Why these stores?
Physics Narrative for 11-14
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|
|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.
Don't think of
light: think of
lighting, or maybe
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.
Don't think of
sound: think of
sounding, or maybe