Conduction, Convection and Radiation

Leave a mug of tea on the table and it goes cold. Warm your hands on it and they heat up. Stand in sunshine and your face glows. In every case energy is on the move — and it always travels the same way round: from the hotter thing to the cooler thing, never the other way, until everything reaches the same temperature.

But how does that thermal energy actually get from hot to cold? Nature has exactly three ways of doing it, and the whole of this page is about telling them apart:

Conduction — energy passed along through a solid, particle to particle.
Convection — energy carried by a moving fluid (a liquid or a gas).
Radiation — energy beamed across a gap as infrared waves, needing nothing to travel through.

They sound similar — all three "make things warmer" — but they work by completely different machinery, and knowing which is which is one of the most useful ideas in all of physics. Learn them by contrast: what each one needs, and what it can't do.

Conduction: passing it hand to hand

Hold a metal spoon in a hot cup of tea and, within a few seconds, the far end you're holding goes warm too — even though that end never touched the tea. That is conduction.

Picture the particles of the spoon. At the hot end, they are jiggling ferociously. They bump into their slower neighbours and pass some of that jiggle on; those neighbours speed up and jostle the next ones, and so the energy is handed down the line — like a whispered message passed along a row of people, or a Newton's cradle sending a click from one end to the other. Crucially, the particles themselves stay where they are: they vibrate harder and pass energy on, but they do not travel along the spoon. Only the energy moves.

Because the particles have to be close enough to knock into each other, conduction works best in solids, where particles are packed tight in a fixed lattice. It is feeble in liquids and gases, whose particles are spread far apart with little to bump into — which is exactly why gases like air are such good insulators.

Metals are the champion conductors, and there is a lovely reason. Inside a metal, the outer electrons of every atom are not tied to any one atom — they roam free through the whole lump as a "sea" of free electrons. These tiny, fast movers pick up energy at the hot end and shuttle it across to the cold end almost instantly, on top of the slower particle-to-particle jostling. Wood, plastic and glass have no free electrons, so their only trick is the slow bumping — which is why a wooden spoon stays cool in the same hot pan that makes a metal one too hot to hold. It's also why a metal railing feels colder than a wooden one on the same frosty morning: the metal conducts heat out of your hand far faster, even though both are at the very same temperature.

Convection: the fluid carries the heat with it

Conduction can't get very far in a liquid or a gas — so fluids use a different, cleverer trick. When you heat the bottom of a pan of water, the water down there warms up, and warm water expands. Spreading the same particles over more space makes it less dense — lighter for its size — so it floats upwards, exactly as a cork bobs up through water. Cooler, denser fluid sinks down to take its place, gets heated in turn, and rises too.

The result is a never-ending loop called a convection current: hot fluid rising, cooling at the top, sinking at the sides, and being reheated at the bottom. Notice the huge difference from conduction — here the warm particles themselves travel, physically carrying their energy along for the ride.

Convection is everywhere once you spot it:

A radiator (badly named — it heats a room mostly by convection, not radiation) warms the air beside it; that air rises to the ceiling, cools, sinks down the far wall and drifts back — stirring the whole room warm.
Sea breezes: on a sunny day the land heats faster than the sea, so warm air rises over the land and cool air sweeps in off the water.
A hot-air balloon floats because the air inside it, heated by the burner, becomes less dense than the cooler air around it.

And here is the catch that defines convection: it needs particles that are free to flow. That means it can only happen in a fluid — a liquid or a gas. It can never happen in a solid, because a solid's particles are locked in place and can't rise or sink.

Radiation: heat that crosses empty space

Now the strangest of the three. Conduction and convection both need stuff — particles to bump, or fluid to flow. But the warmth of the Sun reaches us across 150 million kilometres of empty space, where there are almost no particles at all. It can't be conduction or convection. It is thermal radiation.

Radiation is energy carried by infrared — a kind of electromagnetic wave, the invisible cousin of visible light. Like light, it travels through a vacuum at the speed of light and needs no medium whatsoever. This is the only one of the three that can cross empty space, and it's how every ray of the Sun's heat gets to you.

A few key facts, all worth knowing:

Every object emits it. You, this page, an ice cube — everything above absolute zero glows with infrared. The hotter an object, the more radiation it pours out (and the more it also absorbs from its surroundings).
The surface matters enormously. Dark, matt (dull) surfaces are the best emitters of infrared and the best absorbers of it. Light, shiny surfaces are poor emitters and poor absorbers — they are good reflectors instead, bouncing radiation away.

That surface rule explains a hundred everyday things: a black car gets scorching in the sun while a white one stays cooler; houses in hot countries are painted white to reflect the Sun's heat; the shiny foil blanket wrapped round a marathon runner reflects their own infrared back onto them; and solar hot-water panels are painted matt black to soak up as much radiation as possible.

See all three, side by side

Use the top control to switch between conduction, convection and radiation, and drag the Run it slider to set each one going. Watch the contrast: in conduction the energy hops along a solid bar while the particles stay put; in convection a whole current of fluid loops round, physically carrying the heat; in radiation the energy leaps a completely empty gap as infrared waves.

These three mix-ups cost more marks than anything else in this topic:

Conduction does not move the particles along. It's tempting to imagine hot particles marching from the hot end to the cold end — but they don't. Each particle stays roughly where it is, vibrating harder and knocking the energy on to its neighbour. The energy travels; the particles stay home. (Moving particles that carry their energy with them — that's convection, a completely different thing.)
Convection needs a fluid — it cannot happen in a solid. Rising and sinking only works if the particles are free to flow, so convection is limited to liquids and gases. A solid brick can conduct and it can radiate, but it can never convect, because its particles are locked in place.
Radiation needs no medium at all. Don't say the Sun's heat is "conducted" or "convected" through space — there's practically nothing out there to conduct or convect. Only radiation can cross a vacuum, which is precisely why it's the one that reaches us from the Sun.

A vacuum flask (a "Thermos") keeps a hot drink hot for hours — and a cold drink cold — by cleverly shutting down every one of the three heat-transfer routes at the same time. Look at how it does it:

Conduction and convection are blocked by a vacuum. The flask has a double wall with the air sucked out of the gap between them. With almost no particles in that gap, there's nothing to conduct heat across it and nothing to set up a convection current — both routes are starved of the stuff they need.
Radiation is blocked by silvering. The inner walls are made shiny and mirror-like. A shiny surface is a poor emitter, so little infrared escapes from the hot drink, and the shiny outer wall reflects outside radiation away — so radiation is stopped too.
The stopper at the top, made of plastic or cork (poor conductors, no free electrons), plugs the last easy escape route for conduction and convection out of the neck.

Defeat conduction, convection and radiation together and heat has almost nowhere to go — which is the whole trick behind the flask, and behind the shiny, vacuum-panelled walls of a modern cool-box too.