Energy Transferred by Conduction
Energy and Thermal Physics

Warming with particles: how does it happen?

Physics Narrative for 11-14 Supporting Physics Teaching

Conduction in solids: the particles do not change neighbours

Let's think about conduction in solids first. Here each particle vibrates but ends up going nowhere over the longer term. This is true of every particle, so each particle has fixed neighbours. In solids, only conduction is possible. Warm one end of a solid, and you get lots of vibration at one end, not so much at the other. The vibrations are transmitted from particle to particle, so gradually warming the whole solid object.

This is a good picture for what happens with electrical insulators, or non-metals. But metals, which can conduct electricity, also turn out to be better thermal conductors than non-metals. Why is this?

The clue is in the electrical conduction. Although the atoms are fixed, they share some of their electrons, and these are free to move within the metal. These electrons can shoot off from the hot end, with significant energy in their kinetic store. This energy is shared with the atoms at the cooler end, so warming these atoms more rapidly than if the sole mechanism was transfer of vibrations from atom to atom, as in the electrical insulators.

Overall, in solids the vibrations in one chunk of material are transmitted to the next but without any of the atoms having to move from one chunk to another.

Conduction and convection in liquids and gases, evaporation

For liquids and gases the picture is more complicated. Now the particles can change neighbours but it is still the case that more particle movement in one chunk can lead to more particle movement in the next, without the mass movement of the particles. However this transmission of movement in liquids and gases is not very effective. That is why air is such a good insulator. Your clothing relies on trapped pockets of air to keep you warm.

Where the particles are closer together, as in water, conduction is better precisely because it is easier for the random movements of the particles in one volume to affect the random movements of the particles in the next. But conduction in still not much better in water than it is in air. We rely on this in the design of wetsuits for diving that trap an insulating layer of water next to the skin.

Sometimes there is mass movement of particles, so whole volumes of fluid particles move, taking the energy in their thermal store on the journey. This is called convection, and the flow of such volumes results in convection currents.

Since convection relies on the floating and sinking of such volumes of fluid, the detailed discussion of convection is in the SPT: Motion topic.

The particles in liquids and gases are not all moving at the same speed. If you take some water in a glass, some particles move faster than others and therefore contribute more than an average amount to the thermal store of that water. Random collisions between the particles result in continuous redistribution of energy as particles speed up or slow down. If you selectively remove these faster particles, then the liquid cools. For liquids this process is called evaporation.

Evaporation occurs from liquids at all temperatures as particles escape from the body of the liquid. The fastest moving particles (with the greatest share of the energy in the thermal store) are those that have the greatest chance of escaping. As the fastest moving particles escape, the average speed of the remaining particles necessarily falls (remove the fastest particles and the average speed of those remaining must fall). The average energy per particle of the fluid therefore drops and so the temperature also falls. This fall in temperature is what keeps you cool when sweat evaporates from your skin.

Energy Transferred by Conduction
appears in the relation ΔQ=-kΔθ/Δx
is a special case of Energy Transferred by Heating
is used in analyses relating to Conductive Heating
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