4.Main Content
guidance
The principle of conservation of energy
Energy is conserved. What does this really mean, and why is it true?
Water is more or less conserved. So the amount of water in a reservoir can always be calculated from the amount that was there some time ago, plus the amount that has come in, minus the amount that has gone out (you may have to take account of evaporation as well as water drawn off).
Another way of saying the same thing is that water can’t be made or destroyed. For there to be more, it has to come in; for there to be less it has to go out.
Energy is similar. If you take any volume of space, then the total energy inside that volume at a given time is always the amount that was there earlier, plus the total amount that has come in through the surface, minus the total amount that has gone out through the surface.
Another way of saying the same thing is that energy can’t be made or destroyed. For there to be more, it must have come from somewhere; for there to be less it must have gone somewhere else.
This means that energy can quite correctly be thought of as ‘rather like a fluid’. You may picture it as stored or as flowing. You may sensibly ask where it is, where it is going, where it is coming from. [It is not exactly like water. For example, you can measure an amount (in J) and a rate of loss or gain (in W). But you can’t ask at what velocity the ‘stuff’ flows, because there isn’t any ‘stuff’ whose ‘particles’ would have a velocity. We mention this only to keep a few niggles at bay.]
Energy is ‘rather like a fluid’ because it is conserved, not the other way round. It is a calculated quantity, not observable ‘stuff’. The practical teaching implication here is that it is important to do sums about energy changes – how much in, how much out – and not just to talk generally about it.
The conservation laws, such as the conservation of energy, give physics its backbone. They are not really statements of knowledge but they contain implicit assumptions and definitions. They are however tied to the natural world, and they contain experimental knowledge.
Updated 17 Jul 2009