When an object has been heated, its thermal store of energy increases. As we have seen this can affect the substance in multiple ways – it can increase the temperature or cause the substance to change state. In KS3 and KS4 this is usually the level of understanding that we need, as we are describing the bulk properties of the material – i.e., how it’s behaving.

At times we also want to consider what is happening at a sub-microscopic level (i.e., what is happening to the particles themselves). When we are doing this, it is common to change the language, and the language used does not fit in with the conventions we have used throughout the rest of this module.

This language choice of internal energy is just another label of describing the thermal store of energy. It is included here only because it appears in many schemes of work.

The internal energy of a collection of particles can be thought of as the sum of the kinetic store of energy of all the particles and the electric store of energy in the bonds between the particles (sometimes called potential energy). This is usually written as:

The potential energy changes when the bonding between particles changes; this is associated with state changes.

Students need to take care with absolute temperatures, as it is tempting to think that  is twice the temperature of , is is not, it is only the difference between and  (a change of under 4%).

This visualisation can help students to have a clearer mental model of what ‘bucket’ is being filled up as you heat a substance – going through all the five different stages. As you heat up a solid you are first increasing the speed the particles are moving (as its temperature increases), but then you are overcoming the strong bonds that hold those particles together. This carries on in the liquid phase if you keep heating it. Knowing this should enable students to understand and make comparisons between different situations – for example, that an amount of gas, even if it is the same temperature as when it was a liquid, contains more energy.