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Energy: Home Energy Audit Introduction Energy Transfers and the First Law of Thermodynamics In the 1800's, scientists found, empirically, that rules exist that determine how energy can be transferred. The first of these rules is called the First Law of Thermodynamics. This law is usually stated as, "Energy can neither be created nor destroyed; it can only be transferred from one form to another." This often leads to the re-titling of this law as the Conservation of Energy Principle since it says that energy must be conserved. This statement of the First Law does not say anything about how energy can be transferred, though. It turns out that there are only two ways. This was discovered in 1850 by the English scientist James Joule, who found that heat and work are equivalent methods for changing the energy of an object. In his experimental work, Joule was able to show that he could increase the thermal energy of a pot of water by either placing it over a flame (adding heat), or by stirring it with a paddle (doing work). For this and other important work in this area, the SI unit of energy is called a joule (1 J = 1 kg m 2 /sec 2 ). Using this, we can re-write the First Law mathematically as E = W + Q where E is the change in the energy of an object, W is the work done on the object, and Q is the heat added to an object. In laymen's terms, this means that the only way to change the energy of an object is to exchange either work or heat with it. Energy History The discovery of the laws of thermodynamics was extremely important, as our need to understand energy is fueled by the overwhelming use of energy in human society. From the earliest days, humankind has recognized the need to use energy to condition the environment around it. Wood was needed to heat homes and to cook food. Beasts of burden were needed to plow fields and to provide transportation. When either of these commodities became scarce, hardship prevailed, and solutions were sought. In ancient Rome, for example, the lack of available firewood led to the passing of laws that made it illegal to build a house or structure that would block another person's home from getting sunlight, as this was the primary method of heating homes without fire. Fig. 1: U.S. Oil Consumption (Source: DOE) In the 20th century, fossil fuels (oil in particular) reigned supreme as the energy of choice. Their ubiquitous nature created historically low prices for energy. This led to a substantial increase in the number of mechanized tools used by the average citizen. By the year 2000, the U.S. had a population of about 283 million people that were driving over 200 million passenger vehicles. Almost every home in America has a television, some type of range or stove, and a refrigerator. About 3/4 of all households have their own washer, dryer, and air conditioner. Of course, this cheap price does not come without some political and economic consequences. Energy, and oil in particular, have
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