15000 bài tập tách từ đề thi thử môn Tiếng Anh có đáp án (Phần 18)

Vitamins, taken in tiny doses, are a major group of organic compounds that regulate the mechanisms by which the body converts food into energy. They should not be confused with minerals, which are inorganic in their makeup. Although in general the naming of vitamins followed the alphabetical order of their identification, the nomenclature of individual substances may appear to be somewhat random and disorganized. Among the 13 vitamins known today, five are produced in the body. Because the body produces sufficient quantities of some but not all vitamins, they must be supplemented in the daily diet. Although each vitamin has its specific designation and cannot be replaced by another compound, a lack of one vitamin can interfere with the processing of another. When a lack of even one vitamin in a diet is continual, a vitamin deficiency may result.

The best way for an individual to ensure a necessary supply of vitamins is to maintain a balanced diet that includes a variety of foods and provides adequate quantities of all the compounds. Some people take vitamin supplements, predominantly in the form of tablets. The vitamins in such supplements are equivalent to those in food, but an adult who maintains a balanced diet does not need a daily supplement. The ingestion of supplements is recommended only to correct an existing deficiency due to unbalanced diet, to provide vitamins known to be lacking in a restricted diet, or to act as a therapeutic measure in medical treatment. Specifically, caution must be exercised with fat-soluble substances, such as vitamins A and D, because, taken in gigantic doses, they may present a serious health hazard over a period of time.

Geothermal energy is natural heat from the interior of the Earth that is converted to heat buildings and generate electricity. The idea of harnessing Earth’s internal heat is not new. As early as 1904 , geothermal power was used in Italy . Today, Earth’s natural internal heat is being used to generate electricity in 21 countries , including Russia, Japan, New Zealand, Iceland, Mexico, Ethiopia, Guatemala, EI Salvador, the Philippines, and the United States .Total worldwide production is approaching 9,000 MW (equivalent to nine large modern coal-burning or nuclear power plants)-double the amount in 1980 .Some 40 million people today receive their electricity from geothermal energy at a cost competitive with that of other energy sources. In EI Salvador , geothermal energy is supplying 30% of the total electric energy used. However, at the global level, geothermal energy supplies less than 0,15%of the total energy supply.

Geothermal energy may be considered a nonrenewable energy source when rates of extraction are greater than rates of natural replenishment. However, geothermal energy has its origin in the natural heat production within Earth , and only a small fraction of the vast total resource base is being utilized today. Although most geothermal energy production involves the tapping of high heat sources, people are also using the low-temperature geothermal energy of groundwater in some applications.

Geothermal Systems

The average heat flow from the interior of the Earth is very low, about 0,06% W/m2.This amount is trivial compared with the 177 W/m2from solar heat at the surface in the United States. However, in some areas, heat flow is sufficiently high to be useful for producing energy . For the most part, areas of high heat flow are associated with plate tectonic boundaries. Oceanic ridge systems (divergent plate boundaries) and areas where mountains are being uplifted and volcanic island arcs are forming (convergent plate boundaries) are areas where this natural heat flow is anomalously high. One such region is located in the western, United States, where recent tectonic and volcanic activity has occurred.

 On the basis of geological criteria, several types of hot geothermal systems (with temperatures greater than about 800C , or 1760F)have been defined, and the resource base is larger than that of fossil fuels and nuclear energy combined. A common system for energy development is hydrothermal convection, characterized by the circulation of steam and / or hot water that transfers heat from depths to the surface.

Geothermal Energy and the Environment

 The environmental impact of geothermal energy may not be as extensive as that of other sources of energy , but it can be considerable. When geothermal energy is developed at a particular site, environmental problems include on-site noise, emissions of gas, and disturbance of the land at drilling sites, disposal sites, roads and pipelines, and power plants. Development of geothermal energy does not require large-scale transportation of raw materials or refining of chemicals, as development of fossil fuels does. Furthermore, geothermal energy does not produce the atmospheric pollutants associated with burning fossil fuels or the radioactive waste associated with nuclear energy. However, geothermal development often does produce considerable thermal pollution from hot waste-waters, which may be saline or highly corrosive, producing disposal and treatment problem.

 Geothermal power is not very popular in some locations among some people. For instance, geothermal energy has been produced for years on the island of Hawaii, where active volcanic processes provide abundant near surface heat. There is controversy, however, over further exploration and development .Native Hawaiians and others have argued that the exploration and development of geothermal energy degrade the tropical forest as developers construct roads, build facilities , and drill wells.

In addition, religious and cultural issues in Hawaii relate to the use of geothermal energy. For example, some people are offended by using the “breath and water of Pele” ( the volcano goddess) to make electricity. This issue points out the importance of being sensitive to the values and cultures of the people where development is planned.

Future of Geothermal energy

 At present, geothermal energy supplies only a small fraction of the electrical energy produced in the United States. However, if developed, known geothermal resources in the United States could produce about 20,000 MW which is about 10% of the electricity needed for the western states. Geohydrothermal resources not yet discovered could conservatively provide four times that amounts (approximately 10% of total U.S electric capacity). About equivalent to the electricity produced from water power today.

Although management principles have been implemented since ancient times, most management scholars trace the beginning of modern management thought back to the early 1900s, beginning with the pioneering work of Frederick Taylor (1856-1915) Taylor was the first person to study work scientifically. He is most famous for introducing techniques of time and motion study, differential piece rate systems, and for systematically specializing the work of operating employees and managers. Along with other pioneers such as Frank and Lillian Gilbreth, Taylor set the stage, labeling his philosophy and methods “scientific management’. At that time, his philosophy, which was concerned with productivity, but which was often misinterpreted as promoting worker interests at the expense of management, was in marked contrast to the prevailing industrial norms of worker exploitation.

The time and motion study concepts were popularized by Frank and Lillian Gilbreth. The Gilbreths had 12 children. By analyzing his children’s dishwashing and bedmaking chores, this pioneer efficiency expert, Frank Gilbreth, hit on principles whereby workers could eliminate waste motion. He was memorialized by two of his children in their 1949 book called “Cheaper by the Dozen”.

The Gilbreth methods included using stop watches to time worker movements and special tools (cameras and special clocks) to monitor and study worker performance, and also involved identification of “therbligs” (Gilbreth spelled backwards) – basic motions used in production jobs. Many of these motions and accompanying times have been used to determine how long it should take a skilled worker to perform a given job. In this way an industrial engineer can get a handle on the approximate time it should take to produce a product or provide a service. However, use of work analysis in this way is unlikely to lead to useful results unless all five work dimensions are considered: physical, psychological, social, cultural, and power.