Physics With Kate
Empowering Girls in Physics
Spec-aligned revision resources, group courses, and 1:1 tutoring for GCSE and A Level Physics — built by an experienced teacher and examiner.
Edexcel International GCSE Physics · Section 4 (b) Energy transfers
Conduction, Convection and Radiation
In this lesson you'll learn to: explain conduction using vibrating molecules that
collide, explain how a convection current forms in a liquid or gas, and explain
absorbing and emitting infrared radiation from matt black and shiny white surfaces —
in the exact words the mark scheme is looking for.
▶
Prefer to watch? Scroll to the bottom for the video on this topic
▶
A video walkthrough is coming soon — it will appear at the bottom of this page
▶
Start here — see all three
Three tabs, three ways of moving energy. Heat one end of a metal bar and watch the vibration passed along — then switch the material to trapped air and watch nothing happen. Turn the burner up under a balloon and watch the air thin out until it rises. Put a matt black box and a shiny white box in the Sun, then leave them out on a cold night.
The panel on the right writes the answer as you go, and the mark-scheme key words light up as you make them happen. Those are the words that earn the marks.
Material:
What is happening
Mark-scheme key words — they light up as you see them
▲ On the conduction tab, switch to trapped air and press Heat it. The far end stays at 20 °C — that is what "good insulator" means.
Now learn to write it down
These are wording questions, not calculations. The notes below give you the keyword chain for each method, and the three phrases examiners specifically reject.
✎
Revision notes
The three ways energy is transferred
Every thermal energy question is one of these three, or a mixture of them. The first thing to decide is which one — and that usually follows from the state of matter.
| Method | Happens in | What actually moves | Needs particles? |
|---|---|---|---|
| Conduction | mainly solids | the energy — the particles stay put and vibrate | yes |
| Convection | liquids and gases only | the particles themselves, carrying energy with them | yes |
| Radiation | anything, including a vacuum | infrared waves | no |
⭐ Why convection can't happen in a solid
A convection current needs the particles to be free to move and change places. In a solid they are locked in position, so they can only vibrate — which is conduction.
Conduction
Learn this definition
Conduction is the transfer of energy through a substance when vibrating molecules pass energy on to their neighbours by colliding with them.
Four steps, and there is a mark for each of the key words. Write them in this order:
- The end being heated gains energy, so its vibrating molecules vibrate more.
- These molecules collide with the neighbouring molecules.
- That makes those molecules vibrate more too.
- Step by step this is energy transfer along the material.
💡 Exam tip — the particles do not travel
In a solid the particles stay in the same place. Only the energy moves along. Saying "the hot particles move to the cold end" describes convection, not conduction, and scores nothing.
Good and poor conductors. Metals are good conductors — their particles are packed close together, so collisions happen constantly. Trapped air is a poor conductor, which is the same thing as saying it is a good insulator: its particles are so far apart that they hardly ever collide. That is why bubble wrap, cotton wool, cavity walls and the double wall of a takeaway cup all work by trapping air.
Convection
Learn this definition
A convection current is the circulation set up when warmer, less dense fluid rises and cooler, more dense fluid sinks to replace it.
Six steps, six marking points. A four-mark question wants any four of them, in order:
- The air particles gain KE — the air near the heat source warms up.
- They move further apart, so the air expands.
- The hot air is less dense and rises.
- Energy is transferred upwards with it.
- Cooler, more dense air replaces it, sinking as the warm air rises.
- The process repeats, setting up a convection current.
✗ Examiners reject
"the particles expand""the particles become less dense"
"heat rises"
✓ Write instead
"the particles move further apart""the air becomes less dense"
"the warm air rises"
⭐ Key fact — it is the air that expands, not the particles
A particle is always the same size. Heating gives the particles more kinetic energy so they move further apart, and it is the gas that expands and becomes less dense. This is the single most common way to lose the mark.
Radiation
Learn this definition
Infrared radiation is energy transferred as a wave. It needs no particles at all, so it is the only method that works through a vacuum — which is how energy reaches us from the Sun.
Everything depends on the surface. Two things to know, and they are not the same thing:
| Surface | As an absorber | As an emitter | So it ends up… |
|---|---|---|---|
| Black / matt | good absorber | good emitter | hottest in the Sun; coolest overnight |
| Shiny / white / silver | poor absorber (good reflector) | poor emitter | stays cooler in the Sun; stays warmer overnight |
The mark scheme wants you to say which way round the energy is going:
- A colder object in the Sun is absorbing: a good absorber of radiation reaches a higher temperature.
- A hotter object in a cold room is emitting: a good emitter of radiation falls to a lower temperature.
💡 Exam tip — answer the direction you were asked about
If the question is about something warming up in the Sun, talk about absorbing and ignore emission — the mark scheme says so explicitly. If it is about something hot cooling down, talk about emitting. The same matt black surface is good at both, which is why naming the wrong one still loses the mark.
⭐ Key fact — a vacuum flask uses all three
Trapped air or a vacuum between the walls stops conduction and convection; the shiny silver surface is a poor emitter, so it cuts radiation. A stopper on top stops warm air escaping by convection.
✍
How to write each one
Fill in the gaps and you have a full-mark answer every time.
Conduction. The metal gains energy, so its
vibrating molecules vibrate more.
They collide with neighbouring molecules and make them vibrate more too,
so there is energy transfer through the solid.
They collide with neighbouring molecules and make them vibrate more too,
so there is energy transfer through the solid.
Convection. The air particles gain KE and
move further apart.
The hot air is less dense, so it rises and energy is transferred upwards.
Cooler, more dense air replaces it, setting up a convection current.
The hot air is less dense, so it rises and energy is transferred upwards.
Cooler, more dense air replaces it, setting up a convection current.
Radiation. The black / matt surface is a
good absorber of infrared radiation.
It is the colder object, so it absorbs energy and reaches a higher temperature.
It is the colder object, so it absorbs energy and reaches a higher temperature.
🔢
Model answers
These are the questions from page 8 of the workbook, answered the way the mark scheme wants them. Notice how every marking point is a separate short sentence.
QUESTION 1A takeaway cup has a plastic lid, a double wall with trapped air between the layers, and a shiny silver outer surface. Explain how the design keeps the drink hot.6 marks
Open by naming the effect
The cup reduces the rate of energy transfer to the surroundings, so the
drink stays hot for longer.
Conduction — two points
1. The trapped air between the double walls is a poor
conductor / good insulator.
2. So energy transfer by conduction through the sides is reduced. The plastic lid is a poor conductor too, reducing conduction through the top.
2. So energy transfer by conduction through the sides is reduced. The plastic lid is a poor conductor too, reducing conduction through the top.
Convection — two points
3. The lid traps the air and stops warm air escaping.
4. So energy transfer by convection from the surface is reduced.
4. So energy transfer by convection from the surface is reduced.
Radiation — two points
5. The shiny silver outer surface is a poor emitter of
infrared radiation.
6. So energy transfer by radiation is reduced.
6. So energy transfer by radiation is reduced.
Six marks = six separate points, two per method
Allow IR for radiation. Reject "it stops the cold getting in" — cold is not a
thing that travels.
QUESTION 2A large jacket potato is baking in an oven. The oven transfers energy to the outer surface. Explain how energy is transferred to the centre.3 marks
Name the method first
Energy is transferred through the potato by conduction.
Then the mechanism
The particles at the hot surface vibrate more / gain kinetic energy.
They make the neighbouring particles vibrate more, passing energy on by
collisions, and this transfers energy to the centre of the potato.
conduction · vibrate more · collisions · to the centre
Allow "particles" for "molecules" and "gain KE" for "vibrate more".
Ignore "heat rises" — it earns nothing here.
QUESTION 3(a)A fence rail is painted with alternating matt black and shiny silver stripes. On a sunny day, explain which stripes reach the highest temperature.2 marks
Two sentences, two marks
1. The matt black stripes, because black is a better
absorber…
2. …of (infrared) radiation from the Sun.
2. …of (infrared) radiation from the Sun.
the matt black stripes — good absorber of infrared radiation
You could instead say the silver is a poor absorber / good reflector.
Ignore any reference to emission — the question is about absorbing.
QUESTION 3(b)Explain how a convection current forms in the air just above the warm rail.4 marks
Any four of the six — keep them in order
1. The air near the rail is heated, so its temperature increases (the particles gain KE).
2. The air expands — the particles move further apart.
3. The density of the warm air decreases, so the warm air rises.
4. Cooler, denser air moves in to replace it, and the process repeats, setting up a convection current.
2. The air expands — the particles move further apart.
3. The density of the warm air decreases, so the warm air rises.
4. Cooler, denser air moves in to replace it, and the process repeats, setting up a convection current.
Four clear marking points
Reject "particles expand" and "particles become less dense".
"Cool air sinks" is fine; "heat rises" is not.
🧪
Practical: Keep the Ice!
Page 9 of the workbook is the challenge: keep the largest mass of ice by the end of the session. You weigh the ice at the start, design a container, and weigh it again at the end. The write-up asks you to explain your design against all three methods — so it is the same keywords again, in the negative.
- Conduction. Bubble wrap and cotton wool trap air, a poor conductor. Its molecules are too far apart to vibrate and collide effectively, so there is very little energy transfer to the ice.
- Convection. A lid and a sealed tub trap the air, so warm air cannot rise and be replaced — the convection current never gets going.
- Radiation. Shiny foil, shiny side out, is a poor absorber, so it reflects radiation away instead of letting it warm the ice.
💡 Exam tip — write the negative version
An insulation question is the same six or seven keywords with "reduced", "trapped" or "cannot" in front of them. If you can explain how energy is transferred, you can explain how a design stops it.
📘 Now do it in your workbook
The notes page to fill in, the four exam-style questions above, and the Keep the Ice! practical challenge — with the mark scheme so you can see where every mark comes from.
Edexcel iGCSE Physics Workbook · ENERGY · pages 7–9Free sample = the three pages for this lesson. The full workbook covers the whole of Section 4 Energy resources and energy transfer with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
▶
Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
All three methods explained out loud, with the exact wording each one needs.
All three methods explained out loud, with the exact wording each one needs — the video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook pages above cover every mark.
▲ In GHL you can also use the lesson's built-in video field instead of this embed.
Now put it into practice
Pages 7–9 of the workbook are free. These are wording marks, so the only way to know whether you have them is to write the answers out and check them against the mark scheme.
Proud member · Accredited · Trusted partners
© 2026 Physics With Kate · All rights reserved