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 vehicles into motion, it can change the temperature of objects and it can transform matter from one form to another, e.g., energy will turn solid water (ice) of 0 °C into liquid water of 0 °C. Energy lights our cities, let our planes fly, and runs machinery in factories. It warms and cools our homes, cooks our food, plays our recorded music, and gives us pictures on television.

Quantitatively, energy is a measurable physical quantity of a system and has the dimension M(L/T)² (mass times length squared over time squared). The corresponding SI (metric) unit is joule [= kg(m/s)²]; other measurement units are ergs, calories, watt-hours, Btu, etc. Evidently, all these units have the dimension M(L/T)², and if one meets a physical property of a system with this dimension, one is entitled to call the quantity (part of) the energy of the system.

It is difficult, or maybe impossible, to give an all-embracing definition of energy, because energy 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 manifestations of energy are really the same property, and that in all cases it may rightfully carry the same name (energy). From the middle of the 18th to the middle of 19th century scientists became to realize that the different forms of energy can be converted into each other, and moreover that no energy is lost in the conversion processes.

Let us look at the conventional coal-fired power plant as a practical example of the conversion of energy. Such a plant takes as input coal (carbon) and air (oxygen). These two raw materials combine, i.e., 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 very nice, and would go a long way in solving the energy crisis, if all of the 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 the larger part of the combustion energy is turned into non-useable thermal energy, which in practice is carried off by cooling water. Although the cooling water heated by the electricity plant is of little practical use, it still contains thermal energy that (theoretically not practically) could be used to perform work. It is possible to extract useable energy from the cooling water, if it can be cooled down quickly enough, that is, if a sizeable flow of heat can be generated. This can be done, for instance, by making thermal contact with a supply of ice. If the temperature of the ice would be close to the absolute zero (− 273 °C), we could convert nearly all thermal energy contained in the cooling water into work. The fact that this can be done shows that thermal energy is indeed a form of energy. Clearly, it costs energy to produce ice, so this procedure is not followed in practice and the thermal energy of the cooling water is given off to the environment (often through cooling towers) as a waste product of the electricity plant. In any case, the thermal energy of the cooling water is an important factor in the energy balance of the electricity plant:

combustion energy → electrical energy + thermal energy

Because energy is conserved, the combustion energy is equal to the sum of the electrical and the thermal energy..