User talk:Paul Wormer/scratchbook

Energy is a property of a system that produces action (makes things happen) or, in some cases, has the "potential" to make things happen. For example, energy can put objects into motion, it can change the temperature of objects and it can transform matter from one form to another, such as (solid) ice into (liquid) water. Quantitatively, energy is a measurable physical quantity of a system which can be expressed in joules (the metric unit for a quantity of energy) or other measurement units such as ergs, calories, watt-hours, Btu, etc. Energy lights our cities, powers our vehicles, and runs machinery in factories. It warms and cools our homes, cooks our food, plays our recorded music, and gives us pictures on television. It is difficult, or maybe impossible, to give an all-embracing definition of energy because it exists in many forms, such as kinetic or mechanical energy, potential energy, thermal energy or heat, light, electrical energy, chemical energy, nuclear energy, etc. Indeed, it took scientists a long time to realize that the different appearances of energy can be transformed into each other, so that one is really considering the same property, which may indeed carry the same name (energy). In the middle of the 19th century scientists slowly became to realize that not only the different forms of energy can be converted into each other, but that the different forms of energy could be defined in such a way that energy is not lost in the conversion process.

Let us consider as a practical example of the conversion of energy a conventional coal-fired power plant. Such a plant takes as input coal (carbon) and air (oxygen). These two raw materials can combine, i.e., the coal is burned, and combustion energy, a form of heat, is generated. Combustion energy is converted into electrical energy which is transported to cities and factories through high voltage power lines. It would be vey nice, and would go a long way in solving the energy crisis, if all combustion energy would be converted into electrical energy. Unfortunately, this is not the case, the laws of physics do not allow it. Thermodynamics dictates that a large (as much as 60%) amount of the combustion energy is carried off as heat by cooling water. Although the hot water produced by the electricity plant is of little practical use and sometimes a burden to the environment, it still contains (thermal) energy that (theoretically not pracically) can be used to perform work.