Physics · 2 Thermal physics

2.3.2 Convection

Before you start

Density is mass per unit volume, $ρ = m / V$ , and an object less dense than the liquid around it floats (1.4).
In a solid the particles vibrate about fixed positions; in a liquid or a gas they can move from place to place, so the material can flow (2.1.1, 2.1.2).
When a substance gets hotter, its particles move faster on average (2.1.2).
Gases, and almost all liquids, expand when they are heated at constant pressure (2.2.1).
Gases and most liquids are poor thermal conductors (2.3.1).

Syllabus objectives covered (2)

Know that convection is an important method of thermal energy transfer in liquids and gases
Explain convection in liquids and gases in terms of density changes and describe experiments to illustrate convection

Part 1 of 6

Why liquids and gases need a different method

Thermal energy can be transferred in three ways: conduction, convection and radiation. This lesson is about convection, which is an important method of thermal energy transfer in liquids and gases.
Start from what you know about conduction. Gases and most liquids are poor thermal conductors. If conduction were the only method, a pan of water heated from below would warm its bottom layer and leave the top cold for a long time.
A liquid or a gas has one thing a solid does not: it can flow. Its particles can move from place to place, so warm liquid or gas can travel to another part of the container and take its thermal energy with it.
That is convection. Convection is the transfer of thermal energy by the movement of a liquid or gas, caused by differences in density. The next two sections build up where those density differences come from.
Keep the contrast with conduction clear. In conduction the material stays where it is and energy passes from particle to particle. In convection the material itself moves, carrying the energy.
A fluid is anything that can flow: a liquid or a gas. Convection needs a fluid. It cannot happen in a solid, because the particles of a solid are held in fixed positions and cannot flow. It cannot happen in a vacuum, because there are no particles to move.

Part 2 of 6

Heating a fluid lowers its density

All of convection rests on one fact: when a liquid or gas is heated, its density decreases. Build the reason one link at a time, because each link is something you will write in an answer.
Heat the fluid and its particles gain kinetic energy. On average they move faster.
Faster-moving particles move further apart on average, so the fluid expands. The same particles now take up a larger volume.
The mass has not changed. No particles were added or taken away; only the volume has gone up. Now apply the density equation, which you must recall because Paper 4 does not give it:
$ρ = \frac{m}{V}$
Same $m$ , bigger $V$ , so $ρ$ is smaller. The warm fluid is less dense than the cooler fluid around it.
The trap here is to say the particles change. They do not. Each particle keeps exactly the same size and mass. What changes is the space between the particles.
Cooling works the other way round. The particles slow down and move closer together on average, the fluid contracts, and its density increases. Cooler fluid is denser.
One exception, so that the rule is honest: water between $0$ °C and $4$ °C contracts as it warms. You do not need it for this subtopic, and every example in this lesson uses air or water warmer than that.

Part 3 of 6

Less dense fluid rises: the convection current

You know from 1.4 that an object less dense than a liquid floats up through it. A region of warm fluid behaves the same way. It is less dense than the cooler fluid around it, so it rises, and the cooler, denser fluid sinks.
Picture a pan of water on a hot plate. The water at the bottom is heated by the hot base of the pan. That water expands, becomes less dense and rises. Cooler, denser water from above sinks to the bottom to take its place.
That cooler water is now next to the hot base, so it is heated in turn, becomes less dense and rises too.
Meanwhile the warm water that reached the top moves sideways, passes some of its thermal energy to the cooler water and the surroundings, cools, becomes denser again and sinks.
The result is a continuous loop of moving fluid called a convection current. Every trip round the loop carries thermal energy from the heat source into the rest of the fluid. That is how the whole pan warms up, and not just the bottom layer.
Where you heat a fluid matters. Heat it at the bottom and you get a convection current, because the heated fluid has room above it to rise into. Heat it at the top and the warm, less dense fluid is already at the top, so it stays there. No current forms, and the fluid lower down warms only by slow conduction.
Cooling is the mirror image. Cool a fluid at the top and the cooled, denser fluid sinks, which sets up a current. Cool it at the bottom and the cold, dense fluid simply stays at the bottom.

Part 4 of 6

Experiment: convection in water

Water is transparent, so you cannot see it move. The method is to colour a small amount of it and watch where the colour goes.
Fill a beaker with cold water and leave it to stand until the water is still. Any movement left over from filling it would be confused with convection.
Drop a small crystal of potassium manganate(VII), also called potassium permanganate, down a straw or a glass tube held against the bottom of the beaker at one side, then lift the tube out gently. The tube puts the crystal in one place without colouring the water on the way down. Handle the crystal with tweezers, as it stains skin.
Heat the beaker gently, with a small flame placed directly under the crystal and not under the middle of the beaker.
What you see: a purple stream rises straight up from the crystal, spreads across the top of the water, moves down the far side of the beaker and returns along the bottom towards the crystal.
apparatus diagram — not drawn yet
A beaker of water standing on a tripod and gauze, with a small Bunsen flame under the left-hand side of its base only; a small purple crystal on the bottom of the beaker directly above the flame; a purple stream rising vertically from the crystal to the surface, curving right across the top, down the right-hand side and back along the bottom to the crystal, with arrowheads showing the direction of flow; the rising stream labelled 'hot, less dense water rises' and the falling side labelled 'cooler, denser water sinks'.
The colour marks the water as it moves: up above the flame, across the top, down the cooler side and back. That loop is the convection current.
What it shows: the water moves in a loop, a convection current. The water above the flame is heated, expands, becomes less dense and rises. Cooler, denser water sinks on the far side and flows along the bottom to replace it.
The dye does not cause the movement. It dissolves into the water next to the crystal and marks that water, so you can see where it goes. The current would flow in the same way with no dye at all.
Why heat gently? A large flame makes the water boil, and the bubbles stir the water so that you can no longer follow the path. A small flame gives a slow, steady current. Watch in the first few minutes: after a while the colour spreads through all the water and the path is lost.

Part 5 of 6

Experiment: convection in air

Air is even harder to watch than water, so here smoke is the marker.
Use a box with a glass or clear plastic front and two chimneys in its lid. Stand a lit candle inside the box, directly under one chimney.
Hold a smoking taper, or a piece of smouldering paper, just above the top of the other chimney.
What you see: the smoke is drawn down the chimney that has no candle under it, moves across the inside of the box towards the candle, and comes out of the chimney above the candle.
apparatus diagram — not drawn yet
A box with a glass front and two vertical chimneys on its lid, labelled A on the left and B on the right; a lit candle standing inside the box directly below chimney B; a smoking taper held just above the top of chimney A; arrows showing smoke going down chimney A, across the inside of the box towards the candle, and up out of chimney B; chimney A labelled 'cool, denser air sinks' and chimney B labelled 'hot, less dense air rises'.
The smoke shows where the air goes: down the cold chimney, across the box, and out through the chimney above the candle.
What it shows: the candle heats the air above it. That air expands, becomes less dense and rises out of its chimney. Cooler, denser air from outside sinks down the other chimney to replace it, and carries the smoke with it. The loop is a convection current in air.
A simpler version: hang a spiral cut from paper on a thread above a hot radiator or heater. Warm air rising past the spiral makes it turn. Notice that this shows only the rising part of the current, not the whole loop.

Part 6 of 6

Convection in everyday design, and how to write it up

Once you know 'heated fluid rises, cooled fluid sinks', you can explain where heaters and coolers are put.
Kettle. The heating element is at the bottom. Water next to it is heated, becomes less dense and rises; cooler water sinks to the element to be heated. A convection current carries thermal energy through all the water.
Refrigerator. Many refrigerators have the cooling unit near the top. Air next to it cools, contracts, becomes denser and sinks. Warmer air rises to the top to be cooled in turn. The circulation cools the whole compartment.
Room heater. A heater is placed low down. Warm air rises above it, spreads across the ceiling, cools, sinks on the far side of the room and flows back along the floor to the heater.
apparatus diagram — not drawn yet
Side view of a room with a heater on the floor against the left-hand wall; arrows showing warm air rising above the heater, moving right along the ceiling, falling down the right-hand wall and flowing back left along the floor to the heater; the rising arrows labelled 'warm, less dense air rises' and the falling arrows labelled 'cooler, denser air sinks'.
One heater, low down, keeps the air of the whole room circulating.
Sea breeze. On a sunny day the land warms up faster than the sea. Air above the land is heated, becomes less dense and rises. Cooler, denser air from over the sea moves in to replace it. That moving air is the breeze, blowing from the sea towards the land.
Every one of these explanations is the same chain. Write it one link per sentence: the fluid is heated (or cooled); it expands (or contracts); its density decreases (or increases); it rises (or sinks); cooler, denser fluid sinks to replace it, setting up a convection current.
Each link can carry its own marking point, so a missing link can mean a missing mark. Name the fluid in every sentence: 'the water near the element rises', not 'heat rises'.

Worked examples

Example 1. An electric kettle has its heating element at the bottom. Explain how the element heats all of the water in the kettle, not only the water touching it. (4 marks)

Show the working

Start where the energy goes in: the water touching the element is heated.
Say what heating does to that water. Its particles move faster and further apart, so the water expands.
Turn expansion into density. The mass of that water is unchanged and its volume has increased, so its density decreases.
Now the movement. The hot, less dense water rises, and cooler, denser water sinks to the element to replace it.
Finish with the current. The cooler water is heated in turn, so a convection current circulates and carries thermal energy through all the water.
Check the answer before you move on. Four separate links are there (heated, expands, less dense, rises with cooler water sinking), every sentence names the water, and nothing says 'heat rises' or 'the particles expand'.

Answer: Water next to the element is heated; its particles move faster and further apart, so the water expands and its density decreases. The less dense hot water rises and cooler, denser water sinks to the element to replace it. This sets up a convection current that carries thermal energy through all the water.

Example 2. Describe an experiment to show a convection current in water. State what you would see and what it shows.

Show the working

A Describe answer needs the set-up, the method and the observation. Give the set-up first: a beaker of cold water, left to stand until the water is still.
Say where the marker goes and how: a small crystal of potassium manganate(VII) dropped down a tube so that it sits on the bottom at one side.
Say where the heat goes. Heat gently with a small flame directly under the crystal. 'Under the crystal' matters, because it fixes where the warm water starts to rise.
Give the observation in order: the purple colour rises above the crystal, moves across the top, down the far side and back along the bottom.
Say what it shows: the water is moving in a loop, which is a convection current. If the question also says Explain, add the density chain: water above the flame is heated, expands, becomes less dense and rises; cooler, denser water sinks to replace it.

Answer: Put a crystal of potassium manganate(VII) at the bottom of one side of a beaker of still, cold water and heat gently directly beneath it. The purple colour rises above the crystal, spreads across the top, sinks down the far side and returns along the bottom. This shows the water circulating in a convection current.

Example 3. A pocket of air next to a heater has a mass of $0.60$ kg. Before it is heated its volume is $0.50$ m³; after heating its volume is $0.56$ m³. The rest of the air in the room keeps the density the pocket had before heating. Calculate the density of the air in the pocket before and after heating, and deduce whether the heated pocket rises or sinks.

Show the working

Recall the density equation; Paper 4 does not give it. Mass is in kg and volume in m³, so the density comes out in kg/m³.
$ρ = \frac{m}{V}$
Before heating:
$ρ = \frac{0.60}{0.50} = 1.2 {kg/m}^{3}$
After heating. The mass is still $0.60$ kg, because heating adds energy, not particles. Only the volume has changed.
$ρ = \frac{0.60}{0.56} = 1.07 {kg/m}^{3}$
Compare. The heated pocket has a density of $1.07$ kg/m³ and the surrounding air has $1.2$ kg/m³. The pocket is less dense than the air around it.
A region less dense than the fluid around it rises, in the same way that an object less dense than a liquid floats up through it.

Answer: Before heating $1.2$ kg/m³; after heating $1.07$ kg/m³ ( $1.1$ kg/m³ to 2 significant figures). The heated air is less dense than the surrounding air, so it rises.

Where people go wrong

Mistake: Writing 'heat rises'.

Instead: Write 'the hot water rises' or 'the warm air rises'. It is the warm fluid that moves, and it carries its thermal energy with it.

Thermal energy is not a substance that floats upwards. By conduction and radiation it can be transferred in any direction, including sideways and downwards. Only warm fluid rises, and only because it is less dense.

Mistake: Saying the particles expand, or get lighter, when the fluid is heated.

Instead: The particles keep the same size and mass. They move faster and further apart, so the fluid as a whole takes up more volume.

The density falls because the same mass is spread through a larger volume, not because any particle has changed.

Mistake: Saying hot air rises because it is 'lighter'.

Instead: Say it is less dense.

The heated pocket of air has the same mass, and so the same weight, as before. What has fallen is its mass per unit volume.

Mistake: Thinking that heating a fluid at the top will set up a convection current.

Instead: A current forms when a fluid is heated from below or cooled from above.

Warm, less dense fluid at the top is already where less dense fluid goes, so it stays there. The fluid below it warms only by slow conduction.

Mistake: Thinking the purple dye, or the smoke, is what makes the fluid move.

Instead: The dye and the smoke are markers. The fluid moves because of density differences; the marker only makes the movement visible.

Without a marker the same convection current still flows; you just cannot see it.

Check yourself

Name the two states of matter in which convection takes place, and say why it cannot take place in the third.
Answer
Liquids and gases. It cannot take place in a solid, because the particles of a solid are held in fixed positions and the solid cannot flow.
Why is convection, rather than conduction, the main way a pan of water heated from below warms up all the way through?
Answer
Water is a poor thermal conductor, so conduction alone would be slow. In convection the hot water itself moves up through the pan, carrying its thermal energy with it.
Can convection transfer thermal energy across a vacuum? Give a reason.
Answer
No. A vacuum has no particles, so there is no fluid to move and carry the energy.
Some air is heated at constant pressure. State what happens to its mass, its volume and its density.
Answer
The mass stays the same, the volume increases and the density decreases.
A student writes: 'When water is heated the particles expand, so they rise.' Rewrite the sentence so that it is correct.
Answer
When water is heated its particles move faster and further apart, so the water expands and its density decreases. The less dense warm water rises. (The particles themselves stay the same size.)
A sample of $500$ g of water has a volume of $500$ cm³ when cold and $509$ cm³ when hot. Calculate the density of the hot water and say whether it will rise or sink in the cold water.
Answer
$ρ = m / V = 500 / 509 = 0.982$ g/cm³. The cold water has a density of $500 / 500 = 1.00$ g/cm³, so the hot water is less dense and rises.
In the box-and-chimneys experiment, smoke is held above the chimney that has no candle under it. Describe where the smoke goes and explain why.
Answer
It goes down that chimney, across the box and out of the chimney above the candle. The candle heats the air above it, which expands, becomes less dense and rises out of its chimney; cooler, denser air sinks down the other chimney to replace it and carries the smoke with it.
On a sunny day the land warms up faster than the sea. Explain why a breeze blows from the sea towards the land.
Answer
The air above the land is heated, expands, becomes less dense and rises. Cooler, denser air from over the sea moves in to replace it, so air flows from the sea to the land.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student fills a beaker with cold water and leaves it to stand. She drops a small crystal of a purple dye to the bottom of the beaker, near one side. She then heats the beaker gently with a small flame directly under the crystal.

(a) Describe what the student sees during the first few minutes of heating. 2 marks No calculator

Mark scheme

B1 purple colour rises (straight up) above the crystal / above the flame
B1 colour moves across the top and down the opposite side (and back along the bottom) / circulates in a loop, owtte

(b) Explain, in terms of density, why the coloured water moves in this way. 3 marks No calculator

Mark scheme

B1 water above the flame is heated and expands / its particles move further apart; do not accept 'the particles expand'
B1 its density decreases / it becomes less dense than the cooler water
B1 less dense water rises and cooler, denser water sinks to replace it; 'heat rises' alone is insufficient

(c) Suggest why the student heats the water gently rather than strongly. 1 mark No calculator

Mark scheme

B1 any one: so the water does not boil and bubbles do not stir it / so the current is slow enough to follow / so the colour does not spread through all the water before the path can be seen / so the glass beaker does not crack (safety), owtte

(d) State why convection cannot take place in a solid. 1 mark No calculator

Mark scheme

B1 particles are in fixed positions / cannot move from place to place / a solid cannot flow

Question 2. In a refrigerator, the cooling unit is fixed near the top of the food compartment. The door is kept closed.

(a) Explain how the cooling unit cools the air throughout the compartment. 3 marks No calculator

Mark scheme

B1 air near the cooling unit cools and contracts / its particles move closer together
B1 its density increases / it becomes denser than the air below
B1 cold, denser air sinks and warmer, less dense air rises to the cooling unit to replace it / a convection current is set up

(b) The cooling unit is moved to the bottom of the compartment. Predict, with a reason, how this affects the temperature of food on the top shelf. 2 marks No calculator

Mark scheme

B1 food on the top shelf is warmer / is not cooled as well
B1 cold, dense air stays at the bottom / no convection current forms / warm air stays at the top

Question 3. A pocket of air next to a heater has a mass of $0.36$ kg. When it is heated at constant pressure, its volume increases to $0.33$ m³. The cooler air in the rest of the room has a density of $1.2$ kg/m³.

(a) Explain, in terms of particles, why the volume of the air increases when it is heated. 2 marks No calculator

Mark scheme

B1 particles gain kinetic energy / move faster
B1 particles move further apart (on average); do not accept 'the particles get bigger'

(b) Calculate the density of the heated air. 2 marks

Mark scheme

C1 ρ = m / V or 0.36 / 0.33
A1 1.1 kg/m³ (accept 1.09 kg/m³ or 1.091 kg/m³); unit required

(c) Deduce whether the pocket of heated air rises or sinks. Give a reason for your answer. 2 marks No calculator

Mark scheme

B1 rises
B1 its density is less than the density of the surrounding air (1.2 kg/m³); ecf from (b)

How this is examined

The density chain: heated, expands, less dense, rises, cooler fluid sinks to replace it: a three- or four-mark Explain about a kettle, a heater, a refrigerator or a beaker of water, which may be worded 'in terms of density' One marking point per link, each written as its own statement: the water (or air) near the heater is heated; it expands, or its particles move further apart; its density decreases; it rises and cooler, denser fluid sinks to replace it, forming a convection current. 'Heat rises' does not name the fluid or the density change, so it is insufficient on its own. 'The particles expand' is wrong physics: say the particles move further apart.
Describing the purple-crystal experiment: a two- or three-mark Describe of an experiment to show convection in water, which may ask you to Sketch the path the colour takes Three things you can check your answer against: where the crystal goes (on the bottom at one side), where the heat goes (a small flame directly under the crystal), and what is seen (the colour rises above the flame, moves across the top, down the far side and back along the bottom). For a Sketch, the arrows must make one closed loop, rising above the flame.
Why coolers go at the top and heaters at the bottom: an Explain, Suggest or Predict about where a heater or cooling unit is placed, or what would change if it were moved For a cooler at the top: the air near it cools, becomes denser and sinks, and warmer air rises to replace it, so the whole space circulates. For a device moved to the wrong end: no convection current forms, because the heated fluid is already at the top or the cooled fluid is already at the bottom, so the far end stays warm (or cold).
Density before and after heating, from mass and volume: a two-mark Calculate using $ρ = m / V$ , then a Deduce of whether the heated fluid rises or sinks $ρ = m / V$ recalled (Paper 4 does not give it) and the numbers substituted; the answer with its unit, kg/m³ or g/cm³; the same mass in a bigger volume gives a smaller density, so the heated fluid rises because it is less dense than the fluid around it. A wrong density used correctly in the Deduce still earns that mark by ecf.
Why convection needs a liquid or a gas: a one-mark State of why convection does not happen in a solid or in a vacuum Solid: the particles are in fixed positions and cannot flow. Vacuum: there are no particles to move.

Words to know

Convection: The transfer of thermal energy by the movement of a liquid or gas, caused by differences in density.
Convection current: A continuous circulation of a liquid or gas in which warmer, less dense fluid rises and cooler, denser fluid sinks to replace it.
Fluid: A liquid or a gas: any substance that can flow.
Density: Mass per unit volume, $ρ = m / V$ , measured in kg/m³ or g/cm³.
Thermal expansion: The increase in the volume of a material when its temperature rises.

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