Physics · 2 Thermal physics

2.3.4 Consequences of thermal energy transfer

Before you start

Conduction is the transfer of thermal energy through a material without the material itself moving. It is most important in solids; liquids and gases are poor conductors. Metals are good thermal conductors while plastic, wood and air are poor ones (2.3.1).
Convection happens in liquids and gases: a warmed fluid expands, becomes less dense and rises (2.3.2).
Radiation is the transfer of thermal energy by infrared waves, which need no medium and travel in straight lines (2.3.3).
Heating a substance makes its particles move faster, and a heated gas or liquid expands (2.2).
Density = mass ÷ volume, $ρ = m / V$ , measured in kg/m³. You must recall this equation (1.4).

Syllabus objectives covered (2)

Explain some of the basic everyday applications and consequences of conduction, convection and radiation, including: (a) heating objects such as kitchen pans (b) heating a room by convection
Explain some of the complex applications and consequences of conduction, convection and radiation where more than one type of thermal energy transfer is significant, including: (a) a fire burning wood or coal (b) a radiator in a car

Part 1 of 5

One question to ask at every stage

Every situation in this lesson is a chain. Thermal energy starts somewhere hot and ends up somewhere cooler, and it may pass through several things on the way. Your job is to follow it one stage at a time.
At each stage, ask one question: what is the energy passing through, or across, right now? The answer decides the method.
Through a solid, or from one solid into another solid touching it: conduction.
Carried along by a liquid or a gas that moves: convection. Notice that in convection the fluid itself travels, taking the energy with it.
Across a space, from a hot surface to anything in a straight line from it: infrared radiation. It needs no medium at all.
The Core objective is about everyday cases where you explain one or two of these. The Extended objective is about cases where more than one method is significant at the same time, and there you must say which method happens where. 'The fire transfers heat to the room' names no method and no place, so it explains nothing.
So a good answer has the same shape each time: name the stage, name the method, and give the reason — a good or poor conductor, a fluid that becomes less dense, a large surface area.

Part 2 of 5

Heating a kitchen pan

Start with the chain. The hotplate is hot, the food is cold, and the base of the pan sits between them. Thermal energy must pass through the base, which is a solid, so it passes by conduction.
That is why the base is metal, such as copper or aluminium. Metals are good thermal conductors, so energy passes through the base quickly and the food heats up fast.
The base is also flat. A flat base touches a flat hotplate over its whole area, with no air gaps. Air is a very poor conductor, so any gap between hotplate and pan would slow the energy down.
The handle has the opposite job: you want very little energy to reach your hand. So it is made of plastic or wood, which are poor thermal conductors (insulators). Energy is conducted along the handle only slowly, and it stays cool enough to hold.
Be precise about what an insulator does. It does not stop thermal energy; it transfers it slowly. Slow is enough to keep the handle comfortable while you cook.
Inside the pan, the method depends on the food. Solid food in contact with the base is heated by conduction.
A liquid such as water or soup is heated by convection. Liquid at the bottom is warmed by the base, expands, becomes less dense and rises. Cooler, denser liquid from above sinks to the bottom to be warmed in its turn. This circulation is a convection current, and it carries energy through the whole pan.
Now you can see why the heat source goes under the pan. Warmed liquid rises, so heating from below sets up a convection current through all of it. Heat the liquid only at the top and the warm liquid simply stays at the top, while the bottom warms only by slow conduction through the liquid, which is a poor conductor.

Part 3 of 5

Heating a room by convection

A room heater has a hot surface. The air touching that surface is warmed first, by conduction from the hot metal into the air in contact with it.
The particles of the warmed air move faster and spread further apart, so the air expands. The particles themselves do not get bigger or lighter; there is simply more space between them.
Expansion means the same mass of air now takes up a larger volume, so its density is smaller. Paper 4 gives you no formula sheet, so this equation must come from memory.
$ρ = \frac{m}{V}$
With $m$ unchanged and $V$ larger, $ρ$ falls. The warm air is less dense than the cooler air around it, so it rises.
As the warm air rises, cooler, denser air moves along the floor to take its place next to the heater. That air is warmed in its turn and rises too.
The warm air spreads out under the ceiling, transfers energy to the room and cools. Cooler air is denser, so it sinks on the far side of the room and drifts back along the floor towards the heater. This loop is a convection current, and it carries energy round the whole room.
apparatus diagram — not drawn yet
Side cross-section of a room with a heater against the left wall at floor level; arrows show warm air rising above the heater, travelling right along the ceiling, sinking down the right-hand wall, and returning left along the floor to the heater, forming one closed loop, with the rising part labelled 'warm air, less dense' and the sinking part labelled 'cool air, denser'.
Warm air rises above the heater and cool air sinks on the far side, so one loop carries energy round the whole room.
This is why a heater goes low down, near the floor. The warm air it makes rises through the whole room on its way up. A heater fixed near the ceiling would leave its warm air up there and the lower part of the room cold.
The reverse argument places a cooler. An air-conditioning unit goes high on a wall. The air it cools is denser, so it sinks through the room, and warmer air rises up to the unit to be cooled.
The trap is the name. The panel on the wall of a house that people call a 'radiator' transfers most of its energy to the room by convection, not by radiation. It does emit some infrared, but the convection current is what carries most of its energy round the room.

Part 4 of 5

A fire burning wood or coal: three methods at once

A burning fire is very hot, and conduction, convection and radiation are all significant around it. For the Extended objective you name each one and say where it happens.
Radiation. The burning fuel emits a lot of infrared radiation, outwards in all directions, in straight lines. This is what warms you when you sit in front of the fire.
Two clues tell you it is radiation. You feel the warmth on the side of your body that faces the fire. And if someone steps between you and the fire, you feel much less warm straight away, although the air around you has not changed temperature.
Radiation needs no medium and passes through the air while heating it very little. It is absorbed by whatever it reaches: your skin, the walls, the furniture.
Convection. The air and the gases from burning, just above the fire, are very hot. They expand, become much less dense than the air in the room, and rise.
In a fireplace with a chimney, those hot gases rise straight up the chimney. So much of the energy carried by convection leaves the room, warming the chimney and the air outside rather than you. That is why the people in front of an open fire are warmed mainly by radiation.
apparatus diagram — not drawn yet
Side cross-section of an open fireplace with burning coal on a metal grate, a chimney rising above it and a seated person in front; wavy arrows labelled 'infrared radiation' go outwards from the fire to the person, upward arrows labelled 'hot gases rise (convection)' go up the chimney, low arrows labelled 'cooler air drawn in' move from the room into the base of the fire, and the grate and hearth are labelled 'conduction where the fire touches'.
Radiation reaches the person; convection carries the hot gases up the chimney; conduction happens only where the fire touches a solid.
The rising gases have a second consequence. Cooler air from the room is drawn in at the base of the fire to replace them, and that air brings the oxygen the fuel needs to keep burning.
Conduction. This happens wherever the fire touches a solid. The metal grate under the fuel and the hearth beneath it are heated by conduction from the burning fuel, as well as by absorbing radiation. A metal poker left with its end in the fire becomes hot along its length by conduction.

Part 5 of 5

A radiator in a car: a chain of transfers

The problem first. A running car engine produces a lot of thermal energy. If that energy is not removed, the engine overheats. The cooling system carries it from the engine to the air.
Engine to coolant. A pump pushes a liquid coolant, which is mostly water, through channels in the engine. The hot metal of the engine conducts thermal energy into the coolant flowing past it.
Coolant to radiator. The moving coolant carries that energy to the radiator at the front of the car. A moving fluid that carries energy with it is convection. Here the pump drives the flow, not a difference in density, so describe exactly what happens: the pumped liquid carries the energy with it (convection driven by the pump).
Through the radiator. The radiator is made of many thin metal tubes joined to thin metal fins. Thermal energy is conducted through the walls of the tubes and into the fins. Metal is a good thermal conductor, and thin walls give the energy only a short way to travel.
Radiator to air. The fins give the radiator a very large surface area in contact with the air. Air touching the metal is warmed and carried away, and cooler air takes its place: this is convection, and the larger the surface area, the more energy is transferred to the air each second.
apparatus diagram — not drawn yet
A simple block diagram of a car cooling loop: an engine block on the right with coolant channels, a pipe carrying hot coolant to a radiator on the left drawn as vertical thin tubes with horizontal fins, a return pipe carrying cooled coolant back via a pump to the engine, and a fan behind the radiator with arrows of air passing through the fins; labels 'conduction through tube walls and fins' and 'air carries energy away (convection)'.
Conduction gets the energy into the metal; the moving air carries it away from the large surface of the fins.
Keeping the air moving matters. When the car is moving, air is pushed through the radiator; when it is standing still, a fan draws air through. Either way, warmed air is swept away and cooler air keeps reaching the fins.
The reason that helps: the rate of thermal energy transfer is greater when the temperature difference is greater. Fresh, cool air next to the fins keeps the difference between the metal and the air large.
The hot radiator also emits some infrared radiation, and its large surface area helps that too. But radiation plays a smaller part here; conduction through the metal and convection by the air carry most of the energy away.
Back to the engine. The cooled coolant leaves the radiator and is pumped back to the engine to collect more energy. The whole system is a loop, and an answer that follows it stage by stage, naming the method at each stage, is the answer this objective is asking for.

Worked examples

Example 1. A pan of soup is heated on an electric hotplate. The pan has a flat aluminium base and a wooden handle. Explain how thermal energy reaches the soup at the top of the pan, and why the handle is made of wood.

Show the working

Follow the chain from the hotplate. The first thing the energy passes through is the base, a solid, so that stage is conduction.
Give the reason for the material: aluminium is a metal and a good thermal conductor, so energy passes through the base quickly. The flat base touches the hotplate all over, with no air gaps to slow conduction.
Next stage: the soup touching the base is warmed. Say what that does to it: it expands and becomes less dense.
Less dense soup rises, and cooler, denser soup sinks to the base to be warmed. That circulation is a convection current, and it is how the energy reaches the top of the pan.
Now the handle. Wood is a poor thermal conductor, so energy is conducted along it slowly and the end you hold stays cool.

Answer: Energy is conducted through the aluminium base, which is a good thermal conductor and in full contact with the hotplate. Soup at the bottom is warmed, expands, becomes less dense and rises, while cooler, denser soup sinks to be warmed, so a convection current carries energy to the top. Wood is a poor thermal conductor, so little energy is conducted along the handle and it stays cool enough to hold.

Example 2. An electric heater stands against a wall, close to the floor. Next to the heater, a parcel of air of mass $0.36$ kg has a volume of $0.30$ m³. After it has been warmed by the heater, the same air has a volume of $0.32$ m³. Calculate the density of the air before and after it is warmed, and use your answers to explain why the heater is placed near the floor.

Show the working

Recall the equation: density is mass divided by volume. It is not given on Paper 4.
Before warming, substitute the mass and the first volume.
$ρ = \frac{m}{V} = \frac{0.36}{0.30} = 1.2 {kg/m}^{3}$
After warming, the mass has not changed. Only the volume has, so substitute the new volume.
$ρ = \frac{0.36}{0.32} = 1.125 {kg/m}^{3}$
The data are given to 2 significant figures, so the second density is $1.1$ kg/m³. Writing $1.125$ first shows where it came from.
Now use the numbers. The warmed air is less dense than the air around it, so it rises.
Because the heater is at floor level, the warm air rises through the whole height of the room, and cooler, denser air moves in along the floor to replace it. That sets up a convection current round the whole room.

Answer: Before: $1.2$ kg/m³. After: $1.125$ kg/m³, which is $1.1$ kg/m³ to 2 significant figures. The warmed air is less dense, so it rises; with the heater near the floor, cooler, denser air flows in along the floor to replace it, and a convection current carries energy round the whole room.

Example 3. A living room is heated by an open wood fire in a fireplace with a chimney above it. Explain how conduction, convection and radiation are each involved, and explain why a person sitting in front of the fire is warmed mainly by radiation.

Show the working

Take each method in turn and attach it to a place. Start with radiation, because the question ends on it.
Radiation: the burning wood emits infrared radiation in all directions, in straight lines. It passes through the air and is absorbed by the person, the walls and the furniture.
Convection: the hot gases above the fire expand, become less dense and rise. In this fireplace they rise up the chimney, carrying energy out of the room. Cooler air is drawn in at the base of the fire to replace them.
Conduction: where the fire touches solids. The grate and the hearth are heated by conduction from the burning wood.
Now the 'mainly by radiation' part. Much of the convected energy goes up the chimney and never reaches the person, so the energy that does reach them arrives mostly as radiation.

Answer: Radiation: the fire emits infrared in all directions, which travels through the air and is absorbed by the person and the room. Convection: hot gases above the fire become less dense and rise up the chimney, drawing cooler air in at the base. Conduction: the grate and hearth are heated where the fire touches them. The person is warmed mainly by radiation because much of the energy carried by convection goes up the chimney and out of the room.

Example 4. Explain how a car's cooling system transfers thermal energy from the hot engine to the air, naming the method of transfer at each stage.

Show the working

Lay out the stages before writing anything: engine → coolant → radiator → air → coolant back to engine.
Engine to coolant: the hot metal of the engine conducts thermal energy into the coolant pumped through it. Name it: conduction.
Coolant to radiator: the pump moves the coolant, and the moving liquid carries the energy to the radiator. Name it: convection, driven by the pump.
Through the radiator: energy is conducted through the thin metal walls of the tubes and into the fins. Metal is a good thermal conductor.
Radiator to air: the fins give a large surface area in contact with the air. Air next to the metal is warmed and carried away by convection, helped by the car's motion or the fan, and cooler air replaces it.
Finish the loop: the cooled coolant returns to the engine. You may add that some energy is also emitted from the radiator surface as infrared radiation, a smaller part.

Answer: The engine conducts thermal energy into the pumped coolant, which carries it to the radiator (convection, driven by the pump). There it is conducted through the thin metal tube walls into the fins. The fins give a large surface area, so the air in contact with them is warmed and carried away by convection (helped by the fan or the car's motion), with some energy also emitted as infrared radiation. The cooled coolant returns to the engine.

Where people go wrong

Mistake: Writing 'heat rises'.

Instead: Write 'warm air (or warm water) rises because it is less dense'.

In convection it is the warmed fluid that moves upward, not the energy on its own. Conduction and radiation carry energy in any direction from hot to cold: the radiation from a fire travels sideways and downwards as well as upwards.

Mistake: Saying the particles of air get bigger or lighter when it is heated.

Instead: The particles keep the same size and mass. They move faster and spread further apart, so the air expands.

Density is $ρ = m / V$ . The mass stays the same and the volume increases, so the density falls. Nothing about the particles themselves has changed.

Mistake: Thinking a household 'radiator' heats a room mainly by radiation.

Instead: It heats the room mainly by convection.

Air touching its warm surface is heated, rises and sets up a convection current that carries energy round the whole room. It emits some infrared too, but that is the smaller part; the name is misleading.

Mistake: Saying the fins on a car radiator help because metal is a good conductor.

Instead: The metal conducts the energy into the fins; the fins help because they give a large surface area in contact with the air.

The rate of transfer to the air depends on how much hot surface the air touches. Two different features do two different jobs, and the answer needs both.

Mistake: Thinking an open fire warms the people in a room mainly by convection.

Instead: They are warmed mainly by infrared radiation from the fire.

The hot gases rise straight up the chimney, so much of the energy carried by convection leaves the room. Radiation spreads outwards from the fire in straight lines and is absorbed by the people and objects it reaches.

Check yourself

Explain why a saucepan has a metal base and a plastic handle.
Answer
Metal is a good thermal conductor, so energy is conducted quickly through the base to the food. Plastic is a poor thermal conductor (insulator), so energy is conducted along the handle only slowly and it stays cool enough to hold.
Water in a pan heats all the way through when it is heated from below. Explain why it would not heat evenly if it were heated only at the top.
Answer
Warmed water is less dense and rises. Heated from below, it rises and cooler water sinks to be heated, forming a convection current. Heated at the top, the warm water just stays at the top, and the water below warms only by slow conduction, because water is a poor conductor.
A parcel of air of mass $0.24$ kg has a volume of $0.20$ m³. After warming, its volume is $0.21$ m³. Calculate its density before and after warming, and state whether the warmed air will rise or sink.
Answer
Before: $0.24 \div 0.20 = 1.2$ kg/m³. After: $0.24 \div 0.21 = 1.14$ kg/m³ (about $1.1$ kg/m³). The warmed air is less dense than the air around it, so it rises.
Explain why an air-conditioning unit that cools a room is fixed high on a wall.
Answer
The air it cools is denser, so it sinks through the room. Warmer air rises up to the unit to be cooled, so a convection current carries cool air through the whole room.
By which method does a household 'radiator' transfer most of its energy to the room?
Answer
Convection. Air warmed at its surface becomes less dense, rises and sets up a convection current round the room.
You sit in front of an open fire. A friend stands between you and the fire and you immediately feel much less warm, although the air temperature has not changed. Explain why.
Answer
The fire warms you mainly by infrared radiation, which travels in straight lines. Your friend absorbs or blocks the radiation, so it no longer reaches you. Radiation heats the air very little, so the air temperature stays the same.
Explain how the convection current above a fire helps the fire to keep burning.
Answer
The hot gases above the fire are less dense and rise. Cooler air from the room is drawn in at the base of the fire to replace them, and that air supplies the oxygen the fuel needs to burn.
Give two features of a car radiator that increase the rate at which it transfers thermal energy to the air, and explain each.
Answer
Thin metal fins give a large surface area in contact with the air, so more air is warmed each second. A fan (or the car's motion) keeps cool air flowing past, so warmed air is carried away and the temperature difference between metal and air stays large. (Thin metal tube walls, which conduct energy quickly, are also acceptable.)

Exam questions

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

Question 1. An electric kettle has its heating element fixed near the bottom, inside the water. The handle of the kettle is made of plastic.

(a) Explain why the heating element is near the bottom of the kettle and not near the top. 3 marks No calculator

Mark scheme

B1 water near the element is warmed, expands and becomes less dense
B1 the warm water rises and cooler, denser water sinks to the element
B1 a convection current is set up that heats all the water; ORA an element at the top would leave the warm water at the top and the water below cold

(b) Suggest why the handle is made of plastic. 2 marks No calculator

Mark scheme

B1 plastic is a poor thermal conductor / a thermal insulator
B1 so little thermal energy is conducted to the hand / the handle stays cool enough to hold, owtte

Question 2. A heater stands next to the floor on one side of a room. Next to the heater, a parcel of air has a mass of $0.48$ kg and a volume of $0.40$ m³. After it is warmed by the heater, the same air has a volume of $0.42$ m³.

(a) Calculate the density of the air before it is warmed and after it is warmed. 3 marks

Mark scheme

C1 ρ = m / V or 0.48 / 0.40 or 0.48 / 0.42 seen (implied by both correct answers, which score 3)
A1 before: 1.2 kg/m³; unit required
A1 after: 1.1 kg/m³ (accept 1.14 or 1.143 kg/m³); unit required

(b) Using your answers to (a), explain how the heater warms the whole room. 3 marks No calculator

Mark scheme

B1 the warmed air is less dense than the surrounding air, so it rises; ecf from (a)
B1 cooler, denser air moves in (along the floor) to replace it / the air cools at the top of the room and sinks
B1 a convection current circulates round the room, carrying thermal energy to all of it

Question 3. A room is heated by a coal fire burning in an open fireplace. The smoke and hot gases leave through a chimney directly above the fire.

(a) State the main method of thermal energy transfer by which the fire warms a person sitting in front of it. 1 mark No calculator

Mark scheme

B1 (infrared) radiation

(b) Explain why much of the thermal energy carried away from the fire by convection does not warm the room. 2 marks No calculator

Mark scheme

B1 the hot gases above the fire expand / are less dense, so they rise
B1 they rise up the chimney, carrying the thermal energy out of the room

(c) A metal screen is placed between the fire and the person. Suggest why the person immediately feels much less warm, although the temperature of the air in the room has not changed. 2 marks No calculator

Mark scheme

B1 the person is warmed mainly by radiation, which travels in straight lines from the fire
B1 the screen blocks / absorbs / reflects the radiation so it no longer reaches the person; radiation heats the air very little, so the air temperature is unchanged

Question 4. In a car, a pump moves water-based coolant through channels in the hot engine and then through the radiator at the front of the car. The radiator is made of thin metal tubes joined to many thin metal fins. A fan behind the radiator draws air through it.

(a) Identify the method by which thermal energy passes through the metal walls of the tubes. 1 mark No calculator

Mark scheme

B1 conduction

(b) Explain how the fins increase the rate of transfer of thermal energy from the radiator to the air. 2 marks No calculator

Mark scheme

B1 the fins increase the surface area (in contact with the air)
B1 so more air is warmed at once / more thermal energy is transferred to the air per second (by convection)

(c) The car is stuck in slow traffic on a very hot day and the fan has stopped working. Suggest why the engine may overheat. 2 marks No calculator

Mark scheme

B1 little air flows past the fins, so warmed air is not carried away / cooler air does not replace it
B1 the air is already hot, so the temperature difference between the radiator and the air is small and thermal energy is transferred more slowly

How this is examined

Following the energy through a car radiator and naming the method at each stage: a three- or four-mark Describe or Explain about an engine cooling system, which may include a labelled diagram of the radiator Conduction through the metal walls of the tubes and into the fins; the fins give a large surface area in contact with the air; air next to the fins is warmed and carried away by convection; the fan or the car's motion keeps cooler air reaching the fins. 'Heat is lost to the air' names no method, so it cannot earn a method mark.
Why warm air rises and sets up a convection current: a three-mark Describe or Explain about a room heater, or about a liquid heated in a pan or kettle The fluid near the heat source is warmed, expands and becomes less dense; it rises; cooler, denser fluid sinks or moves in to take its place; the circulation is a convection current. Each of those links is a separate point, so write all of them.
Which method does which job around an open fire: a two- to four-mark Explain about a fire with a chimney, which may ask how a person in front of it is warmed Infrared radiation travels outwards from the fire and is absorbed by the person; the hot gases are less dense and rise up the chimney by convection, carrying energy out of the room; conduction where the fire touches the grate or hearth. Each method must be tied to where it happens.
Choosing the materials of a pan: a two-mark Explain or Suggest about the base and the handle Metal base: a good thermal conductor, so energy reaches the food quickly. Plastic or wooden handle: a poor thermal conductor (insulator), so little energy is conducted to your hand. Each half needs the property and its consequence.
Where to put a heater or a cooler: a one- or two-mark Suggest Heater low down, because the warm air it produces rises through the room; cooler high up, because the cooled air is denser and sinks. Mention density, not just 'hot goes up'.

Words to know

Conduction: Transfer of thermal energy through a material, or between materials in contact, without the material itself moving. It is fast in metals and slow in liquids and gases.
Convection: Transfer of thermal energy by the movement of a liquid or gas: warmed fluid becomes less dense and rises, and cooler, denser fluid sinks to replace it.
Convection current: The continuous loop of rising warm fluid and sinking cool fluid that carries thermal energy round a room or through a liquid.
Infrared radiation: Electromagnetic waves emitted by hot objects. They transfer thermal energy in straight lines and need no medium.
Good thermal conductor: A material, such as a metal, through which thermal energy is conducted quickly.
Thermal insulator: A poor thermal conductor, such as plastic, wood or air. It transfers thermal energy slowly; it does not stop it completely.
Density: Mass per unit volume, $ρ = m / V$ , in kg/m³. A warmed fluid expands, so its density falls.
Coolant: The liquid, mostly water, that a pump moves through a car engine and its radiator to carry thermal energy away from the engine.
Fin: A thin strip of metal attached to a radiator to increase the surface area in contact with the air.

All Physics lessons Next: 3.1 General properties of waves