Overview
What you’ll learn
- State that the SI unit of temperature is the kelvin.
- Describe expansion and contraction and some everyday applications.
- Explain conduction, convection and radiation, and when each can happen.
- State that the rate of radiation depends on colour, texture and surface temperature.
- Explain applications such as insulation, cooking pans and radiant heaters.
In 30 seconds
- 1The SI unit of temperature is the kelvin (K). Everyday thermometers are marked in °C.
- 2Most solids, liquids and gases expand when heated and contract when cooled. Volume changes, so density changes. Mass does not.
- 3Conduction is transfer through a material by particles passing on energy. Metals are good conductors. Air, plastic and wood are poor conductors.
- 4Convection happens in liquids and gases. The warmer, less dense fluid rises and the cooler fluid sinks.
- 5Radiation is infrared transfer. It does not need particles, so it can cross a vacuum. Dull, dark surfaces radiate and absorb better than shiny, light ones.
- 6A higher surface temperature also increases the rate of radiation. Insulation traps air or reflects radiation to slow heat transfer.
Notes · 1 of 5
Temperature and expansion
Temperature is not the same thing as heat energy. Heating changes the space between particles.
The SI unit of temperature is the kelvin (K). School thermometers use degrees Celsius. A change of 1 °C is the same size as a change of 1 K. Ice melts at 0 °C, which is 273 K. You may be asked for the SI unit more often than for the conversion.
When a substance is heated, its particles move faster and, in general, further apart. The substance expands, its volume rises and its density falls. Cooling makes it contract. Mass stays the same if nothing escapes.
- Gaps in railway tracks and bridges let metal expand on a hot day.
- Overhead wires sag more in hot weather because they are longer.
- A tight metal lid can be loosened under hot water because the lid expands.
- A liquid-in-glass thermometer works because the liquid expands more than the glass.
Notes · 2 of 5
Conduction
Energy is passed from particle to particle. The material itself does not have to move along.
In conduction, particles vibrate more when they are heated and pass that energy to their neighbours. Metals also have particles that carry energy quickly, so metals are good thermal conductors. Plastics, wood and trapped air are poor conductors. They are used as insulators.
A cooking pan has a metal base so heat reaches the food, and a plastic or wooden handle so your hand stays cooler. Double glazing works because the air trapped between the panes is a poor conductor.
Notes · 3 of 5
Convection
The fluid itself moves, carrying energy with it.
- 1Heat
The fluid near the heat source gets warmer.
- 2Expand
It expands, so its density falls.
- 3Rise
The warmer, less dense fluid rises.
- 4Replace
Cooler, denser fluid sinks to take its place. A convection current is set up.
Convection needs a fluid that can flow, so it happens in liquids and gases, not in a solid bar. Sea breezes, a room heater that warms the air, and boiling water in a pot all use convection. A solid handle does not have a convection current inside the plastic.
Notes · 4 of 5
Radiation
Infrared radiation travels without particles, including through empty space.
Radiation here means infrared radiation, not nuclear radiation. The Sun warms Earth across space, where there are almost no particles, so the energy cannot have arrived by conduction or convection.
| Factor | Faster radiation or absorption | Slower |
|---|---|---|
| Colour and texture | Dull and dark | Shiny and light |
| Surface temperature | Hotter surface | Cooler surface |
A dull black surface is a good emitter and a good absorber. A shiny white or silver surface is a poor emitter and a poor absorber, so it is a good reflector. That is why a radiant heater has a dull element, and why light-coloured clothes feel cooler in the sun than black clothes.
Notes · 5 of 5
Putting the three processes together
Name the process, then the reason. Many objects use more than one.
- Vacuum flask: the vacuum cuts conduction and convection. Shiny walls cut radiation. The stopper cuts convection of air.
- Radiant heater: energy reaches you by radiation even if the air between you and the heater is cool.
- Roof insulation and woollen clothes: they trap air, and trapped air is a poor conductor.
- Climate: burning fuels and natural processes can change how much heat energy the atmosphere keeps. You only need the awareness that both human activity and natural processes are discussed as causes.
Revise
Key terms
The words examiners expect you to use precisely.
- Kelvin
- The SI unit of temperature. A change of 1 K is the same size as a change of 1 °C.
- Conduction
- Transfer of heat energy through a material by particles passing energy on, without the material flowing.
- Convection
- Transfer of heat energy by a fluid moving. Warmer, less dense fluid rises.
- Radiation
- Transfer of heat energy by infrared radiation. No particles are needed.
- Conductor
- A material that transfers heat energy quickly by conduction. Metals are good conductors.
- Insulator
- A material that transfers heat energy poorly, such as plastic, wood or trapped air.
Show 3 more key termsShow fewer key terms
- Expansion
- An increase in volume when a substance is heated, because particles move further apart.
- Emitter
- A surface that gives out infrared radiation. Dull, dark surfaces emit well.
- Absorber
- A surface that takes in infrared radiation. Dull, dark surfaces absorb well.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying the SI unit of temperature is the degree Celsius.
°C is the everyday scale. The SI unit is the kelvin.
Instead: Temperature: kelvin (K). Heat energy, like other energy, is in joules.
Mistake: Using convection to explain a metal spoon.
The spoon is a solid. The metal does not flow.
Instead: The spoon conducts. Convection is for liquids and gases.
Show 4 more mistakesShow fewer mistakes
Mistake: Saying radiation needs air.
The Sun’s energy crosses space.
Instead: Radiation does not need particles. That is how it differs from the other two.
Mistake: Saying a shiny surface is a good absorber.
Shiny surfaces reflect.
Instead: Dull, dark surfaces absorb and emit better. Shiny, light surfaces do both poorly.
Mistake: Saying trapped air conducts well.
Air is a poor conductor, which is why it insulates when it cannot flow.
Instead: Woollen clothes and cavity walls work because the trapped air conducts poorly.
Mistake: Saying expansion means the object gained mass.
The particles move apart. They are not added.
Instead: Volume increases, density falls, mass stays the same if the object is sealed.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
State the SI unit of temperature
The SI unit of temperature is the
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 40 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 2 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 2.
Mark scheme
PROChapter 2 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What is the difference between conduction, convection and radiation?
Conduction passes energy through a material by its particles, best in metals. Convection is warm fluid rising because it has expanded and become less dense. Radiation is infrared energy that needs no particles and can cross space.
What is the SI unit of temperature?
The kelvin (K). School thermometers show degrees Celsius. A change of one degree Celsius is the same size as a change of one kelvin.
Which surfaces absorb and emit radiation best?
Dull, dark surfaces. Shiny, light surfaces are poor absorbers and poor emitters, so they reflect radiation instead.
Why does a vacuum flask keep a drink hot?
The vacuum between the walls cuts conduction and convection, because those processes need particles. The shiny walls reduce radiation. The stopper stops warm air escaping by convection.
Is heat transfer in the Sec 2 EOY?
Most schools that follow the usual G2/G3 scheme teach this chapter in Sec 2 and include it in the Sec 2 end-of-year paper. A few schools move a topic across the two years, so check your school’s assessment outline.
Up next
Chapter 3: Chemical Changes