Heating and cooling

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Big idea: how thermal energy moves and why things cool

Big idea (key concept): Change. Things heat up and cool down because thermal energy moves. Understanding the change lets us control it.
Related concept: Energy. Heating and cooling are transfers of energy between thermal stores.
Global context: Globalization and sustainability. Heating and cooling buildings uses huge amounts of fuel around the world. Good insulation saves energy, money and carbon dioxide.
Temperature shows how hot something is. It is measured in degrees Celsius (°C) with a thermometer.
Thermal energy is the total energy of all the moving particles in an object. It is measured in joules (J). A bath of water at 40 °C holds far more thermal energy than a cup of water at 40 °C because it has many more particles.
Thermal energy always transfers from a hotter object to a colder one. The transfer stops when both reach the same temperature, called thermal equilibrium.

Conduction

In conduction, thermal energy passes through a solid. The particles at the hot end vibrate more strongly and bump into their neighbours, passing the energy along. The particles themselves stay in place.
Metals are the best conductors. They contain free electrons that move through the metal and carry energy quickly.
Insulators such as wood, plastic and glass conduct badly. Gases such as air are very poor conductors because their particles are far apart.
A metal spoon feels colder than a wooden one at room temperature because metal takes energy from your hand faster.

Conduction along a heated metal bar

Convection

Convection happens in liquids and gases (fluids), where particles can flow.
When part of a fluid is heated, its particles spread out, so the fluid expands and becomes less dense. The warm fluid rises.
Cooler, denser fluid sinks to take its place. This circulating flow is a convection current.
Convection warms a room from a radiator and makes water in a kettle heat evenly. It cannot happen in solids or in a vacuum.

A convection current above a heater

Radiation

Radiation is energy carried by infrared waves. It needs no particles, so it travels through empty space. This is how the Sun's energy reaches Earth.
All objects emit infrared radiation. Hotter objects emit more.
Dull black surfaces are the best emitters and the best absorbers of radiation. Shiny, light surfaces emit and absorb least and reflect radiation instead.
That is why solar water heaters use black panels, and why survival blankets and fire-fighters' suits are shiny.
Safety: never look at the Sun, and use only teacher-approved heat sources.

Dull black and shiny silver containers at the same temperature

Insulation, cooling and saving energy

An insulator slows the transfer of thermal energy. Trapped air is one of the best: it conducts badly and cannot circulate much in tiny pockets. That is why wool, feathers, foam and double glazing work.
A vacuum flask has a vacuum between two walls (no conduction or convection), silvered walls (reflect radiation) and a stopper (stops warm air escaping).
A hot object cools fastest at first, when the temperature difference with its surroundings is biggest, then more slowly. It stops cooling at room temperature. A graph of this is a cooling curve.
In one experiment, a beaker without a lid cooled from 90 °C to 29 °C in 30 minutes. An identical beaker with a lid cooled from 90 °C to 46 °C. The lid reduced the energy loss.
Houses lose energy through the roof, walls, windows and floor. Loft insulation, double glazing and draught-proofing reduce the loss, so less fuel is burned.

Single and double glazing compared

Think like a scientist: which colour of flask keeps water warm for longest?

Four identical flasks are painted black, grey, white and silver. Each is filled with the same volume of hot water at the same temperature. The temperature is read every 5 minutes.
The independent variable is the colour of the flask. The dependent variable is the temperature of the water.
Control variables: the volume of water, the starting temperature, the size and material of the flask, the room and the thermometer.
To make the data reliable, repeat the experiment three times and calculate a mean for each colour.
Inquiry task: plot a cooling curve for each colour. Which colour cools slowest? Is the black flask a good emitter or poor emitter of radiation? Suggest one improvement to the method and explain how it would improve your results.

Four flasks painted different colours to test cooling

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Übungsfragen

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1.What does a thermometer measure?
Easy
- AThe total thermal energy in an object
- BThe mass of a liquid
- CTemperature
- DHow fast a liquid flows
2.A hot drink is left in a cool room. In which direction does thermal energy naturally transfer?
Easy
- AFrom the drink to the cooler air
- BFrom the cooler air to the drink
- CEqually in both directions for ever
- DIt does not transfer at all
3.Which of these materials is the best conductor of thermal energy?
Easy
- AWood
- BPlastic
- CWool
- DCopper
4.Convection can only happen in which type of material?
Easy
- ASolids
- BLiquids and gases
- CMetals
- DA vacuum
5.How does energy from the Sun reach the Earth through empty space?
Easy
- ABy conduction
- BBy radiation
- CBy convection
- DBy evaporation
6.What is a thermal insulator?
Easy
- AA material that makes its own heat
- BA material that transfers thermal energy very quickly
- CA material that cools to 0 °C
- DA material that transfers thermal energy slowly
7.A metal spoon feels colder to touch than a wooden spoon in the same room. What is the best reason?
Easy
- AMetal conducts thermal energy away from your hand faster
- BThe metal spoon is at a lower temperature than the wooden one
- CWood makes more thermal energy than metal
- DMetal absorbs cold from the air
8.A metal rod is heated at one end. How does thermal energy move along the rod?
Medium
- AHot particles travel along the rod to the cold end
- BThe rod expands and pushes the energy along
- CParticles at the hot end vibrate more and pass energy to their neighbours, helped by free electrons
- DCold particles move towards the hot end to cool it

Mehr Fragen

9.Why does warm air rise above a radiator?
Medium
- AIt is lighter because the particles disappear when heated
- BThe radiator pushes the air upwards with a force
- CWarm air particles are attracted to the ceiling
- DIt expands, becomes less dense than the cooler air around it, and floats upwards
10.Two metal containers are at the same temperature. Which surface emits the most infrared radiation?
Medium
- AA shiny silver surface
- BA dull black surface
- CA white surface
- DThey all emit exactly the same amount
11.Why do people in hot, sunny places often wear light-coloured clothes?
Easy
- ALight colours make their own cool air
- BLight colours conduct thermal energy away faster than dark ones
- CLight colours reflect more of the Sun's radiation and absorb less
- DDark colours are made of better insulating material
12.Why does a double-glazed window lose less thermal energy than a single pane?
Medium
- AThe layer of still air or gas between the panes conducts poorly and cannot circulate much
- BThe two panes make their own heat
- CGlass is a better conductor when there are two layers
- DThe gap is filled with a vacuum that creates energy
13.Why does a woollen jumper keep you warm?
Medium
- AWool makes thermal energy from your body
- BWool is a good conductor, so it spreads energy evenly
- CIt traps air in its fibres, and trapped air is a poor conductor and cannot move around
- DWool absorbs cold from the outside
14.A cup of tea cools quickly at first and then more and more slowly. What is the best explanation?
Easy
- AThe bigger the temperature difference with the surroundings, the faster energy is lost
- BThe tea runs out of thermal energy after a few minutes
- CThe cup starts to conduct heat back into the tea
- DCooler tea is a better insulator
15.A hot metal block is put in a beaker of cold water. After a long time they are at the same temperature. Which statement is true?
Easy
- AThe block keeps transferring energy to the water for ever
- BThe block is still hotter because metal holds heat better
- CAll the thermal energy has been destroyed
- DThere is no longer any net transfer of thermal energy between them
16.A bath of water and a small cup of water are both at 40 °C. Which statement is correct?
Hard
- AThe bath is hotter than the cup
- BThe bath has a bigger thermal store because it contains many more particles at the same temperature
- CThe cup has a bigger thermal store because it is easier to heat
- DThey have the same thermal store because the temperatures match
17.A room is warmed by a radiator under a window. Which statement best describes most of the warming of the room?
Hard
- ARadiation spreads warmth to every corner of the room equally
- BWarm air rises from the radiator and cooler air sinks to replace it, forming a convection current
- CConduction carries the energy along the floor
- DThe radiator pushes warm particles to the far wall
18.
In one experiment, two identical beakers each started with 200 cm³ of water at 90 °C in a room at 20 °C. Beaker B had a lid and beaker A had none.
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
Which beaker was better insulated?
Medium
- ABeaker A, because it cooled faster
- BNeither, because both cooled
- CBeaker A, because it reached a lower temperature
- DBeaker B, because it cooled more slowly
19.
Look at the data in this experiment (one trial).
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
At which time did beaker A first read below 50 °C?
Medium
- A15 min
- B10 min
- C20 min
- D30 min
20.
A student looks at the data from the lid experiment (one trial each) and writes: 'A lid always reduces cooling by exactly the same amount.' Which comment best evaluates this?
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
Hard
- AThe conclusion is correct because B is always warmer than A
- BThe conclusion is correct because both beakers started at the same temperature
- CThe data show the lid slows cooling, but the amount changes over time and each beaker was only tested once
- DThe data prove nothing because the beakers cooled
21.A student fills four identical flasks with the same volume of hot water at the same temperature. The flasks are painted black, grey, white and silver. She measures the temperature of the water every 5 minutes. What is the independent variable?
Medium
- AThe temperature of the water
- BThe volume of water
- CThe time between readings
- DThe colour of the flask
22.In the same flask experiment, which of these must be kept the same to make it a fair test?
Medium
- AThe colour of each flask
- BThe starting temperature, the volume of water, the flask size and the room
- CThe water temperature at the end
- DThe time at which each flask is filled
23.Which change would make the flask results more reliable?
Medium
- AUse only the black flask and compare it with a guess
- BMeasure the temperature once at the end
- CRepeat the experiment three times and calculate a mean for each colour
- DUse a different thermometer for each reading
24.Which of these reduce the energy lost from a house? (select all that apply)
Easy
- ALoft insulation
- BDouble-glazed windows
- CDraught-proofing around doors
- DOpening the windows in winter
- EPainting the walls black
25.Which statements about convection are true? (select all that apply)
Medium
- AIt happens in liquids and gases
- BThe warmer fluid becomes less dense
- CCooler, denser fluid sinks
- DIt can happen in a solid metal bar
26.A metal rod is heated at one end. Which of these happen? (select all that apply)
Medium
- AThe particles at the hot end vibrate more
- BEnergy passes from particle to particle along the rod
- CThe particles move along the rod to the cold end
- DFree electrons help to carry energy along
27.Temperature is measured in degrees Celsius (°C).
Easy
True or false?
28.Radiation needs a material to travel through.
Easy
True or false?
29.A dull black surface is a better emitter and absorber of infrared than a shiny silver surface.
Easy
True or false?
30.When you hold an ice cube, cold travels from the ice into your hand.
Medium
True or false?
31.Trapped air is a good insulator because air is a poor conductor and cannot circulate easily when trapped.
Medium
True or false?
32.A vacuum flask has a vacuum between its walls so that conduction and convection are reduced.
Hard
True or false?
33.Match each process to an example.
Easy
- Conduction
- Convection
- Radiation
- Insulation
- Warmth from the Sun on your face
- A woollen hat keeping your head warm
- A metal spoon heating up in hot soup
- Warm air rising above a radiator
34.Match each part of a household item to why it is made that way.
Medium
- Metal base of a saucepan
- Plastic handle of a saucepan
- Foam lagging round a hot water tank
- Silvered walls of a vacuum flask
- Trapped air slows energy loss
- A shiny surface reflects radiation
- A good conductor passes energy to the food
- A poor conductor keeps your hand safe
35.Match each energy-saving measure to how it works.
Medium
- Loft insulation
- Double glazing
- Draught-proofing
- Reflective foil behind a radiator
- Stops warm air leaking out
- Reflects radiation back into the room
- Traps air to slow conduction through the roof
- A sealed gap reduces conduction and convection
36.Match each term to its meaning.
Hard
- Temperature
- Thermal energy
- Thermal equilibrium
- Dissipation
- No net transfer because the temperatures are equal
- Energy spreading out into the surroundings
- How hot or cold something is, measured in °C
- The total energy of the particles, measured in J
37.Put the steps of a convection current in order, starting with the heater.
Easy
- The warm fluid rises
- The heater warms the fluid next to it
- Cooler, denser fluid sinks to take its place
- The warm fluid expands and becomes less dense
38.Put these materials in order from the best conductor of thermal energy to the worst.
Medium
- Glass
- Air
- Copper
- Iron
39.Put these stages of conduction along a metal rod in order.
Hard
- Particles at the hot end vibrate faster
- Energy is passed along the rod
- A flame heats the end of the rod
- Vibrating particles collide with their neighbours
40.Complete the sentence.
Easy
Thermal energy always transfers from a ____ object to a ____ one.
41.Complete the sentence.
Easy
Convection happens in liquids and ____.
42.Complete the sentence.
Medium
Energy transferred by infrared waves, with no particles needed, is called ____.
43.Complete the sentence.
Medium
Trapped ____ is a good insulator in jumpers, double glazing and loft insulation.
44.Explain how a convection current forms when a beaker of water is heated from below.
Medium
45.A vacuum flask keeps hot drinks hot. Describe three features of its design and say which type of energy transfer each one reduces.
Hard
46.Evaluate this method: 'We heated a flask of hot water, put it in a cold room and read the temperature once after 20 minutes.' Suggest two improvements.
Hard
47.A beaker of water is heated from 18 °C to 64 °C. What is the temperature rise?
Easy
48.
Use the table of beaker A (no lid) below.
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
By how many degrees did beaker A cool in its first 10 minutes?
Medium
49.Three thermometer readings of the same cup of tea are 58 °C, 61 °C and 58 °C. What is the mean temperature?
Medium
50.
Using the table below, what was the mean rate of cooling of beaker A over the first 10 minutes? Give your answer in °C per minute.
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
Hard
51.An example house loses 8000 J of thermal energy every second through its roof and walls. After insulation it loses 5200 J every second. By what percentage has the energy loss fallen?
Hard
52.
Look at beaker A in the table.
Time (min) Beaker A, no lid: temperature (°C) Beaker B, with lid: temperature (°C)
0 90 90
5 70 79
10 56 70
15 46 62
20 38 56
25 33 50
30 29 46
How much more did beaker A cool in the first 5 minutes than in the last 5 minutes (25 to 30 min)?
Hard

Lernnotizen

## The big idea - Heating and cooling are **changes** caused by the transfer of **energy** from hotter to colder places. ## Temperature and thermal energy - **Temperature** (°C) shows how hot something is; **thermal energy** (J) is the total energy of the particles. - Energy transfers from **hot to cold** until **thermal equilibrium**. ## Three ways to transfer energy - **Conduction:** vibrating particles pass energy along a solid; metals are best (free electrons). - **Convection:** warm fluid expands, becomes less dense and rises; cooler fluid sinks (liquids and gases only). - **Radiation:** infrared waves, no particles needed; dull black surfaces are best emitters and absorbers. ## Insulation and saving energy - Trapped air, a vacuum and shiny surfaces slow transfers. - Loft insulation, double glazing and draught-proofing cut energy loss and save fuel. - Objects cool fastest when the temperature difference is largest. ## Think like a scientist - Change one variable, measure one, control the rest, repeat and take the mean.

Temperature: How hot or cold something is, measured in °C with a thermometer.
Thermal energy: The total energy of the moving particles in an object, measured in joules.
Thermal equilibrium: When two objects are at the same temperature and there is no net transfer of energy.
Conduction: Transfer of energy through a solid as particles vibrate and pass energy to their neighbours.
Free electrons: Electrons in a metal that can move and carry energy quickly.
Convection: Transfer of energy in a liquid or gas as warmer, less dense fluid rises and cooler fluid sinks.
Convection current: The circular flow of a fluid caused by heating.
Radiation: Transfer of energy by infrared waves, which needs no particles.
Emitter: A surface that gives out radiation; dull black surfaces are the best.
Insulator: A material that slows the transfer of thermal energy.
Cooling curve: A graph of temperature against time for a cooling object.
Dissipation: Energy spreading out into the surroundings, where it is hard to use again.

Karteikarten

What is the difference between temperature and thermal energy?

Temperature shows how hot something is (°C). Thermal energy is the total energy of all its particles (J).

In which direction does thermal energy transfer?

From a hotter object to a colder one.

What is thermal equilibrium?

When two objects are at the same temperature, so there is no net energy transfer.

How does conduction work?

Particles at the hot end vibrate more and pass energy to neighbouring particles.

Why are metals good conductors?

They have free electrons that carry energy quickly through the metal.

Name two good insulators.

Wood, plastic, wool, foam, or trapped air.

Why does hot fluid rise?

It expands, so it becomes less dense than the cooler fluid around it.

What is a convection current?

The circular flow of a fluid as warm fluid rises and cooler fluid sinks.

Can convection happen in a solid or a vacuum?

No. It needs a liquid or gas that can flow.

What is radiation?

Energy carried by infrared waves, which does not need any particles.

Which surfaces emit and absorb radiation best?

Dull black surfaces.

Which surfaces reflect radiation best?

Shiny, light-coloured surfaces.

Why is trapped air a good insulator?

Air conducts badly and cannot circulate much when trapped in small pockets.

How does a vacuum flask keep drinks hot?

The vacuum stops conduction and convection, the silvered walls reflect radiation, and the stopper traps air.

When does a hot object cool fastest?

At the start, when the difference in temperature with the surroundings is largest.

Name three ways to reduce energy loss from a house.

Loft insulation, double glazing and draught-proofing.

How do you make a result more reliable?

Repeat the experiment and calculate a mean.

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Time (min)	Beaker A, no lid: temperature (°C)	Beaker B, with lid: temperature (°C)
0	90	90
5	70	79
10	56	70
15	46	62
20	38	56
25	33	50
30	29	46

Time (min)	Beaker A, no lid: temperature (°C)	Beaker B, with lid: temperature (°C)
0	90	90
5	70	79
10	56	70
15	46	62
20	38	56
25	33	50
30	29	46

Time (min)	Beaker A, no lid: temperature (°C)	Beaker B, with lid: temperature (°C)
0	90	90
5	70	79
10	56	70
15	46	62
20	38	56
25	33	50
30	29	46

Time (min)	Beaker A, no lid: temperature (°C)	Beaker B, with lid: temperature (°C)
0	90	90
5	70	79
10	56	70
15	46	62
20	38	56
25	33	50
30	29	46