☕ Kettle
The heating element is at the bottom. The water it heats expands, becomes less dense and rises. Cooler, denser water sinks to the element and is heated in turn. A convection current heats all the water.
Energy Transfers » Convection
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.6, 4.7
- How thermal energy is transferred through liquids and gases by convection
- How a convection current forms, step by step, and why it cannot happen in a solid
- Why heaters go at the bottom and coolers go at the top
- How convection explains sea breezes, land breezes, hot air balloons and chimneys
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Unlock This CourseWhat convection is
In Conduction you saw that liquids and gases are poor conductors. Yet a whole pan of water gets hot on a hob, and one small heater can warm a whole room. Something else must be carrying the energy. That something is convection.
In conduction, the energy is passed along while the material stays where it is. In convection, the hot material itself moves and takes its energy with it.
Key terms:
- Fluid: a substance that can flow. Liquids and gases are fluids.
- Convection: the transfer of thermal energy by the movement of a fluid, in which the warmer, less dense fluid rises and the cooler, denser fluid sinks.
- Convection current: the continuous circulating flow of a fluid caused by convection.
'Less dense' means lighter for its size. A full lesson on Density comes later. Here you only need the words less dense and denser.
How a convection current forms
Think of a pan of water on a hob. The spec asks you to describe convection, so learn this chain of steps in order:
- The water at the bottom, nearest the heat, is heated. Its particles gain energy, move faster and move further apart.
- This water expands. The same mass of water now takes up more space, so it becomes less dense than the cooler water around it.
- The warm, less dense water rises.
- Cooler, denser water sinks to take its place at the bottom.
- This cooler water is heated in turn and rises. The water that rose earlier cools near the top, becomes denser and sinks again.
- The water keeps going round and round. This is a convection current, and it carries energy to every part of the pan.
Exactly the same thing happens in a gas such as air. Only the name of the fluid changes.
Describing it as a diagram
Draw a beaker of water with a flame under the bottom left-hand corner. Draw an arrow going up the left-hand side, labelled 'warm, less dense water rises'. Carry the arrow across the top to the right. Draw an arrow going down the right-hand side, labelled 'cooler, denser water sinks'. Finish with an arrow along the bottom, back towards the flame. The arrows should make one closed loop. Label the loop 'convection current'.
Why convection cannot happen in a solid
Convection needs the material to flow, so that the warm part can rise and the cool part can sink. The particles in a solid are held in fixed positions. They can vibrate, but they cannot move from place to place. A solid cannot flow, so convection cannot happen in solids. It happens only in fluids: liquids and gases.
Heaters at the bottom, coolers at the top
Warm fluid rises and cool fluid sinks. So a heater works best low down, and a cooler works best high up. You can see this in four things around the home.
🚿 Hot-water tank
The heater is at the bottom of the tank for the same reason. The hottest water collects at the top, which is why the pipe to the hot taps leaves from the top of the tank.
🏠 Room heater or radiator
The air next to the heater is warmed. It expands, becomes less dense and rises towards the ceiling. Cooler, denser air sinks and flows along the floor towards the heater. The convection current spreads energy around the whole room.
🥛 Fridge
The freezer compartment is at the top. The air next to it is cooled. It contracts, becomes denser and sinks. Warmer, less dense air from lower down rises to take its place and is cooled in turn.
Worked example: a heater in the wrong place
A water tank has its heater near the top. Explain why only the water at the top gets hot.
Step 1: The water next to the heater is heated, expands and becomes less dense.
Step 2: Less dense water rises, but it is already at the top, so it stays there.
Step 3: The cooler, denser water stays at the bottom. No convection current forms below the heater.
Step 4: Energy can only reach the bottom by conduction, and water is a poor conductor. So the water at the bottom stays cold for a long time.
Sea breezes and land breezes
Convection currents can be huge. On a sunny day at the coast, the land warms up more quickly than the sea. At night, the land cools down more quickly than the sea. This sets up a convection current in the air that changes direction between day and night.
| Sea breeze (daytime) | Land breeze (night) | |
|---|---|---|
| Which is warmer? | The land | The sea |
| Where does air rise? | Over the land: the air is warmed, expands and becomes less dense | Over the sea: the air is warmed, expands and becomes less dense |
| What replaces it? | Cooler, denser air from over the sea | Cooler, denser air from over the land |
| Breeze at ground level | From the sea towards the land | From the land towards the sea |
A breeze is named after the place it blows from. A sea breeze comes from the sea, which is why a beach often feels breezy and fresh on a hot afternoon.
Hot air balloons and chimneys
🎈 Hot air balloon
A burner heats the air inside the balloon. The air expands, and some of it is pushed out through the opening at the bottom. The air left inside is less dense than the cooler air outside, so the balloon rises. To come down, the pilot lets the air inside cool.
🔥 Smoke up a chimney
A fire heats the air and smoke above it. These hot gases expand, become less dense and rise up the chimney. Cooler, denser air from the room flows in at the bottom to take their place. This also brings the fire a fresh supply of air.
Common mistakes
- Writing 'heat rises'. Heat does not rise. The warm, less dense fluid rises. Always name the fluid: 'the warm air rises'.
- Saying the particles expand or get lighter. The particles stay the same size and mass. They move further apart, so the fluid as a whole expands and becomes less dense.
- Leaving out the middle of the chain. 'The air is heated so it rises' misses the marks for expands and less dense.
- Forgetting the second half. A current needs cooler, denser fluid to sink and take the place of the fluid that rose.
- Saying the fridge 'lets cold in' or 'cold sinks'. Write that the cooled air becomes denser and sinks.
- Getting the breeze the wrong way round. Work out where the air is warmest. It rises there, and the breeze blows towards that place.
Exam-style question
A cabin is heated by a wood-burning stove. The stove stands on the floor in one corner and has a metal chimney.
(a) State the two states of matter in which convection can take place. [1 mark]
(b) Describe how convection transfers energy from the stove to the air in the whole cabin. [4 marks]
(c) Explain why convection cannot take place in the metal wall of the stove. [2 marks]
(d) The owner keeps food in a cool box with one ice pack. Suggest whether the ice pack should be placed on top of the food or underneath it. Explain your answer. [3 marks]
Model answer
(a) Liquids and gases (1).
(b) The air next to the stove is heated and expands (1). It becomes less dense than the surrounding air, so it rises (1). Cooler, denser air sinks and moves in to take its place (1). This air is then heated in turn, so a convection current forms that carries energy around the cabin (1).
(c) The particles in a solid are held in fixed positions / can only vibrate (1), so the metal cannot flow and the warmer part cannot rise (1).
(d) On top of the food (1). The air next to the ice pack is cooled, becomes denser and sinks (1). Warmer, less dense air rises to the ice pack and is cooled in turn, so a convection current cools all the food (1).
Exam tip
Part (b) has 4 marks, so write four linked steps: heated and expands, less dense so rises, cooler denser air sinks to replace it, convection current. In part (d) the cool box is not in your notes, but 'suggest' means apply what you know. It is the fridge idea again: the cold thing goes at the top.