Section 1 of 4The Convection Current
1
A fluid near a heat source gains thermal energy and its particles move further apart.
2
The heated fluid expands, so a given volume of it has less mass and it becomes less dense than the fluid around it.
3
The less dense fluid rises, while cooler, denser fluid moves in underneath to take its place.
4
The fluid cools at the top, contracts, becomes denser and sinks, which completes the circulation.
| Stage | Density | Movement |
|---|---|---|
| Heated fluid | Lower | Rises |
| Cooled fluid | Higher | Sinks |
Section 2 of 4Convection in Action
Convection in Action
Heating a room
- A radiator near the floor warms the air above it, which rises and circulates around the room.
Potassium permanganate
- Coloured crystals placed in heated water spread through the whole beaker as convection currents form.
Hot water tanks
- Hot water is less dense, so it rises to the top of the tank, while cooler water sinks to the bottom.
Main process
- Convection is the main way thermal energy moves through liquids and gases, while conduction dominates in solids.
Section 3 of 4Why Convection Works
Why Convection Works
Why fluids circulate
- Fluids can flow freely, so the heated fluid can physically carry its thermal energy to other places.
Why solids do not
- Solids cannot circulate because their particles are fixed in place, so energy moves through them only by conduction.
Kettle example
- Water in a kettle circulates by convection, so the whole volume of water heats up rather than only the water at the base.
Room heating
- Warm air from a heater rises and creates a convection current that gradually warms the rest of the room.
Section 4 of 4Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Convection Energy transfer through a fluid by the movement of the fluid itself.
- Convection current A circulating movement of a fluid caused by differences in density due to temperature.
- Density Mass per unit volume of a substance.
- Fluid A liquid or gas, which can flow.
- Expansion An increase in volume when a material is heated.
Common Mistakes
- Saying heat rises; it is the hot, less dense fluid that rises.
- Describing convection in a solid, where particles stay in fixed positions and cannot circulate.
- Explaining that particles grow larger when heated; the particles keep their size and move further apart.
- Describing a convection current without linking it to changes in density.