Heat Transfer — Conduction, Convection & Radiation

Conduction carries heat through a material without the material itself moving. Molecules at the hot end vibrate harder, knock into their neighbours, and pass the energy along the lattice. In metals a cloud of free electrons carries energy as well, which is why metals conduct so much better than anything else — copper is roughly eight times better than stainless steel, and hundreds of times better than plastic or air.

The rate depends on the thermal conductivity k of the material, the temperature difference across it, the area available and the thickness to be crossed. Watch the far-end temperature climb: with copper it arrives quickly, with steel it takes far longer, and a poor conductor barely passes anything at all — which is the whole idea behind lagging and firewall insulation.

In maintenance this is why a metal component picks up heat from anything it touches, why a heat sink is bolted tight with a thermal compound to fill the air gaps, and why you never judge a component's temperature by touching it: a good conductor at 60 °C feels far hotter than plastic at the same temperature, because it moves heat into your hand so much faster.

• Try this: Run copper to the far end, then switch to stainless steel. How much longer does the same rise take?
• Try this: Halve the heat input. Does the far end reach a lower temperature, take longer, or both?
• Try this: Why does the bar heat progressively rather than all at once?

Convection carries heat by moving the fluid itself. Fluid next to the heat source expands, becomes less dense, and is pushed up by the denser cool fluid around it. That cool fluid takes its place, is heated in turn, and a circulating convection current is set up. Convection therefore needs a fluid that is free to move — it cannot happen in a solid, and it stops if the fluid is trapped in small pockets, which is exactly how foam and fibre insulation work.

The result is a temperature gradient: the warmest fluid collects at the top and the coolest sits at the bottom. That is why a cabin's ceiling is warmer than its floor, why hot air rises from a heater and cold air falls from a cold window, and why a fire spreads upward first.

Aircraft use it constantly: fuel tank venting, oil cooling, cabin air distribution and battery cooling all depend on either natural convection or a fan providing forced convection. Turn the heat up and the loop moves faster, carrying more heat per second.

• Try this: Follow one particle round a full loop. Where does it rise, and where does it sink?
• Try this: Turn the heat up. What happens to the speed of the loop and to the temperature spread top to bottom?
• Try this: Why can convection not take place inside a solid metal bar?

Radiation needs no material at all. Every surface above absolute zero emits infra-red electromagnetic waves, which travel through a vacuum at the speed of light and warm whatever absorbs them. This is the only way the sun's heat reaches us across empty space, and the reason you feel the warmth of a hot engine casing before you touch it.

Two rules matter. First, the energy radiated climbs extremely steeply with temperature — as the fourth power of absolute temperature, so doubling the temperature in kelvin radiates sixteen times as much. Second, the intensity reaching a target falls off with the square of the distance: move twice as far away and you receive a quarter as much. Watch the readouts as you move the plate.

Surface finish matters too. Dark, dull surfaces absorb and emit well; bright, polished ones reflect and emit poorly. That is why heat shields, engine fire zones and thermal blankets are polished or foiled, and why a matt black surface is chosen when heat needs to be shed rather than kept.

• Try this: Set the distance to 1 m, note the intensity, then move to 2 m. Why does it fall to a quarter and not a half?
• Try this: Raise the heat input. Why does the radiated power climb so much faster than the temperature?
• Try this: Which of the three modes could still deliver heat across a vacuum, and why can the other two not?