Conservation and dissipation

Saving energy

We are all familiar with the SAVE IT campaigns which urge people to SAVE ENERGY. You might be forgiven for thinking:

Hold on! Energy is conserved! Why do we need to save it? It saves itself!

In fact, you'd be right in thinking along these lines. There is an instructive paradox here which follows from the different usage of words in science and everyday contexts. In scientific terms it is not energy but fuels that get used up and it is these that we should make every effort to conserve. Energy is conserved (and there is nothing that we can do to avoid this conservation) but we need to make every effort to conserve fuels.

If we burn a piece of coal, the energy that was in the chemical store becomes spread out amongst other stores, most of which will be thermal stores of energy, as the surroundings undergo heating. It is very difficult to do anything useful with these dispersed stores of energy. There is a difference between having plenty of something useful around and being able to do something useful with it. A few sticks can be used to make a cheerful fire, but with sawdust scattered over a square kilometre of forest floor, it is rather harder to perform the same trick (not impossible, but needing significant input before getting something back). You may have the same quantity of wood, yet probably only the sticks are thought of as viable fuel.