2 Convection
- Syllabus
- 0625–2026–2027
- Topic
- 2.3.2
- Level
- —
Learning objectives
Convection transfers energy through moving fluids
Convection is thermal energy transfer by the bulk movement of a fluid. It is an important transfer method in liquids and gases because their particles can move from place to place.
| Medium | Can convection occur? | Reason |
|---|---|---|
| liquid | yes | the liquid can flow and carry internal energy |
| gas | yes | the gas can flow and carry internal energy |
| solid | no | its particles do not move through the solid as a bulk flow |
| vacuum | no | there is no matter to move |
Convection circulates water heated in a pan and air heated in a room. The fluid itself moves, carrying energy from one region to another.
Do not say ‘heat rises’. Warmer, less-dense fluid may rise, while cooler, denser fluid moves down to replace it; thermal energy is carried with the moving fluid.
Explain and demonstrate a convection current
A convection current is a continuous circulation caused by density differences within a liquid or gas.
| Step | What happens |
|---|---|
| 1 | a region of fluid is heated, expands and becomes less dense |
| 2 | the warmer, less-dense fluid rises |
| 3 | cooler, denser fluid moves down to replace it |
| 4 | continued heating and cooling maintain a circulating current that transfers energy |
| Fluid | Demonstration | Observation and conclusion |
|---|---|---|
| liquid | place a small coloured tracer near the bottom at one side of a beaker of water and heat gently below it | the coloured water rises above the heater, moves across the top and descends on the cooler side, showing a convection current |
| gas | use a convection box with two chimneys; warm the air below one chimney and introduce smoke at the other | smoke is drawn down the cool chimney, crosses the box and rises out above the heater, tracing the air current |
Cooling can drive the same cycle in reverse locally: water beside ice cools, becomes denser and sinks; warmer water rises to replace it. The result is still a convection current.
Rising and sinking are consequences of density differences, not separate transfer methods. A complete explanation links temperature change to density change, fluid motion, replacement and circulation.