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Conductors, Semiconductors and Insulators

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Matter

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Matter consists of one or more elements. Elements are substances that cannot be divided into other substances. An atom is the smallest particle of an element, which has all the properties of the element and can take part in chemical reactions.

In each atom there is a nucleus, containing a number of protons (each proton has a positive electrical charge) and a number of neutrons, having no electrical charge. The nucleus is surrounded by a number of electrons. Each electron has a negative electrical charge. The electrons orbit around the nucleus. An atom has as many protons as it has electrons. This results in a zero electrical charge of the atom. Electrons orbit around the nucleus in one of the several possible orbits. These orbits are called shells.

In a silicon atom there are fourteen protons in the nucleus. Fourteen electrons orbit around the nucleus in three orbits. The first or inner shell contains two electrons and cannot hold any more electrons. In the second shell there are eight electrons. It cannot hold additional electrons either. In the third shell there are only four electrons. This shell can hold more electrons. The outer shell of any atom is called its valence shell. The number of electrons in the valence shell is known as the valency of the atom.

Materials can be classed in three groups according to their electrical properties – conductors, semiconductors and insulators. Metals such as silver, copper, and aluminium have many free electrons. This makes it easy for current to flow through them. Therefore, these metals are called conductors.

Materials such as glass, rubber, and many plastics have practically no free electrons. This makes it very difficult for current to flow through them. These materials are known as insulators and are used in many fields ranging from the covering on conductors to the dielectric in capacitors.

London's Underground

Bands were playing and the gentlemen in hats were preparing to make speeches about their great achievement. They made the first underground railway travel in the world – a distance of almost four miles.

It was the year 1863 and on that first historic day 30,000 Londoners used this new and strange way of travel. Now more than a hundred years later the London Underground carries more than a million passengers every day. The total 56 number of passengers carried by the London Underground each year is enormous, and it is constantly growing.

In the early days the trains were driven by steam locomotives which burnt coal.

It is said that the train staff and porters asked permission to grow beards and moustaches – as an early form of smog mask.

Now the atmosphere "underground" is considered even better than that outside as germs cannot grow in the dry air – and the trains, of course, are electric.

The deep tunneling came later, in 1890. Tunneling a tube through miles of clay, and sometimes sand and gravel is no easy task, and it was James Henry Greathead who developed the method which was to make most of London's tube tunnels possible.

London transport's experience with tunnels brought them another record. One of the longest continuous railway tunnel in the world is the 17 1/2 mile tunnel on the Northern line.

There are numerous escalators which help to keep the traffic moving. The first was installed in 1911. One of them at Leicester Square is over 80 feet in length. On long escalators the speed is changeable. The "up" escalator runs at full speed when carrying passengers, but when empty it runs at half speed. Many of the new escalators have automatic control making a more frequent service throughout the day possible.

Safety was always one of the main concerns of London transport. In spite of the fact that trains often follow each other within seconds, it is said that the London Underground is the safest form of transport in the world. Automatic signalling is operated by the trains themselves. A programme machine controls routes: if changes are necessary, they are made automatically and with lighting speed. No accidents can happen through human weakness.

New automatic driving systems are now being used. The air in the Underground is changed every quarter of an hour, and the temperature all year round is maintained at 69 – 79 degrees Fahrenheit.

Though the quality of service is high there is still significant overcrowding on certain lines at peak periods. This problem is being discussed and additional improvement of service is planned for the future.

A number of station modernization schemes are being completed. The aim is to provide a more attractive environment for the passengers. The usage of modern materials makes maintenance easier. This modernization is coordinated with the renewal of lifts and escalators and the introduction of new ticketing machines. The fare in the Underground depends on the distance, but the lowest is 50 pence.

The Underground is becoming much cleaner and brighter and therefore more pleasant to use, carrying more passengers than at any time in its long history.

Crime levels in London are generally reflected by crime levels on London's transport. According to the statistics the crime level in the Underground is rather high. To ensure safety of passengers and the staff the transport authorities added more police officers to the Underground System to guarantee as much protection as possible.

Task1. Agree or disagree using That's right..., or That's wrong....

Example: a) If changes are necessary, they are made automatically with lighting speed. That's right, according to the text if changes are necessary they are made automatically with lighting speed. b) Safety signalling will be controlled by the drivers themselves. That's wrong, according to the text safety signalling will be controlled by coded electrical impulses.

1. It was fifty years ago that Londoners used a new way of travel. 2. The first underground travel in the world took place in 1863 in London. 3. The London Underground is not a very safe form of transport. 4. People who made the first underground travel did not consider it a great achievement. 5. In the early days the trains were driven by electricity. 6. The first escalator was installed 30 years ago. 7. The first trains were driven by steam locomotives. 8. The first escalator was installed in 1911. 9. The temperature in the Underground is very changeable. 10. The deep-level tubes came in 1890. 11. On long escalators the speed is not changeable. 12. The trains follow each other within seconds. 13. The temperature is maintained at 40 degrees by Fahrenheit.

Дата добавления: 2014-12-17