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2017年职称英语考试《卫生类》概括大意专项练习(4)

  第3部分:概括大意与完成句子。下面的短文后有2项测试任务:(1)第23~26题要求从所给的6个选项中为第3~6段每段选择1个最佳标题;(2)第27~30题要求从所给的6个选项中为每个句子确定1个最佳选项。

  Maglev Trains

  A few countries are using powerful electromagnets to develop high-speed trains, called maglev trains. Maglev is short for magnetic levitation1, which means that these trains float over a guide way using the basic principles of magnets to replace the old steel wheel and track trains.

  If you’ve ever played with magnets, you know that opposite poles attract and like poles repel each other. This is the basic principle behind electromagnetic propulsion. Electromagnets are similar to other magnets in that they attract metal objects, but the magnetic pull is temporary. You can easily create a small electromagnet yourself by connecting the ends of a copper wire to the positive and negative ends of an AA-cell battery. This creates a small magnetic field. If you disconnect either end of the wire from the battery, the magnetic field is taken away.

  The magnetic field created in this wire-and-battery experiment is the simple idea behind a maglev train rail system. There are three components to this system: A large electrical power source, metal coils lining a guide way or track, and large guidance magnets attached to the underside of the train.

  The big difference between a maglev train and a conventional train is that maglev trains do not have an engine-at least not the kind of engine used to pull typical train cars along steel tracks4. The engine for maglev trains is rather innoticeable. Instead of using fossil fuels, the magnetic field created by the electrified coils in the guideway walls and the track combines to propel the trains.

  The magnetized coil running along the track, called a guideway, repels the large magnets on the train’s undercarriage, allowing the train to levitate between 1 to 10 cm above the guideway. Once the train is levitated, power is supplied to the coils within the guideway walls to create a unique system of magnetic fields that pull and push the train along the guideway. The electric current supplied to the coils in the guideway walls is constantly alternating to change the polarity of the magnetized coils. This change in polarity causes the magnetic field in front of the train to pull the vehicle forward, while the magnetic field behind the train adds more forward thrust.

  Maglev trains float on a cushion of air, eliminating friction. This lack of friction allows these trains to reach unprecedented ground transportation speeds of more than 500 kph, or twice as fast as the fastest conventional train. At 500 kph, you could travel from Paris to Rome in just over two hours.

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