Shifting and conserving energy
Physics Narrative for 11-14
From one store to another
The next step in this energy story involves recognising that when one energy store empties, another energy store elsewhere must be filling. The energy is just shifted from store to store. Energy is not used up, but is conserved.
For example, combining chemicals to lift an ornament up onto a shelf gets the job done, but raising this ornament also sets up another kind of store. This store involves the ornament positioned higher in the Earth's gravitational field. From the energy perspective, this process is seen in the following way:
Energy is shifted from the chemical store of your muscles to the gravitational store of the ornament in the Earth's gravitational field.
You can then work with this gravitational store by allowing the ornament to fall to do another job. At this point, we must admit that there are few, if any, useful jobs we can think of that are based on falling ornaments, but pile drivers work in just this way as they hammer steel foundations into the ground. The general point is that the second store can be emptied to fill yet another store of energy as a job gets done.
These ideas can lead to a whole host of questions:
- Can emptying the second store (associated with the ornament in a higher position) do an equivalent job to that done by emptying the first (the chemical store associated with the muscles of your arm)?
- If not, what happens to the energy not shifted from the first store to the second?
- Is it possible to reduce this shortfall?
These are very practical questions concerned with getting the best results from designed devices and with not squandering the resources available to use. It was concerns such as these that led to the law of Conservation of Energy and to the subsequent growth in importance of the idea of energy.