Getting the job done
So far we have concentrated on combining chemicals to get jobs done. However burning fuels with oxygen is not the only way to get these jobs done. It is possible to use other devices to drive the car along the road, or to lift a book onto the shelf. These might work by allowing a spring to relax or something to fall.
We do not have many examples of rubber band powered cars, although the alternative technology centre in Machynlleth does have a transport system powered directly by falling water. Electric trains, for which the wires supplying the electric current are connected to a hydro-electric generating station are another example of a falling-object driven transport system. This time the connection is indirect; there is an electric circuit connecting one change to another.
It does not seem appropriate to refer to either a relaxing spring or a falling object as a fuel (even though they get jobs done).
In just the same way as fuels, we refer to these as having energy stores associated with them:
This idea of stores of energy is fundamental to our treatment of energy. There are a number of changes associated with emptying energy stores. These are clues as to which kind of store will be relevant to the energy description.