열역학 법칙(1,2법칙)과 환경 문제
1. 열역학 제 1법칙
∙ 에너지는 그 형태를 달리하면서 변화하지만, 그 양은 전체적으로 일정하다는 에너지 보전법칙
∙ 물리학에서의 에너지 보존 법칙(-保存法則, 영어: law of conservation of energy)은 외계에 접촉이 없을 때 고립계에서 에너지의 총합은 일정하다는 것으로 물리학의 바탕이 되는 법칙 중 하나다. 가끔 에너지 보존의 법칙이라고도 불린다. 이 법칙에 따르면 에너지는 그 형태를 바꾸거나 다른 곳으로 전달할 수 있을 뿐 생성되거나 사라질 수 없다. 항상 일정하게 유지된다는 것이다. 롤러코스터에서 중력에 의한 퍼텐셜에너지가 운동에너지로 변환되거나 화약의 화학에너지가 총알의 운동에너지로 변환되는 것이 그 예이다. 20세기, 에너지 보존 법칙은 알버트 아인슈타인의 특수 상대성이론을 통해 질량-에너지 보존 법칙으로 확장되었다. 특수 상대성이론에 따르면 질량은 에너지의 한 종류이고 기준 관성계에 따라 측정되는 값이 다를 수는 있지만 같은 관성계에서 시간의 변화에 대해서 불변이다. 열역학에 있어서의 에너지 보존 법칙은 열역학 제1법칙(熱力學第一法則, 영어: the first law of thermodynamics)이라고 불린다.
환경문제의 적용
∙ 자연자원으로 추출된 물질과 에너지는 궁극적으로 동일한 양의 폐기물로 전환된다.
방안
① 자연자원의 사용량을 감소
② 자연자원의 사용량을 유지하되, 오염처리기술을 활용한 저감 또는 재활용기술을 적용
2. 열역학 제 2법칙
∙ 에너지의 형태가 전환되는 과정에서 무질서도(사용할 수 없는 양) 즉 entropy가 증가되는 현상
∙ 물리학에서 열역학 제2법칙(second law of thermodynamics)은 열적으로 고립된 계에서 매 시각마다 계의 거시상태의 엔트로피를 고려하였을 때, 엔트로피가 더 작은 거시상태로는 진행하지 않는다는 법칙이다. 이 법칙을 통해 자연적인 과정의 비가역성과 미래와 과거 사이의 비대칭성을 설명한다. 하지만 엔트로피가 감소된 거시상태가 될 확률은 극히 낮을 뿐 불가능은 아니다.
∙ 열역학 2법칙을 통해 차가운 부분에 한 일이 없을 때, 열이 차가운 부분에서 뜨거운 부분으로 흐르지 않는 이유와 열원(reservoir)에서 열에너지가 모두 일로 전환될 때, 다른 추가적인 효과를 동반하지 않는 순환과정(cycle)은 존재하지 않는다는 점에 대해 설명할 수 있다.
∙ 열역학 제2법칙의 모순처럼, 고립계가 아닌 계의 엔트로피는 감소하는 것으로 볼 수도 있다. 예를 들어 에어컨은 방 안의 공기를 차갑게 해주어서 공기의 엔트로피를 감소시킨다. 하지만 방 안으로부터 방출되거나 에어컨이 작동함에 따라 흡수되는 열은 항상 그 계의 공기의 엔트로피의 감소보다 많은 양의 엔트로피를 생성한다. 따라서 전체 계의 총 엔트로피는 열역학 제2법칙에 의하듯 증가한다.
∙ 역학에서 열역학의 기본 관계를 사용하여 표현된 제2법칙은 계의 일을 할 수 있는 능력의 한계를 나타낸다. 또한 열역학 2법칙에 따르면 총 일의 생산에 있어, 열의 이동은 뜨거운 열원에서 차가운 열원으로 향하므로 영구 기관은 존재할 수 없다.
∙ 열역학 제1법칙이 과정 전, 그리고 후의 에너지를 양적(量的)으로 규제하는 반면, 열역학 제2법칙은 에너지가 흐르는 방향을 규제한다.
환경문제의 적용
∙ 인간활동으로 인하여 무가치한 에너지, 즉, 엔트로피가 증가되어 폐기물로 변화된다
방안
① 재생불가능한 자원의 사용을 저감한다(화석연료의 사용 저감)
② 신재생에너지(태양열, 수력, 풍력)의 사용으로 전환함으로 인해 엔트로피 증가를 없앤다.
laws of thermodynamics, environmental problems
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.