ConvectionCambridge IGCSE Physics: Revision notes
Section 1
What Is Convection?
Convection is an important method of thermal energy transfer in liquids and gases (fluids). Unlike conduction, convection involves the actual movement of the substance itself — the heated fluid physically moves and carries its thermal energy with it. Convection cannot happen in solids, because their particles cannot flow from place to place.
Section 2
How Does Convection Happen? (Density Changes)
Convection occurs because of changes in density within a fluid:
- When part of a fluid is heated, its particles gain kinetic energy and move further apart.
- This makes the heated fluid less dense than the surrounding, cooler fluid.
- The less dense, warm fluid rises, while the denser, cooler fluid sinks to take its place.
- The sinking cool fluid is then heated in turn, and the cycle repeats, setting up a circulating flow called a convection current.
This explains why warm air or warm water rises, and cooler air or water sinks.
An experiment to illustrate convection: place a crystal of potassium permanganate at the bottom of a beaker of water and gently heat the water directly beneath the crystal. As the water above the crystal warms, it becomes less dense, rises, and carries a visible purple trail upward, showing the convection current.
Do not say convection happens because 'heat rises' on its own — it is the change in density of the heated fluid that causes it to rise, displaced upward by denser, cooler fluid sinking.
Section 3
Everyday Application: Heating a Room by Convection
A room heater warms the air immediately around it. This warm air becomes less dense and rises, while cooler, denser air from the rest of the room sinks to take its place near the heater. This sets up a convection current that circulates air around the whole room, gradually warming it throughout — which is why room heaters are usually placed near the floor, so the convection current can carry warm air upward through the room.
If asked to explain how a room heater warms a whole room, describe the full cycle: air heats near the heater, becomes less dense, rises, cooler air sinks to replace it, and the cycle repeats.
Section 4
A More Complex Example: A Car Radiator
Some everyday situations involve more than one method of thermal energy transfer working together. In a car's cooling system, a radiator uses convection as well as conduction:
- Hot coolant fluid from the engine flows through the radiator, and thermal energy is conducted through the metal radiator fins to their outer surfaces.
- Air passing over and around the radiator fins is heated; this warmed air becomes less dense and rises, setting up convection currents that carry the heat away into the surroundings, helped by air flow from the car's movement or a fan.
Recognising which transfer method (or combination of methods) is happening in a given situation is a common exam skill.
A car radiator relies on conduction (through the metal fins) and convection (of the surrounding air) working together to remove heat from the engine coolant.
Must Know
- Convection is the transfer of thermal energy through a fluid (liquid or gas) by movement of the fluid itself.
- Convection cannot happen in solids, because their particles are fixed in place.
- Convection is caused by density changes: heated fluid expands, becomes less dense, and rises; cooler, denser fluid sinks to replace it.
- This circulating flow is called a convection current.
- A room heater warms a whole room by setting up a convection current: warm air rises, cooler air sinks and is heated in turn.
- A car radiator uses both conduction (through metal fins) and convection (of surrounding air) to remove heat from the engine.
That's the notes covered.
Carry on to the next subtopic.