How does thermal energy travel?
Conduction, convection and radiation
Physics · 3A · 12 October 2026
The sea is at 20 °C.
The air is at 20 °C.
You step in. In the water you feel cold. In the air, you don't.
Why?
- Water is colder
- Water drains energy faster
- Only in our head
Before the new topic
Three words that are easy to mix up
temperature T
A property of an object, linked to the average kinetic energy of its particles (K or °C).
internal energy
The total energy of all its particles (kinetic and potential): it is stored in the object (J).
thermal energy transfer (heat, Q)
Energy that moves from a hotter place to a colder place because of a temperature difference. It is a process, not something stored (J).
Three ways to transfer thermal energy
conduction
through a material, particle to particle, without the material moving
convection
through a fluid, carried by the moving fluid itself
radiation
by infrared waves, with no material needed at all
All three need a temperature difference. We take them one at a time.
1 · Conduction
Remember the four cubes?
On 8 September: glass, metal, wood, plastic. All at room temperature, all read the same on a thermometer.
- The metal cube felt the coldest
- Same temperature: so what is the finger really detecting?
Thermal conduction
Definition
Thermal conduction is the transfer of thermal energy through a material, from the hotter part to the colder part, without the material moving.
- a thermal conductor passes energy on well (metals)
- a thermal insulator passes very little (wood, plastic, air)
- it needs a temperature difference: energy flows from high T to low T
The animation
How to read it: conduction
- Change: the material slider (wood → copper); the heater temperature; then the race view
- Observe: the atoms only vibrate in place; thermometers A, B, C rise one after the other; the graph shows when each reaches 30 °C
- In a metal: the bright dots are free electrons; brighter = more kinetic energy; a flash marks a collision that passes energy on
- It tells us: energy travels along the rod, the atoms do not
Time scale is compressed: real wood is far slower than shown.
Why are metals such good conductors?
non-metals
- the hot end vibrates more
- atoms collide with their neighbours and share energy
- energy passes on, atom after atom: slow
metals (Supplement)
- the same vibrations, plus
- delocalised (mobile) electrons: free to move through the metal
- fast electrons from the hot end collide with cooler atoms and give them energy: fast
How good a conductor is it?
| Material | κ (W m⁻¹ K⁻¹) | compared with air |
|---|---|---|
| Copper | ≈ 400 | ≈ 15 000 × |
| Steel | 15 – 50 | ≈ 600 – 2000 × |
| Glass | ≈ 1 | ≈ 40 × |
| Water | ≈ 0.6 | ≈ 23 × |
| Wood | ≈ 0.15 | ≈ 6 × |
| Air | ≈ 0.026 | 1 |
κ = thermal conductivity, typical values near room temperature (beyond the 0653 syllabus; the book orders materials from best conductor to best insulator).
Back to the sea
Water conducts about 23 times better than air
Skin at about 33 °C, water at 20 °C: energy leaves the skin much faster than into air at 20 °C.
Same temperature, different rate of energy transfer.
Is conduction the whole story?
Insulators: air is the key
- Air is a very poor conductor: trapped air insulates
- Glass wool, wool, polystyrene, feathers: mostly air held in small spaces
- Two thin layers of clothing are warmer than one thick layer: the air between them
- Why only trapped air? Free air also carries energy by convection (next block)
2 · Convection
An electric kettle: where is the heating element?
- At the top
- In the middle
- At the bottom
At the bottom: hot water rises and cold water sinks to replace it, so the whole kettle warms. With the element at the top, only the top layer would get hot.
Convection
Definition
Convection is the transfer of thermal energy through a fluid by the movement of the fluid itself.
- a fluid can flow: liquids and gases
- a convection current: moving fluid that carries energy from a warmer place to a cooler one
- not in solids: their particles stay in fixed positions
Conduction: the material does not move. Convection: the material moves.
The animation
How to read it: convection
- Change: source = heater or ice cube; position (bottom ↔ top); power; flow arrows on or off
- Observe: hot particles (red) are further apart and rise; cold (blue) are closer and sink; the arrows show the circulation; top and bottom thermometers
- Compare: heater at the bottom, then at the top (no current); ice at the top, then at the bottom
- It tells us: the energy travels because the fluid itself moves, and the motor is the difference in density
Particle model: spacing between particles shows density. Not to scale.
Why does hot fluid rise?
1
The fluid is heated: its particles move faster
higher average kinetic energy
2
It expands: the same mass takes up more volume
particles further apart
3
\rho=\dfrac{m}{V}\ \text{decreases}\ (m\ \text{constant},\ V\ \text{larger})
less dense, not "lighter": the mass has not changed
4
The denser, colder fluid around it pushes it upwards
cold fluid sinks to take its place: a convection current
Convection at work
heater in a room
warm air rises from the heater, cooler air flows in to replace it: the room is shared out
fridge
freezer at the top: cold air sinks and cools the food below; warm air rises to be cooled
cavity wall
air in the gap carries energy across by convection; foam in the gap stops the air moving
The foam conducts a little: but it stops the convection, so overall it helps.
Back to the sea, again
The water next to your skin is always replaced
The thin layer of water that has warmed up moves away; cold water takes its place. Your skin never gets a warm coat.
In air the same happens with the wind: a breeze removes the thin warm layer of air next to the skin.
Water: conduction × 23, plus convection. Still, the sun warms us on the beach: how does that energy get here?
3 · Radiation
The Sun is 150 million km away. Between us: almost nothing.
How does thermal energy reach the Earth?
- By conduction
- By convection
- By radiation
Conduction and convection both need a material to pass through. Space is almost empty: only radiation can cross it.
Infrared radiation
- emitted by all objects: the hotter the object, the more it emits each second
- an electromagnetic wave: it travels through a vacuum, in straight lines
- it warms the object that absorbs it
- invisible to the eye, felt by the skin, seen by a thermal camera
- a hot object and a cold one both emit and absorb: the net flow goes from hot to cold
Beyond the book
Every object above 0 K emits. In which part of the spectrum?
| Object | T (K) | peak λ | region |
|---|---|---|---|
| human skin | 310 | 9.3 µm | infrared |
| red-hot iron | 1000 | 2.9 µm | infrared (and a red glow) |
| lamp filament | 3000 | 0.97 µm | near infrared (and visible) |
| the Sun | 5800 | 0.50 µm | visible light |
Wien's law: \lambda_{\max}=b/T, b=2.898\times10^{-3}\ \mathrm{m\,K}. Hotter, shorter.
The syllabus says "mainly infrared": true at everyday temperatures. Thermal radiation is a continuous spectrum whose peak moves with T.
Colour and finish of the surface
matt (dull) black
- good absorber
- good emitter
- poor reflector
shiny silver or white
- poor absorber
- poor emitter
- good reflector
A surface that is good at absorbing is good at emitting. Always in pairs.
Experiment (Experimental Skills P3.01 of the book)
Two cans: matt black and shiny silver
Same water, same lids, same distance from the lamp. Which can's temperature rises faster? And if both start hot, which cools faster?
- Matt black, both times
- Shiny silver, both times
- Black heats faster, silver cools faster
The same surface is better at absorbing and at emitting: black heats faster and cools faster.
The animation
How to read it: radiation
- Change: experiment (lamp heats the cans / hot cans cool); lamp temperature; finish of the lower can; air or vacuum
- Observe: the rays (more and darker when more energy flows), the rays reflected by a shiny can, the two curves T against t and the rates in °C/s
- Air against vacuum: in air the particles also carry energy away by conduction and convection, whatever the colour; in vacuum only radiation is left
- It tells us: energy crosses empty space; and the surface decides how much is absorbed and emitted
Simple model, illustrative numbers. The black can has the same value in every run.
Air or vacuum: what changes?
In vacuum the can cools more slowly, and colour matters entirely. A vacuum flask is built on this.
Beakers in a room at 20 °C
- A emits more than it absorbs: it cools. B: equal, T constant. C absorbs a little more than it emits: it warms.
Radiation in everyday life
car sunshield
shiny: reflects the Sun, the car stays cooler; the black dashboard absorbs and gets too hot to touch
white house, light clothes
poor absorbers: they reflect the radiation of the Sun
campfire
the warmth on your face is almost all radiation; the air above warms by convection
clear night, frost
the ground radiates to space and gets little back: it cools below the air
Putting it together
The vacuum flask
- ① vacuum between the walls: no conduction, no convection
- ② silvered surfaces: reflect infrared, so little radiation
- ③ stopper (plastic): stops convection and evaporation at the top
- ④ glass and supports: poor conductors
A solar water heater
Why is …
- the panel painted black?
a matt black surface is a good absorber of infrared - the back insulated?
to reduce loss by conduction (and convection) - cold water in at the bottom, hot out at the top?
the warmed water rises: convection current
After Question P3.31 of the coursebook.
Which mechanism is it?
1
Could it happen in space?
yes → radiation (the only one that needs no material)
2
Is the material itself moving?
yes, a liquid or a gas → convection
3
Energy passing through a still material?
→ conduction (usually a solid)
Often more than one acts at the same time: name them all.
Back to the sea, last time
So why does 20 °C water feel colder than 20 °C air?
- Conduction: water conducts ~23 times better than air
- Convection: the water next to the skin is replaced all the time (wind does it in air)
- Radiation: in air, infrared exchange with the room is a real share of the loss; in water it is dwarfed
One more thing
You get out of the shower, or out of the sea. The air is at 20 °C.
Wet, you feel even colder than in the water. That is why we dry ourselves.
- The water on the skin is colder than the air
- The water on the skin evaporates
- The towel is warm
Evaporation takes energy from the skin. Is it conduction, convection or radiation? Neither: next slide.
Evaporation: a fourth way to lose energy
- The most energetic particles escape from the surface of the liquid
- The ones left behind have a lower average kinetic energy: the liquid cools (here, the skin)
- Faster with higher T, larger surface area, air moving over the surface (wind, fan)
- A towel removes the water film: there is nothing left to evaporate from the skin
- Wet clothes insulate badly: water replaces the trapped air, and it also evaporates
Beyond the syllabus: an order of magnitude
Is it really that much?
1
Q=mL=0.010\ \mathrm{kg}\times 2.4\times10^{6}\ \mathrm{J\,kg^{-1}}=2.4\times10^{4}\ \mathrm{J}
evaporating just 10 g of water takes about 24 kJ (L: energy per kg to evaporate, ≈ 2.4 MJ/kg near skin temperature)
2
t=\dfrac{Q}{P}=\dfrac{2.4\times10^{4}\ \mathrm{J}}{100\ \mathrm{W}}=240\ \mathrm{s}
our whole body gives out about 100 W at rest: that is 4 minutes of all of it
L: specific latent heat of vaporisation of water (coming up in the lesson on changes of state). 100 W: figure quoted in the coursebook.
In short
- Conduction: energy passes through a still material, by vibrations and (in metals) free electrons
- Convection: a fluid carries energy by moving; hot fluid is less dense and rises
- Radiation: infrared, no material needed; good absorbers are good emitters
- The skin senses the rate of energy loss, not the temperature; evaporation is a fourth way to lose it
Tomorrow: mock test on everything so far. Friday: midterm.
If there is time · 1
Give the two states of matter in which convection can happen.
- solids and liquids
- liquids and gases
- solids and gases
Liquids and gases (fluids): they can flow. In solids the particles stay in fixed positions.
If there is time · 2
Electric water heater: describe how a heater at the bottom heats all the water. [3]
- water near the heater is heated: its particles gain kinetic energy and it expands
- it becomes less dense, so it rises
- colder, denser water sinks to take its place: a convection current carries energy through all the water
If there is time · 3
A hot metal cube has four different faces
All faces are at the same temperature. An infrared detector is held the same distance from each face.
- Which face gives the highest reading? Which the lowest?
- Why must the distance be the same for every face?
- Answers: matt black highest, shiny silver lowest; the reading depends on distance too, so it must be controlled (fair test).