Laws of thermodynamics
Thermodynamics, Laws of
Thermodynamics, Laws of
One way of understanding the environment is to understand the way matter and energy flow through the natural world. For example, it helps to know that a fundamental law of nature is that matter can be neither created nor destroyed. That law describes how humans can never really "throw something away." When wastes are discarded, they do not just disappear. They may change location or change into some other form, but they still exist and are likely to have some impact on the environment.
Perhaps the most important laws involving energy are the laws of thermodynamics. These laws were first discovered in the nineteenth century by scientists studying heat engines. Eventually, it became clear that the laws describing energy changes in these engines apply to all forms of energy.
The first law of thermodynamics says that energy can be changed from one form to another, but it can be neither created nor destroyed. Energy can occur in a variety of forms such as thermal (heat), electrical, magnetic, nuclear, kinetic, or chemical. The conversion of one form of energy to another is familiar to everyone. For example, the striking of a match involves two energy conversions. In the first conversion, the kinetic energy involved in rubbing a match on a scratch pad is converted into a chemical change in the match head. That chemical change results in the release of chemical energy that is then converted into heat and light energy.
Most energy changes on the earth can be traced back to a single common source: the sun. The following describes the movement of energy through a common environmental pathway, the production of food in a green plant: solar energy reaches the earth and is captured by the leaves of a green plant. Individual cells in the leaves then make use of solar energy to convert carbon dioxide and water to carbohydrates in the process known as photosynthesis . The solar energy is converted to a new form, chemical energy, that is stored within carbohydrate molecules.
"Stored" energy is called potential energy. The term means that energy is available to do work, but is not currently doing so. A rock sitting at the top of a hill has potential energy because it has the capacity to do work. Once it starts rolling down the hill, it uses a different type of energy to push aside plants, other rocks, and other objects.
Chemical energy stored within molecules is another form of potential energy. When chemical changes occur, that energy is released to do some kind of work.
Energy that is actually being used is called kinetic energy. The term kinetic refers to motion. A rock rolling down the hill converts potential energy into the energy of motion, kinetic energy. The first law of thermodynamics says that, theoretically, all of the potential energy stored in the rock can be converted into kinetic energy, without any loss of energy at all.
One can follow, therefore, the movement of solar energy through all parts of the environment and show how it is eventually converted into the chemical energy of carbohydrate molecules, then into the chemical energy of molecules in animals who eat the plant, then into the kinetic energy of animal movement, and so on.
Environmental scientists sometimes put the first law into everyday language by saying that "there is no such thing as a free lunch." By that expression, they mean that in order to produce energy, energy must be used. For many years, for example, scientists have known that vast amounts of oil can be found in rock-like formations known as oil shale , but the amount of energy needed to extract that oil by any known process is much greater than the energy that could be obtained from it.
Scientists apply the first law of thermodynamics to an endless variety of situations. A nuclear engineer, for example, can calculate the amount of heat energy that can be obtained from a reactor using nuclear materials (nuclear energy) and the amount of electrical energy that can be obtained from that heat energy.
However, in all such calculations, the engineer also has to take into consideration the second law of thermodynamics. This law states that in any energy conversion, there is always some decrease in the amount of usable energy.
A familiar example of that law is the incandescent light bulb. Light is produced in the bulb when a thin wire inside is heated until it begins to glow. Electrical energy is converted into both heat energy and light energy in the wire.
As far as the bulb is concerned, the desired conversion is electrical energy to light energy. The heat that is produced, while necessary to get the light, is really "waste" energy. In fact, the incandescent lightbulb is a very inefficient device. More than 90% of the electrical energy that goes into the bulb comes out as heat. Less than 10% is used for the intended purpose, making light.
Examples of the second law can be found everywhere in the natural and human-made environment. And, in many cases, they are the cause of serious problems.
If one follows the movement of solar energy through the environment again, the amazing observation is how much energy is wasted at each stage. Although green plants do convert solar energy to chemical energy, they do not achieve 100% efficiency. Some of the solar energy is used to heat a plant's leaf and is converted, therefore, to heat energy. As far as the plant is concerned, that heat energy is wasted. By the time solar energy is converted to the kinetic energy used by a school child in writing a test, more than 99% of the original energy received from the sun has been wasted.
Some people use the second law of thermodynamics to argue for less meat-eating by humans. They point out how much energy is wasted in using grains to feed cattle. If humans would eat more plants, they say, less energy would be wasted and more people could be fed with available resources.
The second law explains other environmental problems as well. In a nuclear power plant, energy conversion is relatively low, around 30%. That means that about 70% of the nuclear energy stored in radioactive materials is eventually converted not to electricity, but to waste heat. Large cooling towers have to be built to remove that waste heat. Often, the waste heat is carried away into lakes, rivers, and other bodies of water. The waste heat raises the temperature of this water, creating problems of thermal pollution .
Scientists often use the concept of entropy in talking about the second law. Entropy is a measure of the disorder or randomness of a system and its surroundings. A beautiful glass vase is an example of a system with low entropy because the atoms of which it is made are carefully arranged in a highly structured system. If the vase is broken, the structure is destroyed and the atoms of which it was made are more randomly distributed.
The second law says that any system and its surroundings tends naturally to have increasing entropy. Things tend to spontaneously "fall apart" and become less organized. In some respects, the single most important thing that humans do to the environment is to appear to reverse that process. When they build new objects from raw materials, they tend to introduce order where none appeared before. Instead of iron ore being spread evenly through the earth, it is brought together and arranged into a new automobile , a new building, a piece of art, or some other object.
But the apparent decrease in entropy thus produced is really misleading. In the process of producing this order, humans have also brought together, used up, and then dispersed huge amounts of energy. In the long run, the increase in entropy resulting from energy use exceeds the decrease produced by construction. In the end, of course, the production of order in manufactured goods is only temporary since these objects eventually wear out, break, fall apart, and return to the earth.
For many people, therefore, second law of thermodynamics is a very gloomy concept. It suggests that the universe is "running down." Every time an energy change occurs, it results in less usable energy and more wasted heat.
People concerned about the environment do well, therefore, to know about the law. It suggests that humans think about ways of using waste heat. Perhaps there would be a way, for example, of using the waste heat from a nuclear power plant to heat homes or commercial buildings. Techniques for making productive use of waste heat are known as cogeneration .
Another way to deal with the problem of waste heat in energy conversions is to make such conversions more efficient or to find more efficient methods of conversion. The average efficiency rate for power plants using fossil fuels is only 33%. Two-thirds of the chemical energy stored in coal , oil, and natural gas is, therefore, wasted. Methods for improving the efficiency of such plants would obviously provide a large environmental and economic benefit.
New energy conversion devices can also help. Fluorescent light builds, for example, are far more efficient at converting electrical energy into light energy than are incandescent bulbs. Experts point out that simply replacing existing incandescent light bulbs with fluorescent lamps would make a significant contribution in reducing the nation's energy expenditures.
[David E. Newton ]
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