Conduction and Convection

Thermal conduction — the transfer of energy through a material, from the hotter end to the cooler end, without the material itself moving.

How it works in any solid. Particles at the hot end vibrate more vigorously. They collide with their neighbours and pass some of that energy along, and so on through the material.

Why metals are far better conductors. Metals also contain free (delocalised) electrons. These move through the whole structure, colliding with ions and transferring energy very much faster than vibration alone can. This is the whole reason a metal spoon in hot soup becomes too hot to hold while a wooden one does not.

Insulators — materials with no free electrons and often with trapped air. Air is a poor conductor because its particles are far apart and collide rarely. Wool, fibreglass, polystyrene and feathers all work by trapping air.

Convection — the transfer of energy through a fluid (a liquid or a gas) by the movement of the fluid itself.

The mechanism, in order:

1. Fluid near the heat source warms up.
2. The particles move faster and spread apart, so the fluid expands.
3. The same mass now occupies more volume, so its density falls.
4. The less dense warm fluid rises, and denser cool fluid sinks to take its place.
5. That fluid is warmed in turn, and a circulating convection current is set up.

The step examiners look for is the density one. "Hot air rises" is an observation; "hot air is less dense, so it rises" is the explanation.

Convection cannot happen in a solid, because the particles are fixed in position and cannot flow.

Example: a radiator warms a room mostly by convection, not radiation, despite the name. Air heated at the radiator rises to the ceiling, spreads across, cools, sinks at the far wall and returns along the floor. This is why radiators sit under windows — the rising warm air meets the cold draught coming down the glass.