托福红delta-test3阅读Which sentence below best expresses the essential information in the highlighted sentence in paragra答案解析【小站备考】

The Greek word “systema” means union, and scientists use the word “system” to describe a collection of several components that are linked to one another by functional relationships. Everything outside the system is known as the surroundings. Most scientific literature is a description of the components of a system, their relationships with one another, and their relationships with other systems. Although each science has its own systems with their own subject matter and networks of relationships, the formal characteristics of systems are similar for all sciences. The scientific discipline called general systems theory formulates principles that are valid for systems in general, no matter the elements involved and the relations or forces among them.

Systems can be divided into two types: closed systems and open systems. A closed system receives no supply of energy from outside and transfers no energy outwards. An open system receives energy from its surroundings and transfers it out again.

A closed system is isolated from its surroundings. The energy supply of a closed system is limited and is progressively used up by the processes operating within the system. The ability of the system to function decreases as the available energy is exhausted. Without any additional energy supplied from the outside, the system’s processes stop altogether and no further change is possible in the system. A mill wheel supplied with water from a non-refillable container is a closed system. Once the container of water is empty, the wheel no longer turns because there is no water to turn it. In a truly closed system, the water would have to be collected below the mill wheel in a second container to ensure that the system did not supply any energy to the outside.

Some scientists argue that there are few truly closed systems in nature, and many define closed systems more broadly as those allowing energy but not mass to cross the system boundary. By this definition, the Earth system as a whole is a closed system. The boundary of the Earth system is the outer edge of the atmosphere, and except for the occasional meteorite, virtually no mass is exchanged between the Earth system and the rest of the universe. However, energy in the form of solar radiation passes from the sun, through the atmosphere to the Earth’s surface, which in turn radiates energy back out to space across the system boundary. Hence, energy passes across the Earth’s system boundary, but mass does not, making it a closed system.

In an open system, energy and mass can be transferred between the system and its surroundings. Living organisms are open systems. They absorb light energy or chemical energy in the form of organic molecules and release heat and metabolic waste products, such as carbon dioxide, to the surroundings. Generally, relationships exist between the components of a system and its surroundings, that is, other systems. Each open system is part of a larger system that receives and gives off energy. In an open system, the energy is continually resupplied from sources outside the system. In the example of the mill wheel, if the non-refillable water container is replaced by a reservoir fed continuously by a stream, it becomes an open system because the energy supply is renewed from the outside.

The natural environment is made up of open systems. These can behave as closed systems temporarily if the energy supply is halted for a period. If, for example, the stream to the reservoir supplying the mill dries up for a long period, the energy consumption of the mill wheel cannot be balanced by new energy supply. The water in the reservoir is used up, and if the dry period is long enough, the mill wheel stops turning. Eventually, the stream may flow again, filling the reservoir and turning the mill wheel again. This occurs because the stream-reservoir-mill system is itself a part of the Earth’s much larger systems of water circulation and water budget, which include condensation, precipitation, run-off, and evaporation. The water systems receive their energy supply from the Earth’s heat budget, which in turn receives its energy supply from the sun’s radiation.

托福红delta-test3阅读Which sentence below best expresses the essential information in the highlighted sentence in paragra答案解析

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