One of the reasons energy is hard to conceptualize is that it is constantly changing from one form to another. When this happens, an energy conversion takes place. During these conversions, energy is changing between potential and kinetic forms of energy. (For more information, refer to What is Energy? Section B. Two Main Forms of Energy.)
For example, to operate a wind-up toy, kinetic energy from winding the toy is converted to elastic potential energy in the toy’s spring mechanism. After the spring is released, the elastic potential energy is converted back to kinetic energy when the toy moves.
During the conversion process, all the original forms of energy are turned into other forms of energy. No matter how the energy changes form, careful measurement and accounting of the energy before and after a conversion takes place shows that energy is always conserved. Despite repeated investigations over nearly 200 years, no exception to this has ever been found. As a result, scientists are confident enough in the conservation of energy principle to explain energy transfers and conversions that they refer to this principle as the First Law of Thermodynamics.
The following statements are various ways of expressing the first law of thermodynamics:
As a classroom teacher, you might want to know of an experiment that demonstrates the first law of thermodynamics to students. While such demonstrations exist, they are too advanced for the common classroom. Sometimes it helps to think of an analogy. Imagine you have a closed room that contains a jigsaw puzzle.
The room represents the universe, and the completed jigsaw puzzle is a concentrated source of energy (or an energy resource). Let’s say an agitated and angry cat is in the room, and this cat jumps on the puzzle, scattering the pieces all over the room, in the corners, under couches, in drawers, etc. It might look like the puzzle pieces are gone or missing, but with careful searching, all the pieces can be found.
Likewise, energy might appear to run out or get lost (e.g., when you run out of gas), but with careful examination, it is found to still exist. It might be in different forms, such as heat or sound waves, but it is not lost.
This information pertains to how energy is not destroyed, but what about energy not being created? We often talk about energy supply, development, production, and generation. All these involve converting one form of energy into another rather than energy being literally created. On Earth, the incoming energy from the sun is in relative balance with the amount of heat loss from our planet. This holds true for the universe as a whole. In other words, over the years, the amount of energy in our universe has remained constant. While energy is constant, its form and quality are continually changing. This leads us to the second law of thermodynamics.