Convection in liquids and gases
Key idea: Convection carries energy as a liquid or gas flows. In natural convection, warmer, less dense fluid rises and cooler, denser fluid sinks, setting up a current.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Follow natural convection and compare flow driven by a fan.
- Compare densities using the same mass and different volumes.
- Choose which air rises and explain its lower density.
- Trace the dye’s complete loop through the water.
- Compare bottom heating with warm water above the ice.
- Explain why the coil is low and the outlet high.
- Trace room circulation and the daytime sea breeze.
- Draw the whole night-time loop with the temperatures reversed.
- Explain how rising warm air lifts the glider.
- Explain why solids and a vacuum have no convection current.
- Compare energy transfer before and after the wax melts.
- Check the container’s volume before predicting the density change.
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ContinueWater and air are poor conductors, yet a kettle heats all its water within minutes and an air-conditioner cools a whole room. They rely on convection: the warmer liquid or gas itself moves, carrying its energy with it. What drives the movement is a change in density, mass per unit volume, ρ = m/V, from Density of solids and liquids.
Convection: density changes make a fluid circulate
A fluid is a liquid or a gas. Its particles are not fixed in place, so the fluid can flow. Convection is the transfer of energy by the movement of the fluid itself: fluid moves from one place to another and carries its internal energy with it. Both warmer and cooler fluid can move in a convection current.
In natural convection, differences in density drive the flow. (A fan or pump can also drive a fluid around; that is forced convection.) Natural convection starts when part of the fluid is heated:
- Heating: energy is transferred to the fluid near the heat source. Its particles gain kinetic energy and move faster.
- Expansion: the particles move further apart, so that part of the fluid expands. It takes up a larger volume.
- Lower density: its mass has not changed but its volume is larger, so its density, ρ = m/V, decreases.
- Rising and sinking: the warmer, less dense fluid rises. Cooler, denser fluid sinks to take its place.
- Circulation: the cooler fluid is heated in turn and rises, while the warmer fluid moves away from the heat source, cools, becomes denser and sinks again. This circulating flow is a convection current.
Worked example 1
How much does the density change?
Problem
1.00 kg of air at 20 °C takes up about 0.83 m³. Heated to 60 °C at the same pressure, the same 1.00 kg of air takes up about 0.94 m³. Find its density before and after heating.
Show full solution
Before heating
Method
Divide mass by volume.Reason
Density is mass per unit volume.Working
Answer
ρ = 1.00 ÷ 0.83 ≈ 1.20 kg/m³
After heating
Method
Repeat with the new volume.Reason
The mass has not changed.Working
Answer
ρ = 1.00 ÷ 0.94 ≈ 1.06 kg/m³
Interpret
Working
Answer
The mass stayed the same, but the volume grew, so the density fell. The heated air is less dense than the 20 °C air around it, so it rises through it.
Say what rises and why. “Heat rises” is not an explanation: it is the warmer fluid that rises, because it is less dense than the cooler fluid around it. The particles do not get bigger; they move faster and spread further apart.
Check your understanding 1
Air next to a heater has density 1.09 kg/m³; the cooler surrounding air has density 1.16 kg/m³. Which air rises, and which explanation accounts for this?
Show answer
The warm air near the heater rises. Its density, 1.09 kg/m³, is less than that of the surrounding cooler air, 1.16 kg/m³, so it rises through it while the denser air sinks to replace it.
Convection in a liquid
Worked example 2
Seeing a convection current with a dye
Problem
A crystal of purple potassium permanganate is dropped to the bottom of a beaker of water, close to one side. The beaker is heated gently just under the crystal. Predict the path of the purple dye, and explain it.
Show full solution
Find where the water is heated
Method
Locate the heated water.Reason
Convection starts where the fluid is heated.Working
Answer
At the bottom, just above the flame, where the dye is dissolving.
Follow the warm water up
Method
Describe what the heated water does.Reason
Heated water expands and becomes less dense than the water around it.Working
Answer
It rises, carrying the purple dye straight up.
Follow it across and down
Method
Describe the water at the top.Reason
Water that cools becomes denser.Working
Answer
It moves across the surface away from the flame, transfers energy to the cooler water and the air, cools and sinks down the far side.
Complete the loop
Working
Answer
Cooler water moves along the bottom back towards the flame to replace the rising water. The dye traces a loop: up above the flame, across the top, down the far side and back along the bottom.
Worked example 3
Why the kettle element is at the bottom
Problem
Explain why the heating element of a kettle is at the bottom.
Show full solution
Follow the heated water
Method
Describe the water next to the element.Reason
Heated water expands and becomes less dense.Working
Answer
It rises.
Follow the cooler water
Method
Describe the water above.Reason
Cooler water is denser.Working
Answer
It sinks to the element and is heated in turn, so the convection current carries energy to all the water.
Compare with an element at the top
Working
Answer
The heated water would already be at the top and would stay there. Without stirring, there would be little natural convection carrying energy to the water below, and conduction through water is slow.
The top half of the figure shows the same idea in a test tube. Weighted gauze holds ice at the bottom, and the water is heated near the top. The hot, less dense water stays at the top, so no convection current carries energy down to the ice, and water conducts energy only slowly. The water at the top can boil while the ice at the bottom has still not melted.
Check your understanding 2
In a hot-water storage tank, the heating coil is near the bottom and hot water is drawn off from the top. Explain both choices.
Show answer
Heating at the bottom sets up a convection current: the water heated at the coil becomes less dense and rises, and cooler, denser water sinks to the coil to be heated in turn, so all the water in the tank is heated. The hottest water is the least dense, so it collects at the top, which is where the hot-water pipe draws from.
Convection in air: heaters, air-conditioners and sea breezes
A heater is placed low down. Air next to it is heated, expands, becomes less dense and rises. Cooler, denser air from the rest of the room sinks and flows along the floor to the heater to replace it. The convection current carries warmed air around the whole room. Without a fan, a heater near the ceiling tends to leave the warmest air at the top and warms the lower air less effectively.
An air-conditioner is placed high up. It cools the air next to it. That air contracts, becomes denser than the air around it and sinks. Warmer, less dense air from lower down rises towards the air-conditioner to replace it and is cooled in turn. (Its fan also pushes the air around.) The freezer compartment at the top of some refrigerators cools the space below in the same way.
Worked example 4
A daytime sea breeze
Problem
On a sunny day the land warms up faster than the sea. Explain why a cool breeze blows from the sea onto the land.
Show full solution
Over the land
Method
Describe the air above the warm land.Reason
Heated air expands and becomes less dense.Working
Answer
The air rises.
Over the sea
Method
Compare the air over the sea.Reason
The sea is cooler, so the air above it is cooler.Working
Answer
Cooler, denser air
At the beach
Method
Describe the flow near the ground.Reason
Denser air moves in to replace the rising air.Working
Answer
Air flows from over the sea onto the land: the sea breeze.
High above
Working
Answer
The risen air moves out over the sea, cools, becomes denser and sinks, completing the convection current.
Check your understanding 3
At night the land cools faster than the sea. Predict the surface breeze direction. On paper, sketch land and sea, then show surface flow, rising air, higher return flow and sinking air. Label why air rises or sinks and compare the whole loop below.
Show answer
From the land to the sea. Now the sea is warmer than the land, so the air above the sea is heated, expands, becomes less dense and rises. The cooler, denser air over the land moves out over the sea to replace it. Check the simplified circulation sketch: near the surface, land → sea; rising air over the warmer sea; higher return flow sea → land; sinking cooler air over the land. The four arrows must connect into a circulation, rather than ending with the rising arrow.
Check your understanding 4
A glider pilot looks for places where the ground is heated strongly by the Sun, such as a car park. Explain why the glider can rise over them.
Show answer
The hot ground heats the air above it. That air expands, becomes less dense and rises in a current of rising warm air, which carries the glider up with it.
Why there is no convection in solids
In a solid the particles vibrate about fixed positions and cannot flow past one another. A heated part of a solid does become a little less dense, but it cannot rise and be replaced by denser material. With no bulk flow there is no convection current in the stationary solid. Energy can still travel through it by conduction.
Convection also cannot happen in a vacuum, because there is no fluid there to move.
Check your understanding 5
A solid block of wax is heated from below. Explain why no convection current forms in the solid wax, but one forms once the wax has melted.
Show answer
The particles of the solid wax are in fixed positions and cannot flow, so heated wax cannot rise; energy can pass through the stationary solid by conduction. Liquid wax is a fluid: the heated wax at the bottom expands, becomes less dense and rises, and cooler, denser wax sinks to replace it.
Check your understanding 6 Check whether expansion is possible
A sealed, rigid container holds 0.80 kg of gas in 1.00 m³. It is heated uniformly; its mass and volume stay fixed. Calculate its average density before and after heating. Why does the whole gas not become less dense?
Show answer
Both densities are 0.80/1.00 = 0.80 kg/m³. Heating makes the particles move faster, but the rigid container prevents the whole gas from expanding. With the same mass in the same volume, its average density stays the same. In ordinary convection, a heated region can expand relative to the cooler surrounding fluid and become less dense.
Common mistakes
- Writing “heat rises”. The warmer fluid rises because it is less dense, and it carries energy with it.
- Saying the particles expand when heated. They move faster and spread further apart, so the fluid expands.
- Saying cold air sinks because it is full of cold. Cooler air sinks because it is denser than the warmer air around it.
- Describing convection as energy passed from particle to particle. That is conduction; in convection the fluid itself flows.
- Saying convection happens in solids. A solid cannot flow, so convection needs a liquid or a gas.
- Saying the warm fluid rises and the process stops. It moves away, cools and sinks again, forming a continuous convection current.
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Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
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