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So far, we have learned that energy is a measure of the capability of an object or system to do work, and we have also learned about the basic different forms of energy.
But these concepts still don't quite do justice to the full concept of energy, for energy has a number of very special additional properties we have not fully discussed yet. If you think about these carefully, and don't take them for granted, you'll realize that they don't follow from simple intuition. Rather, these properties had to be discovered or proven somehow. We'll explore briefly how these properties were proven in the next section. In this section, we'll first review them:
These properties are;
Energy can be transferred from one object or system to another through the interaction of forces between the objects (unlike the condition of, say, being the color red, which is intrinsic to the object in question).
Energy comes in multiple forms: kinetic, potential, thermal (heat), chemical, electromagnetic, and nuclear energy. (as discussed in the previous section).
In principle, energy can be converted from any one of these forms into any other, and vice versa, limited in practice only by the Second Law of Thermodynamics (we discuss the Second Law, that is "entropy", in a later section).
Energy is always conserved, that is, it is never created anew or destroyed - this is called the First Law of Thermodynamics. Thus, when an object does work on another object, the energy can only be converted and/or transferred, but never lost or generated anew. In a sense, energy is like perfect money - transferred but always preserved, assuming no inflation or deflation!
Although most people are aware of these facts nowadays and take them for granted, these are really amazing properties if you stop and think about them. How was anyone ever able to prove such properties? These properties go far beyond the intuitive concept of energy given at the beginning of this primer. You may find this hard to see now, because we generally take these ideas for granted. But for thousands of years, people didn't have a clearly defined concept of energy, and didn't know, for example, that there is a definition of "energy" which refers to a quantity that is always conserved.
Moreover, even after kinetic energy and potential energy became understood, it still took people centuries to figure out that heat is just another form of energy.
Before our present understanding of physics evolved, it was still a logical possibility that the Universe might have been constructed quite differently, such that energy, in the sense of power to modify the world, would not have been conserved and/or things in even everyday life might have been controlled by some kind of supernatural beings. We can now see easily that such a world would likely look very different from our own, because the basic properties of energy are actually responsible for "constraining" many aspects of our world: Everything from the branching structures of trees to the way that our bodies and the planets move are all strongly constrained by the properties of energy.
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