Conservation law
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A conservation law is a statement used in Physics - a thing that one person who studies physics might say to another - that says that the amount of some thing does not change in time. That thing could be as simple as mass or charge, or something that has to be calculated, like energy, or angular momentum.
For example, if I say that mass is conserved, I would be making a conservation law. I would be saying that if I could measure the mass of the universe right now, I'd know what its mass will be tomorrow, because it will not change.
For a long time, people thought that a law just like that would be right. They also thought that there was a different law that said the same thing about the amount of energy in the universe. Later on Albert Einstein said that they were almost right, but also mass could change into energy (or the other way around). If this happened, it would be against the laws I just said, because if you change mass into energy, the total amount of mass goes down, and the total amount of energy goes up. But Einstein said we could still have a law if we added all the mass to all the energy. He said that even though the mass changes or the energy changes, the sum when you add them together does not change. So now we have just one conservation law for mass and energy together.
Of course, mass is measured in kilograms, and energy is measured in joules, so you cannot add them together right away, but Einstein told us how to add them together. Most people have heard of the equation E = mc2, and what it means is that before we can add the mass to the energy, we have to multiply the mass by the speed of light and then by the speed of light again.
Some of the things that we think right now are conserved are:
- mass and energy added together
- charge
- angular momentum
- momentum
Conservation laws can come in two types, global, or local. A global conservation law just says that the total amount of something in the universe does not change in time. A local conservation law says a little bit more than that. It says that if the amount of something changed in one place, it's because it moved in to or out of that place, and we can measure that movement.
Conservation laws are helpful for people when they do problems in Physics, because if they know that a thing is conserved, it gives them an extra bit of mathematical information about the thing they are doing the problem about.
