ENERGY

"The energy of a system expresses the ability of that system to do some useful work or generate heat. Energy can be in many forms; e.g., mechanical energy, chemical energy, heat energy, electrical energy, etc. The different forms of energy can be converted into each other. It is a fundamental law of nature that energy can never be converted from one form to another 100%, some of the energy is always converted into heat energy during the conversion. Also, heat can never be converted 100% into any other form of energy. "