Why these stores?

Calculating changes to stores

The different stores that are presented here cover all of the processes that you may care to describe from the energy perspective.

There is, however, a deeper significance to selecting these stores. For each of the chosen stores it is possible to calculate, and therefore quantify, the change in energy as processes happen and shifts in energy occur.

To see if the store is being filled or emptied, look for a physical clue:

Store filled when …	physical clue
elastic	material stretched or squeezed, or relaxed
kinetic	object speeds up or slows down
chemical	atoms combine
thermal	material warms up
vibration	amplitude increases or decreases
nuclear	nuclear fission or fusion
gravity	massive objects separate, or fall together
electric and magnetic	magnets or charged particles alter separation

Later these physical clues will lead to measurements that'll enable you to calculate the changes of the energy in the store.

What about light and sound?

The list of stores is likely to be familiar to you from previous experiences teaching about energy. One big difference is that here, we are referring to different stores of energy rather than different kinds of energy (we find find the description kinds rather too slippery to be helpful).

You may also have noticed that we do not list a light energy store. The reason for this is that whilst it is quite natural to refer to a fuel such as domestic gas as having a chemical store of energy, it is not helpful to talk about light as an energy store. The fuel can do the job of heating your home, but what can a beam of light do? Ultimately, the beam of light interacts with the surroundings (being absorbed, reflected or scattered) and produces a very slight warming effect. In other words it leads to the filling up of a thermal store of energy. We shall return to the case of light in episode 03 when we introduce the idea of power in pathways. It turns out that light is better thought of as a mechanism for shifting energy around rather than as a store … but more of that later.

Teacher Tip: Don't think of light: think of lighting, or maybe radiating.

In the SPT: Sound topic, we argued that sound was a good label for the whole process of vibrations travelling from source to detector. We hope you can see the link between this and light. When you go to the store, you buy your light bulbs and sound systems for their power output, usually quoted in watts. That measure reports the accumulations in a store in each second, not the quantity in the store. You might find it hard to think of sounding, rather than sound, but once the song is over (the process has finished), all that's happened, on the energy front, is that some thermal stores have filled. Much more on this in the SPT: Radiations and radiating topic.

Teacher Tip: Don't think of sound: think of sounding, or maybe radiating.