The Law of Conservation of Energy says that in our universe the amount of energy remains constant. Energy cannot be created nor destroyed; it only changes form from one kind of energy to another.
Conservation is a defining feature of energy. Let’s say that one could total up all the energy in our universe. Then, one could write this number and say that here’s what we mean by “energy,”: this amount of the ability to do work. This is, really, the major thing we know about what energy is. It’s a number that tells us the amount of work that can be done. In addition, we do know a lot about what energy does as opposed to is.
Example of what energy does. For example, when Alice throws a ball, her arm uses chemical energy in her body (calories). The chemical energy transforms into the energy of motion in her arm (kinetic energy), which then transforms into the energy of motion in the ball as it moves through the air. When the ball lands, the energy of motion transforms into heat energy which the ball imparts to air molecules it disturbs and the dirt it lands on.
Alice probably wouldn’t notice the heat energy; but, in fact, the ball would have encountered some air resistance along the way (friction), jiggling the air molecules, which is another way of saying, it would heat them. And, when it landed, the ball would have moved some grains of sand and dirt, which is to say, would have imparted motion, jiggling them. When our fingers sense heat, they are really feeling that particles are moving quickly. And so on–the energy would continue to move on, transforming from one kind to another.
Convertibility of energy with mass. Energy can convert to matter and matter can convert to energy. This is the meaning the Einstein’s famous equation: E = mc^2. The Law of Conservation of Energy says that the total amount of matter and energy is constant. Since physicists know that the two can convert into each other, when they speak of the Law of Conservation of Energy, the term energy includes both matter and energy. In other words, the combined total of both is a constant number.
No Law of Conservation of Matter. Prior to Einstein’s Special Theory of Relativity, published in 1905, which yielded the equation, E = mc^2, physicists believed that there was also a Law of Conservation of Matter. Physicists had believed, at least since the time of Newton, that the amount of matter in the universe was constant. They believed that matter could not be created nor destroyed. In fact, in the 1950’s, when I was in grade school, I was taught both the Law of Conservation of Energy and the “Law of Conservation of Matter.” Apparently, no one had let my fifth-grade science teacher in on what E = mc^2 means.
No conservation of elements. In addition, prior to the discovery of radioactivity, physicists believed that there was a specific unchanging number of each type of atom in our universe. They believed that alchemists had been sadly mistaken: one type of atom, such as lead, could not be converted into another type of atom, such as gold.
Physicists Sir Earnest Rutherford and Frederick Soddy were the first to publish a paper (1902) stating that radioactivity involves the conversion of one element into another. Reportedly, upon making this discovery, Soddy said, “It’s transmutation!” Rutherford shot back, “Don’t say that, Soddy, they’ll have our heads as alchemists!”
In fact, physicists can now transmute one element into another, just as Nature does. The problem that alchemists of the Middle Ages ran into is that they couldn’t access enough energy to convert one type of atom into another. When physicists do this type of conversion they must use the huge quantities of energy generated in particle accelerators.