Heat Transfer: Conduction, Convection and Radiation — KS3 Physics Guide

Thermal energy (heat) always transfers from a warmer region to a cooler one — never the other way round — until both regions reach the same temperature. At KS3, you need to know the three mechanisms by which this transfer happens: conduction, convection, and radiation. Each works differently, involves different states of matter, and explains different everyday phenomena.

What is thermal energy transfer?

When particles have more energy, they move (vibrate, translate, or rotate) faster. This energy can be passed on to neighbouring particles — that is heat transfer. Heat transfer stops when all parts of a system reach the same temperature (thermal equilibrium). The three mechanisms differ in how the energy is moved and whether a material medium is needed.

1. Conduction

Conduction is the transfer of thermal energy through a material by direct contact between neighbouring particles, without the particles themselves moving from place to place.

How does conduction work?

In a solid, particles are held in fixed positions and vibrate. When one end of a solid is heated, those particles vibrate more energetically and pass some of that vibrational energy to their neighbours. The energy passes along the solid particle by particle — like a "Mexican wave" of vibration — without any particle physically travelling the length of the solid.

In metals, conduction is much faster because metals have free electrons — electrons not bound to individual atoms — that can move freely and carry energy quickly through the metal.

Conductors and insulators

Material | Type | Why
Copper | Good conductor | Many free electrons carry energy rapidly
Aluminium | Good conductor | Free electrons, lightweight
Glass | Poor conductor (insulator) | No free electrons, tightly bonded structure
Wood | Insulator | No free electrons, porous structure
Air | Very poor conductor | Particles far apart, energy passes slowly
Expanded polystyrene | Very good insulator | Traps pockets of air (the real insulator)

Real-world examples of conduction

A metal spoon left in hot soup becomes hot along its handle.

Touching a metal bench on a cold day feels colder than touching a wooden bench at the same temperature, because metal conducts heat away from your hand faster.

Double-glazed windows use a gap (air or gas) between the panes to reduce conductive heat loss from a building.

2. Convection

Convection is the transfer of thermal energy through a fluid (liquid or gas) by the bulk movement of the fluid itself, driven by differences in density.

How do convection currents form?

A region of fluid is heated (e.g. by a radiator or a flame beneath a pan).

The heated fluid expands, becoming less dense than the surrounding fluid.

The less dense fluid rises.

Cooler, denser fluid moves in to take its place from the sides and below.

The cooler fluid is then heated in turn.

The cycle continues, forming a convection current — a circulating loop of fluid.

Key point: Convection only works in fluids (liquids and gases), not in solids, because the particles must be free to move in bulk.

Real-world examples of convection

Central heating: A hot water radiator heats the air directly around it; that warm air rises, cool air sinks in to replace it, and a convection current circulates warm air around the room.

Sea breezes: Land heats up faster than the sea during the day. Warm air rises over the land; cooler air from the sea flows in — a coastal breeze.

Thermals: Birds of prey and gliders use rising columns of warm air (thermals) created by convection over warm ground.

Boiling a kettle of water: Warm water at the base rises, cool water descends, distributing heat through the entire volume.

3. Radiation

Radiation (also called thermal radiation or infrared radiation) is the transfer of thermal energy by electromagnetic waves — specifically infrared (IR) waves — without needing any matter at all.

Key properties of radiation

Travels through a vacuum (no medium required).

Travels at the speed of light (3 × 10⁸ m/s in a vacuum).

All objects emit and absorb infrared radiation; hotter objects emit more radiation at higher frequencies.

The amount of radiation emitted or absorbed depends on the colour and texture of the surface.

Which surfaces are best at emitting and absorbing radiation?

Surface | Absorption | Emission
Dull, black (matt) | Best absorber | Best emitter
Shiny, white (polished) | Worst absorber (best reflector) | Worst emitter

A matt black surface both absorbs and emits radiation most effectively. A shiny silver surface reflects radiation and emits very little — that is why emergency blankets are silver-coated.

Real-world examples of radiation

The Sun's energy reaches Earth through 150 million km of (mostly) empty space — only radiation can do this.

Grills and toasters heat food by infrared radiation emitted from the hot element.

Infrared thermometers measure temperature by detecting the IR radiation emitted by an object.

Wearing dark clothes in summer causes you to absorb more solar radiation and feel warmer; pale clothes reflect more radiation and keep you cooler.

Comparing the three methods

Feature | Conduction | Convection | Radiation
Requires a medium? | Yes (solid works best) | Yes (fluids only) | No (works in vacuum)
Particle movement | Vibration in place | Bulk flow of fluid | No particle movement
Works in solids? | Yes | No | Yes (surface to surface)
Works in fluids? | Yes (but slowly) | Yes | Yes
Works in a vacuum? | No | No | Yes
Main application | Conduction along solids | Heating rooms/fluids | Sun's energy, grills

Reducing heat transfer in buildings

Architects and engineers use all three mechanisms when designing energy-efficient homes:

Loft insulation (glass wool or mineral wool): fibres trap air pockets, reducing conduction and convection through the roof.

Double glazing: the air or gas gap reduces conduction; the sealed gap prevents convection between panes.

Cavity wall insulation: foam or fibres fill the gap between the inner and outer walls, reducing convection currents in the cavity.

Reflective foil behind radiators: reflects infrared radiation back into the room instead of letting it heat the wall.

How heat transfer fits the KS3 national curriculum

The Department for Education's KS3 science programme of study requires pupils to understand "heating and thermal equilibrium: temperature and thermal equilibrium" and "the differences in heat transfer in solids and fluids through conduction, convection, and radiation." BBC Bitesize KS3 Physics covers conduction, convection, and radiation with diagrams and worked applications as part of the Year 8 energy topic.

Common mistakes

Mistake 1 — Saying convection happens in solids. Convection requires particles to move in bulk — only possible in fluids. In solids, particles are fixed and can only vibrate; thermal energy travels by conduction instead.

Mistake 2 — Saying "heat rises." Heat does not have direction. What rises is the warm fluid, which is less dense. The phrase "heat rises" is shorthand for convection and should be explained correctly in an exam.

Mistake 3 — Thinking radiation requires air. Infrared radiation travels through a vacuum. The Sun heats the Earth across 150 million km of near-empty space. If radiation needed a medium, the Earth would be permanently frozen.

Mistake 4 — Confusing "good insulator" with "good reflector." A good insulator (like wool or polystyrene) mainly reduces conduction. A good reflector (like shiny silver foil) mainly reduces radiation. Both reduce heat loss, but by different mechanisms.

Frequently asked questions

What is the difference between conduction and convection?

Conduction transfers energy through a solid (or a fluid at a microscopic level) by passing vibrations between neighbouring particles without the particles themselves moving far. Convection transfers energy through a fluid by the bulk movement of the fluid — entire parcels of warm fluid rise and cool fluid sinks. Conduction works best in solids (especially metals); convection only occurs in liquids and gases. Think of conduction as handing a parcel down a line of people standing still, and convection as someone walking the parcel across the room.

Why do metals feel colder than wood at the same temperature?

Both a metal bench and a wooden bench may be at exactly the same temperature — say, 10 °C on a winter day. But when you touch the metal, it conducts heat away from your hand very quickly (metal is a good conductor), so your skin temperature drops rapidly and you perceive the metal as "cold." Wood is a poor conductor, so heat transfers away from your hand much more slowly, and the wood feels warmer. The temperature of the object is the same; the sensation differs because of different rates of conduction.

Why does radiation not need a medium?

Radiation is an electromagnetic wave, not a mechanical vibration of particles. Electromagnetic waves are oscillating electric and magnetic fields that can propagate through empty space — there is no need for matter to be present. In contrast, conduction and convection both require matter to carry the energy (either by vibration or by bulk movement of particles). This is why the Sun can heat the Earth across the vacuum of space through radiation, while conduction and convection play no role over that distance.

How does a vacuum flask keep drinks hot?

A vacuum flask (Thermos) is designed to reduce all three types of heat transfer. The vacuum between the inner and outer walls eliminates conduction and convection (no medium to carry energy). The silvered inner walls reflect infrared radiation back into the flask (reducing radiation loss). The stopper reduces convection of warm air out of the flask. The combined effect keeps hot drinks hot (and cold drinks cold) for hours.

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