13.1 Answer the questions

1. Into what groups are materials divided?

2. What do conductors do?

3. What materials are common conductors?

4. What metals have high mechanical strength?

13.2 Test. Choose the correct variant

1. All materials are divided into

   1. conductors and semiconductors

   2. conductors, insulators and semiconductors

2. Copper has

   1. high mechanical strength

   2. low mechanical strength

3. Carbon has

   1. a positive coefficient of resistance

   2. a negative coefficient of resistance

4. Copper is commonly used to produce

   1. heaters

   2. brushes

14 Insulators

Insulating materials have a very low conductivity. They offer extremely high resistance to the flow of current. Insulators are used in electric devices to isolate conductors. Thus they should have a high dielectric strength and a high resistivity. Their mechanical properties are also important for practical use.

Insulators are divided into gaseous, liquid, solid. They are also divided into groups according to their heat resistance.

The main gaseous insulator is air. The dielectric strength of air is extremely low; it is lower than the strength of most liquid and solid dielectrics. Liquid insulators are mineral oils, synthetic liquids, resins, and others. Mineral oils are used in oil transformers, cables and capacitors. In transformers, oil is used to insulate current conducting parts. As to resins, at low temperatures they are amorphous. When heated, they become first plastic, then liquid. Resins are the most important components of many plastics. Plastics are used as wire and cable insulation. Solid insulators are paper, cloth, plastics, porcelain, and clastomers. Plastics are widely used in electrical engineering as insulating and structural materials.

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14.1. Answer the questions

1. What materials offer high resistance?

2. Where are insulators used?

3. What properties of insulators are important for practice?

4. Into what groups are insulators divided?

5. What is the main gaseous insulator?

6. What are liquid insulators?

14.2 Test. Choose the correct variant

1. Insulators offer

   1. extremely high resistance

   2. extremely low resistance

2. They should have

   1. a high dielectric strength

   2. a low electric strength

3. Their mechanical properties

   1. are important for practice

   2. are not important for practice

4. Commonly used insulating materials are

   1. liquid

   2. gaseous

   3. solid

15 Transmission of electric power

Electric power is generated at power plants and it must be transmitted to other parts of the country. Thick wires carry electric power over long distances. The wires are not always made of copper, often they are made of other metals.

A number of wires put together form one thick cable. A cable consists of a number of wires put together and a pair consists of two insulated conductors forming a metallic circuit. Thus the basic transmission element of communications is a long transmission line. A cable is the main part of this long transmission line. Nowadays the electricity is transmitted over long distances and the length of transmitting power lines varies from area to area.

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