Before you read this, I suggest you read post 16.35.
In post 16.34 we saw that heat flows spontaneously from hot things to cold things – this leads to the idea of entropy that was discussed in more detail in posts 16.35 and 16.38.
Conduction (post 19.14) is one mechanism for the flow of heat. It is especially important in solids where molecules are close together (post 16.37) and so likely to collide, transferring their kinetic energy by inelastic collisions. Since heat is simply kinetic energy on the molecular scale (post 16.35), these collisions lead to transfer of heat.
In fluids (liquids and gases) the molecules are free to move (post 16.37). It is very unlikely that all the molecules with high kinetic energy will move together – they are much more likely to move randomly (post 16.38). They will then mix with molecules that have lower kinetic energy. So, heat (kinetic energy on the molecular level) becomes more evenly distributed.
This process, where heat is transferred by molecular motion, is called convection.
There are two forms of convection – forced convection and free convection.
In forced convection, work (post 16.20) is done to move the molecules. An example is when we blow across the surface of a hot drink, to cool it. We blow air to create movement of the hot vapour above the drink. More of the liquid then evaporates to replace the hot vapour. As a result, the liquid loses heat (latent heat of vaporisation – see post 16.37).
Free convection is a spontaneous process – no work needs to be done. Molecules with high kinetic energy simply mix with other molecules, as described above.
In post 17.31, we saw how rising columns of warm air, called thermal columns or thermals, enable a glider to gain altitude. Radiation from the sun heats the earth that, in turn, heats the air. The hot air molecules can’t move downwards, into the earth, and so they tend to rise.
The creation of thermals is usually explained by hot air expanding, so that a fixed mass of gas has an increased volume – so it has a lower density (post 16.44) than cold air. As a result, the hot air floats (post 17.6) above the cold air. If you read post 16.35, you will see that this expansion is simply a consequence of increased molecular motion – so the explanations in the previous paragraph and this paragraph are simply different ways of saying the same thing.
Sometimes it is more convenient to think of matter as a collection of molecules; other times it’s simpler to think of it as a continuum with bulk properties like density. We can use whichever approach is simplest to understand a phenomenon or solve a problem. For example, it’s easier to think of flotation (post 17.6) and fluid flow (post 17.15) as properties of bulk matter and not think about molecules. But to understand how soap works (post 16.48) or diffusion through membranes (post 18.27), we have to think about molecules. In the case of free convection, both approaches work equally well.
19.14 Fick’s law of diffusion and conduction of heat