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The laws of thermodynamics are amongst the simplest, most elegant, and most impressive products of modern science. Most physical laws are designed to explain processes which humans experience in nature. The laws of thermodynamics on the other hand, were developed to explain the absence of perpetual motion (a human concept) in nature. The first law of thermodynamics states that "energy can never be created or destroyed, but it can be transformed from one form into another." An automobile engine can be used to illustrate the first law. Energy stored in gasoline is transformed into useful work used to move the car, heat the car, create frictional energy, and produce waste energy in the exhaust products. The sum of the energy of these four is exactly equal to the energy of the gasoline. The first law of thermodynamics thus seems to indicate that we could never run out of energy. Unfortunately, things are not quite so simple, and this is outlined by the second law of thermodynamics. The second law of thermodynamics says that: "Every time energy is transformed from one state to another, a penalty is exacted. The penalty is a reduction in the amount of energy available to perform useful work in the future." The term for this penalty is "entropy". The two laws of thermodynamics can be stated together in one simple sentence: "The total energy content of the universe is constant, and the total entropy is continually increasing." In other words, "there is no such thing as a free lunch; and there are always crumbs under the table." Entropy is thus a measure of the amount of energy which is no longer capable of performing work. An increase in entropy means the reduction of energy available to do work in the future. Increasing entropy can also be considered as a change from an ordered state to a disordered state. Every event in the natural world results in an increase in entropy. Time is not symmetrical, it only moves in one direction, therefore entropy will always increase. One example of increasing entropy is water falling over a dam. When the water is above the dam it has some potential energy due to gravity, which can be used to generate electricity or turn a wheel to perform some useful task. Once the water has fallen to the level below the dam, its total energy is the same - as the fall warms the water increasing its thermal energy - but it no longer has the same capacity to do work. The water has moved from what is referred to as an "available" or "free" energy state (high grade energy) to an "unavailable" or "bound" energy state (low grade energy). This change in the energy state of the water as it falls over the dam is an increase in entropy. Unavailable energy is most often in the form of thermal energy. Thermal energy is the result of random and disorderly motions of huge numbers of individual atoms and molecules. Whereas kinetic energy, such as that of a moving object is the result of the orderly motion of all the atoms in the object. Conversion of other forms of energy (kinetic, electric etc.) into thermal energy is simple but converting thermal energy into other forms is difficult and can never be performed completely. In order to harness thermal energy, two reservoirs of heat at different temperatures are required. In such a 'heat engine', heat from the high temperature reservoir (high grade energy) flows to the low temperature reservoir (low grade energy). The maximum possible efficiency at which the heat engine can work is defined by the equation E=(T2-T1)/T2, where E is the efficiency, T1 is the temperature of the cold reservoir (Kelvin) and T2 the temperature of the hot reservoir. From the equation, it can be seen that unless the cold reservoir is at a temperature of absolute zero (and no such reservoirs exist), the efficiency of the heat engine will always be less than 1, meaning there can never be a complete conversion of thermal energy into another form of energy. This is illustrated in a thermal electric power plant (Figure 2). In this case, the hot reservoir has a temperature of about 773 K (500° C) and the cold reservoir 293 K (20° C), resulting in a maximum efficiency of about 60%, although in actual power plants the efficiency is rarely above 40% due to frictional and other losses. FIGURE 2: Schematic diagram of a steam-electric power plant (53K). |
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