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Hydroelectric Power Hydroelectric power is America’s leading renewable energy resource. Of all the renewable power sources, it’s the most reliable, efficient and economical. Water is needed to run a hydroelectric generating unit. It’s held in a reservoir or lake behind a dam, and the force of the water being released from the reservoir through the dam spins the blades of a turbine. The turbine is connected to the generator that produces electricity. After passing through the turbine, the water re-enters the river on the downstream side of the dam. Hydroelectric plants convert the kinetic energy within falling water into electricity. The energy in moving water is produced in the sun, and consequently is continually being renewed. The energy in sunlight evaporates water from the seas and deposits it on land as rain. Land elevation differences result in rainfall runoff, and permit some of the original solar energy to be harnessed as hydroelectric power. Hydroelectric power is at present the earth’s chief renewable electricity source, generating 6% of global energy and about 15% of worldwide electricity. Hydroelectric power in Canada is plentiful and provides 60% of their electrical requirements. Usually regarded as an inexpensive and clean source of electricity, most big hydroelectric projects being planned today are facing a great deal of hostility from environmental groups and local people. The earliest recorded use of water power was a clock, constructed around 250 BC. Since then, people have used falling water to supply power for grain and saw mills, as well as a host of other uses. The earliest use of flowing water to generate electricity was a waterwheel on the Fox River in Wisconsin in 1882. The first hydroelectric power plants were much more dependable and efficient than the plants of the day that were fired by fossil fuels. This led to a rise in number of small to medium sized hydroelectric generating plants located wherever there was an adequate supply of falling water and a need for electricity. As demand for electricity soared in the middle years of the 20th century, and the effectiveness of coal and oil power plants improved, small hydro plants became less popular. The majority of new hydroelectric developments were focused on giant mega-projects. Hydroelectric plants harness energy by passing flowing water through a turbine. The water turbine rotation is delivered to a generator, which generates electricity. The quantity of electricity that can be produced at a hydroelectric plant relies upon two variables. These variables are (1) the vertical distance that the water falls, called the "head", and (2) the flow rate, calculated as volume over time. The amount of electricity that is produced is thus proportional to the head product and the flow rate. So, hydroelectric power stations can normally be separated into two kinds. The most widespread are "high head" plants and usually employ a dam to stock up water at an increased height. They also store water at times of rain and discharge it during dry times. This results in reliable and consistent electricity generation, capable of meeting demand since flow can be rapidly altered. At times of excess electrical system capacity, usually available at night, these plants can also pump water from one reservoir to another at a greater height. When there is peak electrical demand, the higher reservoir releases water through the turbines to the lower reservoir. "Low head" hydroelectric plants usually exploit heads of just a few meters or less. These types of power station use a weir or low dam to channel water, or no dam at all and merely use the river flow. Unfortunately their electricity production capacity fluctuates with seasonal water flow in a river. Around 2003 people believed almost universally that hydroelectric power was an environmentally safe and clean means of generating electricity. Hydroelectric stations do not release any of the usual atmospheric pollutants emitted by power plants fuelled by fossil fuels so they do not add to global warming or acid rain. Nevertheless, recent studies of the larger reservoirs formed behind dams have implied that decomposing flooded vegetation could give off greenhouse gases equal to those from other electricity sources. The clearest result of hydroelectric dams is the flooding of huge areas of land. The reservoirs built can be exceptionally big and they have often flooded the lands of indigenous peoples and destroyed their way of life. Numerous rare ecosystems are also endangered by hydroelectric power plant development. Damming rivers may also change the quantity and quality of water in the rivers below the dams, as well as stopping fish migrating upstream to spawn. In addition, silt, usually taken downstream to the lower parts of a river, is caught by a dam and so the river downstream loses the silt that should fertilize the river’s flood plains during high water periods. Theoretical global hydroelectric power is approximately four times larger than the amount that has been taken advantage of today. Most of the residual hydro potential left in the world can be found in African and Asian developing countries. Exploiting this resource would involve an investment of billions of dollars, since hydroelectric plants normally have very high building costs. Low head hydro capacity facilities on small scales will probably increase in the future as low head turbine research, and the standardization of turbine production, reduce the costs of low head hydroelectric power production. New systems of control and improvements in turbines could lead in the future to more electricity created from present facilities. In addition, in the 1950’s and 60’s when oil and coal prices were very low, lots of smaller hydroelectric plants were closed down. Future increases in the prices of fuel could lead to these places being renovated. When do high head power plants use surplus electricity to transfer water to a second reservoir
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第三篇 Common-cold Sense You can’t beat it, but you don’t have to join it. Maybe it got the name "common cold" because it’s more common in winter. The fact is, though, being cold doesn’t have anything to do with getting one. Colds are caused by the spread of rhinoviruses, and, at least so far, medical science is better at telling you how to avoid getting one than how to get rid of one. Children are the most common way cold Viruses are spread to adults, because they have more colds than adults — an average of about eight per year. Why do kids seem so much more easily to get colds than their parents Simple. They haven’t had the opportunity to become immune to many cold viruses. There are more than 150 different cold viruses, and you never have the same one twice. Being infected by one makes you immune to it — but only it. Colds are usually spread by direct contact, not sneezing or coughing. From another person’s hand to your hand and then to your nose or eyes is the most common route. The highest concentration of cold viruses anywhere is found under the thumbnails of a boy, although the viruses can survive for hours on skin or other smooth surfaces. Hygiene is your best defense. Wash your hands frequently, preferably with a disinfectant soap, especially when children in your household have colds. But even careful hygiene won’t ward off every cold. So, what works when a coughing, sneezing, runny nose strikes The old prescription of two aspirins, lots of water, and bed rest is a good place to start. But you’ll also find some of the folk remedies worth trying. Hot mixtures of sugar (or honey), lemon, and water have real benefits. Children have more colds because
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Most people don’t enjoy facing the difficult situations that sometimes occur with coworkers in the workplace. Such situations may arise from honest disagreements over design or engineering issues, personnel or benefits matters, management decisions or actions, or from any other situation where human impressions and objectives differ. There could be double trouble for engineers who are more likely to feel at home with electrons and bytes (信息组), and behave in highly predictable ways, than with coworkers, who often appear arbitrary and unpredictable. For those of us who have internalized the strict and measurable rules of the physical world, dealing with other people can be both disappointing and frustrating. Yet how you manage situations of conflict with your coworkers could have a significant impact on your career, often even more than your engineering prowess or your design skills. Those who deal successfully with potential conflicts are far more likely to receive added responsibilities and promotions, in addition to the pay increases and respect that come with them. On the other hand, not dealing successfully with conflict can potentially relegate you to a career, backwater, with technical challenges and high pay passing you by. Why is dealing with conflict an important skill today It’s primarily because there’s more of it now than in the past. Workers Of all types are more likely to speak up for their own ideas or actions, rather than follow the dictating corporate chain of command. Conflict also sometimes arises as a result of unclear company goals, or when those goals aren’t shared equally by all. Rather than working for a single common good, employees and managers seek individual goals, such as promotion, job security, experience, money, and even the proverbial free lunch. Not only is actual conflict greater today, but even the potential for interpersonal conflicts in the workplace is far greater than at any time in the past. One reason for this is increased time-to-market pressures. The need to rapidly make decisions, establish an engineering direction, and meet project milestones adds elements of tension and stress to an already difficult endeavor. This makes the workplace a potential minefield for interpersonal conflict. It’s especially apparent to an engineer in a position of responsibility, like a project leader or an engineering manager. For an engineer who must work with others to complete a project, the need to manage conflict can spell the difference between success and failure. Which of the following statements is true according to the passage
A controversy erupted in the scientific community in early 1998 over the use of DNA (deoxyribonucleic acid) fingerprinting in criminal investigations. DNA fingerprinting was introduced in 1987 as a method to identify individuals based on a pattern seen in their DNA, the molecule of which genes are made. DNA is present in every cell of the body except red blood cells. DNA fingerprinting has been used successfully in various ways, such as to determine paternity (父亲的身份) where it is not clear who the father of a particular child is. However, it is in the area of criminal investigations that DNA fingerprinting has potentially powerful and controversial uses. DNA fingerprinting and other DNA analysis techniques have revolutionized criminal investigations by giving investigators powerful new tools in the attempt to prove guilt, not just establish innocence. When used in criminal investigations, a DNA fingerprint pattern from a suspect is compared with a DNA fingerprint pattern obtained from such material as hairs or blood found at the scene of a crime. A match between the two DNA samples can be used as evidence to convict a suspect. The controversy in 1998 stemmed from a report published in December 1991 by population geneticists Richard C. Lewontin of Harvard University in Cambridge, Mass, and Daniel L. Hartl of Washington University School of Medicine in St. Louis, Mo. Lewontin and Hartl called into question the methods to calculate how likely it is that a match between two DNA fingerprints might occur by chance alone. In particular, they argued that the current method Cannot properly determine the likelihood that two DNA samples will match because they came from the same individual rather than simply from two different individuals who are members of the same ethnic group. Lewontin and Hartl called for better surveys of DNA patterns. In response to their criticisms, population geneticists Ranajit Chakraborty of the University of Texas in Dallas and Kenneth K. Kidd of Yale University in New Haven, Conn. , argued that enough data are already available to show that the methods currently being used are adequate. In January 1998, however, the Federal Bureau of investigation and laboratories that conduct DNA tests announced that they would collect additional DNA samples from various ethnic groups in an attempt to resolve some of these questions. And, in April, the National Academy of Sciences called for strict standards and system of accreditation (鉴定合格) for DNA testing laboratories. Before DNA fingerprinting is used, suspects ______ .
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