Conduction, Convection and Radiation
Three ways thermal energy moves. Each has its own mechanism, and the marks are for explaining the mechanism — not just naming the process.
1. Conduction
Conduction is the transfer of thermal energy through a substance without the substance itself moving.
It happens mainly in solids.
The mechanism — two stages, and good answers give both:
- Particles at the hot end vibrate more (they gain kinetic energy)
- They collide with neighbouring particles, passing energy along
- In metals only: free (delocalised) electrons gain kinetic energy, move through the metal and transfer energy rapidly by colliding with other particles
Free electrons are why metals are such good conductors. Vibration alone is slow; the mobile electrons carry energy far faster. Any full-mark conduction answer for a metal must mention them.
Give the detailed mechanism, not just “the heat travels through”. Tutors flagged this specifically — the marks are for vibration, collision and free electrons.
Conductors and insulators:
- Good conductors: metals — copper, aluminium, silver
- Poor conductors (insulators): wood, plastic, glass, air, water
Gases and liquids are poor conductors because their particles are far apart, so collisions are less frequent. Trapped air is one of the best insulators, which is why wool, fibreglass and foam work.
Why a metal feels colder than wood at the same temperature:
The metal conducts thermal energy away from your hand quickly, so your skin cools rapidly and you sense “cold”. The wood conducts slowly, so your hand stays warm. Both are at the same temperature — the difference is the rate of energy transfer from you.
This was a recorded misunderstanding, and it is a favourite exam question. The object is not colder; it removes energy from you faster.
2. Convection
Convection is the transfer of thermal energy by the movement of the fluid itself — the heated particles carry energy with them.
It happens in liquids and gases only, never in solids, because the particles must be free to move.
The mechanism — the full chain, and every link is a mark:
- The fluid near the heat source is heated
- Its particles move faster and spread further apart, so the fluid expands
- The same mass now occupies a larger volume, so its density DECREASES
- The less dense warm fluid RISES
- Cooler, denser fluid sinks to take its place
- This sets up a circulating convection current
Hot fluid rises because it is LESS DENSE, not because the particles are lighter. A recorded error was attributing it to the “mass of particles” — the mass of each particle is unchanged. What changes is how spread out they are, and therefore the density.
Cold air is denser because its particles are closer together, not because they weigh more.
Convection currents go UP, then round. Warm fluid rises, cools at the top, sinks at the sides, and returns. Describing convection as sideways, or reversing the direction, were both recorded errors — get the rise at the heat source right and the rest follows.
Everyday examples: heating water in a pan, a room heated by a radiator, sea breezes, hot-air balloons.
A heater is placed near the FLOOR so the warm air rises and circulates through the whole room; a freezer compartment goes at the TOP so cold dense air sinks through the fridge.
3. Thermal radiation
Thermal radiation is the transfer of energy by infrared electromagnetic waves.
It is unique among the three:
Radiation needs NO medium — it is the only way energy reaches us from the Sun, across the vacuum of space.
All objects emit and absorb infrared radiation. The hotter the object, the more it emits.
Surfaces matter — and this is heavily examined:
| Surface | Emitter | Absorber | Reflector |
|---|---|---|---|
| Matt black | best | best | worst |
| Shiny silver / white | worst | worst | best |
Matt black is the best absorber AND the best emitter. Shiny silver is the best reflector and the worst at both. Confusing shiny and matt surfaces for absorption was a recorded error — and note that a good absorber is also a good emitter, which students often find counter-intuitive.
Uses: dark solar panels absorb efficiently; the shiny backing of a radiator reflects energy into the room; survival blankets are silver to reflect body radiation back.
Being a good thermal CONDUCTOR is unrelated to being a good RADIATOR. Conduction depends on the material (free electrons); radiation depends on the surface (colour and texture). A shiny metal is an excellent conductor and a poor radiator. Confusing the two was recorded directly.
4. Reducing heat loss — the vacuum flask
The classic question, because it needs all three mechanisms.
| Feature | Reduces |
|---|---|
| Vacuum between the walls | conduction and convection — no particles to transfer energy |
| Silvered surfaces | radiation — reflects infrared back |
| Plastic stopper / lid | convection (and evaporation) — traps the fluid |
| Plastic supports | conduction through the casing |
The vacuum stops conduction AND convection, because both need particles. It cannot stop radiation, which is why the silvering is needed as well. A recorded confusion was over how an air gap helps: trapped air is a poor conductor and, if the gap is thin enough to prevent circulation, it also limits convection — but a vacuum stops both outright.
In houses: loft insulation and cavity wall insulation trap air; double glazing uses a gap between panes; carpets and curtains reduce conduction and convection.
5. Which process is it?
| Clue | Process |
|---|---|
| Through a solid, nothing moving | Conduction |
| A fluid circulating, warm rising | Convection |
| Across a vacuum or through empty space | Radiation |
| Involves surface colour | Radiation |
| Involves free electrons | Conduction |
| Involves density changes | Convection |
Conduction in liquids is possible but poor — if a liquid is being heated and circulating, it is convection. Confusing the two in liquids was recorded.
6. Mistakes that cost marks
Naming the process without explaining the mechanism.
Omitting free electrons from a metal conduction answer.
Saying hot fluid rises because it is “lighter” rather than less dense.
Getting the direction of convection currents wrong.
Saying convection happens in solids.
Confusing shiny and matt surfaces for absorbing and emitting.
Forgetting that a good absorber is a good emitter.
Confusing a good conductor with a good radiator.
Saying radiation needs a medium.
Saying a metal is “colder” than wood rather than a faster conductor.
Frequently asked questions
What is conduction? Transfer of thermal energy through a substance by particle vibration and collision, plus free electrons in metals — without the substance moving.
Why are metals good conductors? They contain free electrons that move through the metal transferring energy rapidly.
What is convection? Transfer of thermal energy by the movement of the fluid, driven by density differences.
Why does hot air rise? It expands, becoming less dense than the surrounding air.
Can convection happen in solids? No — the particles cannot move freely.
What is thermal radiation? Transfer of energy by infrared waves, requiring no medium.
Which surface is the best absorber? Matt black — and it is also the best emitter.
Which is the best reflector? Shiny silver or white.
Why does metal feel colder than wood? It conducts energy away from your hand faster — both are at the same temperature.
How does a vacuum flask reduce heat loss? The vacuum stops conduction and convection; the silvering reflects radiation; the stopper stops convection and evaporation.
Quick revision checklist
- I can explain conduction with vibration, collision and free electrons
- I know why metals conduct well and gases badly
- I can explain why metal feels colder than wood
- I can give the full convection chain ending in density
- I know hot fluid rises because it is less dense
- I know convection needs a fluid
- I can describe convection current directions
- I know radiation is infrared and needs no medium
- I know matt black absorbs and emits best
- I know shiny silver reflects best
- I don’t confuse conductors with radiators
- I can explain every feature of a vacuum flask
- I can identify which process a situation involves
These notes cover conduction, convection and radiation in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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