User talk:Paul Wormer/scratchbook: Difference between revisions

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'''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, lets 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 <font style="font-family: sans-serif"> M(L/T)<sup>2</sup></font> (mass times length squared over time squared). The corresponding [[SI]] (metric) unit is [[joule]] [= kg(m/s)<sup>2</sup>]; other measurement units are ergs, calories, watt-hours,  Btu, etc.  Evidently, all these units have the dimension  <font style="font-family: sans-serif"> M(L/T)<sup>2</sup></font>,  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,<ref>Strictly speaking there is a distinction between heat and thermal energy. The distinction is that an object possesses thermal energy while heat is the transfer of thermal energy from one object to another. However, in practice, the words "heat" and "thermal energy" are often used interchangeably</ref> 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 because of its relatively low temperature, it still contains thermal energy that (theoretically not practically) could be used to  perform work. At lower ambient temperatures a larger part of the thermal waste energy is converted into useful electrical energy and in the hypothetical case of  zero  K (&minus;273 °C)  ambient temperature  all of the thermal  energy in the warmed cooling water is converted into electrical energy, which shows  that thermal energy is indeed  a form of energy. In any case, the thermal energy of the cooling water is important in the energy balance of the electricity plant:
::''Combustion energy &rarr; electrical energy + thermal energy''
Because energy is conserved, the combustion energy is equal to the sum of the electrical and the thermal energy.<ref> This is somewhat simplified, in practice  part of the combustion energy is lost to the hot combustion flue gases (carbon dioxide, nitrogen, water vapor, etc.)  that leave the plant. </ref>.
In the now following article the different manifestations of energy will be discussed in more detail.
==Note==
<references />

Revision as of 10:35, 8 March 2009

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