Conduction, Convection and Radiation

There are exactly three ways heat gets from a hotter place to a colder one, and telling them apart is worth marks on its own. By the end of this lesson you should be able to describe the mechanism of each, say which ones need a material to travel through and which does not, rank everyday materials as conductors and insulators and justify the ranking from particle spacing, calculate how fast heat flows through a slab of material, and explain why a metal handrail feels colder than a wooden one at the same temperature.

Conduction: passing it along without moving the material

Hold one end of a metal rod in a flame and the other end becomes too hot to hold, although no piece of metal has travelled from one end to the other. What has happened is that the particles at the hot end are vibrating violently, and each one collides with its neighbours and shares some of that vibration with them; those neighbours pass it on in turn, and a wave of increased vibration works its way along the rod. Each particle stays roughly where it was. That is conduction: energy handed from particle to particle through a material without the material itself going anywhere. It is the dominant mechanism in solids, because a solid's particles are locked in place and touching, which is ideal for handing energy on and hopeless for anything else.

Metals do it far better than anything else, and for a second reason on top of the vibration. In a metal, some of the electrons — the light, negatively charged particles that belong to each atom — are not tied to any particular atom at all but are free to move throughout the whole piece of metal. When one end is heated those free electrons pick up energy and, because they are free and extremely light, they carry it across to the cool end directly and very fast, rather than passing it hand to hand. That is why metals conduct heat one or two orders of magnitude better than non-metallic solids, and why the two properties of conducting heat well and conducting electricity well always turn up together.

Gases are the worst conductors, and the particle picture says why: their particles are far apart with a great deal of empty space between them, so they rarely meet and have few opportunities to hand energy on. Liquids sit in between — their particles are touching, like a solid's, but disordered and not held in a lattice. So the ranking of conductors follows the ranking of how closely and how rigidly the particles are packed, with metals promoted above the rest by their free electrons. Written as numbers, in watts per metre per degree, copper is about 400, steel about 50, glass about 1, water 0.6, wood about 0.15, and still air 0.025 — copper conducts around sixteen thousand times better than air.

Why a metal handrail feels colder than a wooden one

Leave a steel bar and a wooden batten in the same unheated room overnight and they end up at exactly the same temperature, because anything left long enough in one place ends up matching its surroundings. Touch them the next morning and the steel feels bitterly cold while the wood feels almost neutral. Your hand is not measuring their temperature. Nerve endings report the rate at which heat is leaving your skin, and the steel is drawing it away hundreds of times faster than the wood can, so your skin cools quickly and reports cold. On a hot day the same effect runs in reverse and metal feels hotter than wood at the same temperature, because it delivers heat into your hand just as efficiently. The property being sensed is conductivity, not temperature, which is exactly why a thermometer and a fingertip disagree.

How fast heat conducts through something

That comparison can be made properly quantitative, and it turns the whole subject of insulation into arithmetic. The rate at which heat crosses a slab of material — joules per second, which is watts — grows with the material's conductivity, grows with the area available to cross, grows with the temperature difference driving it, and falls as the slab is made thicker. All four appear in one expression.

The useful thing about that arrangement is that the conductivity and the thickness sit in different places, so a poor conductor and a thick layer buy you the same result — which is why the cheapest insulation in the world is a thick layer of trapped air.

Convection: moving the material itself

The second mechanism needs a fluid, meaning a liquid or a gas, and it works by carrying the warmed material bodily to somewhere else. Heat one region of a fluid and its particles move faster and push each other further apart, so the same mass of fluid now occupies a larger volume — and density is mass divided by volume, so its density has fallen. From the matter and fluids module, a less dense fluid floats upwards through a denser one. So the warmed region rises, cooler and denser fluid flows in underneath to take its place, is warmed in turn, and rises as well, while the material that rose earlier cools higher up, becomes denser again and sinks. The result is a continuous circulation called a convection current, and it is a far more effective way of moving heat through a fluid than conduction, because a fluid conducts badly and a current can carry energy metres in seconds.

This is why heating appliances are placed low and cooling elements high. A kettle's element sits at the bottom, so the water it warms rises and the whole kettle is stirred; put the element at the top and the hot water would simply stay there. A room heater on the floor sets up a circulation that warms the whole room, and the object we call a radiator does most of its work this way rather than by radiating, which makes its name misleading. The freezing compartment goes at the top of a refrigerator so that the air it chills sinks through the rest of the cabinet. And convection is impossible in a solid, because nothing can flow, and impossible in a vacuum, because there is nothing to flow.

Radiation: needing nothing in between

The third mechanism is the odd one out, because it needs no material at all. Every object emits electromagnetic waves because of the motion of its particles, and for objects at everyday temperatures the emission is mostly infrared — the band of the electromagnetic spectrum just beyond the red end of visible light, covered in the energy and waves module. Being electromagnetic waves, they travel at the speed of light, they carry energy, and they cross a vacuum perfectly happily. When they strike another object they are absorbed and their energy becomes internal energy there, which is to say the object warms up. The Sun's energy reaches us this way across 150 million kilometres of near-empty space, which by itself rules out both other mechanisms: there is nothing there to conduct through and nothing there to circulate.

Two properties of radiation are worth fixing now. It travels in straight lines from its source and can be reflected, so it can be blocked by an obstacle and bounced back by a shiny one, neither of which is true of conduction. And every object emits it all the time, hot or cold — a block of ice radiates, just far less than a radiator does. What you feel standing in front of a fire is the net effect: you absorb more from the fire than you emit back towards it.

Defeating all three

Insulation is the practical test of whether you understand the three mechanisms, because a good insulator has to attack each one separately. Take a vacuum flask. The gap between its double walls is evacuated, which removes both conduction and convection at a stroke — no particles, so nothing to hand energy along and nothing to circulate. Both facing surfaces are silvered, which cuts radiation across the gap. The stopper closes the top, where convection would otherwise carry warm air straight out, and the inner vessel is supported on small plastic pads rather than metal ones, because any solid bridge between inside and outside would conduct around the vacuum.

Building insulation works on trapped air, and the word trapped is doing the work. Air is an excellent insulator at 0.025 watts per metre per degree, but only if it cannot circulate; a wide open gap simply sets up a convection current and moves heat across efficiently. So loft insulation, foam, cavity-wall fill and a woollen jumper all do the same thing — they hold air still in a great many small pockets. Double glazing follows the same logic: two panes with a narrow sealed gap of air or argon between them, narrow enough that convection cannot get going. And it explains why wet clothing is dangerous rather than merely uncomfortable: water displaces the trapped air, and water conducts about twenty-four times better, so the insulation largely disappears.

Why it matters in aviation

All three mechanisms are at work on one aircraft at the same time. In a piston engine, conduction carries heat out of the cylinder walls into the fins cast onto them, and forced convection then removes it into the passing airflow — forced, because the air is being driven past rather than left to circulate on its own, which is why cooling depends on airspeed as well as on power setting and why a long slow climb asks more of an engine than a cruise does. Radiation matters wherever a surface faces something much hotter or much colder than itself, which in an engine installation means the hot sections and the structure around them.

Ice protection is the same physics used on purpose rather than coped with. A thermal anti-icing system routes hot air bled from the engine compressor through the inside of a leading edge, and the heat conducts outwards through the skin to the surface where ice would otherwise form; an electrically heated element does the same job by passing a current through a resistive layer bonded into the structure. In both cases what is being designed is a rate of heat flow through a known thickness of material to a known area, which is exactly the quantity this lesson puts a formula to.

What the test asks

The headline distractor is the metal one: an item describes two objects in the same room, one metal and one not, and offers the metal as being at a lower temperature. Both are at room temperature, and the metal only feels colder because it conducts heat out of your hand faster. Expect also a mechanism-identification item where radiation and convection are the two plausible options, and the deciding detail is whether there is any material in the path at all — vacuum, or a gap, means radiation. A third distractor claims that convection happens in solids, usually phrased as heat rising through a metal bar; nothing can flow in a solid, so it cannot. And on insulation, watch for the option that credits the vacuum in a flask with stopping all three mechanisms. It stops two of them, and radiation crosses a vacuum without difficulty, which is what the silvering is for.