B) Cover the left column and translate the Russian words back into English. cheap дешевый 'copper медь decrease ['dikri:s] уменьшение load нагрузка make smb (smth) заставить

4* Put down the Russian for:

bad resistance

^ire conductors

silver wire conductors

temperature 'decrease

temperature 'increase

-ai-

5. Translate into Russian. Mind since.

1. Copper conductors are widely used since they are much cheaper than silver ones.

2. A minimum voltage drop is produced in copper wire conductors sinc^ they have a low resistance.

3. A bulb connected to an open circuit does not light since an open circuit] has no current.

Conductors and Insulators

Conductors are materials having a low resistance so that currenl easily passes through them. The lower the resistance of the material, thq more current can pass through it.

The most common conductors are metals. Silver and copper are thd best of them. The advantage of copper is that it is much cheaper than silver. Thus copper is widely used to produce wire conductors. One ol the common functions of wire conductors is to connect a voltage sourc<| to a load resistance. Since copper wire conductors have a very low re sistance a minimum voltage drop is produced in them. Thus, all of th<| applied voltage can produce current in the load resistance.

It should be taken into consideration that most materials change th^ value of resistance when their temperature changes.

Metals increase their resistance when the temperature increase! while carbon decreases its resistance when the temperature increases] Thus metals have a positive temperature coefficient of resistance whild carbon has a negative temperature coefficient. The smaller is the temj perature coefficient or the less the change of resistance with the changl of temperature, the more perfect is the resistance material.

Materials having a very high resistance are called insulators. Cui^ rent passes through insulators with great difficulty.

The most common insulators are air, paper, rubber, plastics.

Any insulator can conduct current when a high enough voltage is| applied to it. Currents of great value must be applied to insulators in on" der to make them conduct. The higher the resistance of an insulator, th| greater the applied voltage must be.

When an insulator is connected to a voltage source, it stores electrid charge and a potential is produced on the insulator. Thus, insulatorj have the two main functions:

1. to isolate conducting wires and thus to prevent a short between sm and

2. to store electric charge when a voltage source is applied.

Find answers to these questions in the text above:

1. What materials are called conductors?

2. What is the advantage of copper compared with silver?

3. What is the most common function of wire conductors?

4. Why is a minimum voltage drop produced in copper conductors?

5. What is the relation between the value of resistance and the temperature in carbon?

6. What materials are called insulators?

7. What are the most common insulators?

8. What are the two main functions of insulators?

Complete the sentences using the correct variant:

1. Insulators are materials having a) low resistance.

b) high resistance.

2. Current passes through conductors a) easily.

b) with great difficulty.

3. Copper and silver are a) common conductors.

b) common insulators.

4. Air, paper and plastics are a) common insulators.

b) common conductors.

5. In case a high voltage is applied to a) it does not conduct current, an insulator b) it conducts current.

6. Insulators are used a) to store electric charge.

b) to reduce voltage.

c) to prevent a short between conducting wires.

7. Metals increase their resistance a) when the temperature de-

creases, b) when the temperature in­creases.

58-

-89-

a) when the temperature in­creases.

b) when the temperature de­creases.

c) a positive temperature coefl ficient of resistance.

d) a negative temperature coed ficient of resistance.

8. Complete the sentences using while. Follow the model on page 13.

1. Conductors have a low resistance.............

2. Current passes through insulators with great difficulty................

3. Metals are common conductors.............

4. To make insulators conduct, currents of great value must be applia

5. Carbon decreases its resistance when the temperature increases ....

6. Metals have a positive temperature coefficient of resistance ....

9. Pair work. Put these questions to your groupmate, and ask him/her t< answer them:

1. What is the difference between conductors and insulators?

2. How does current pass through insulators?

3. What materials are commonly used to produce insulators?

4. What materials are commonly used to produce conductors?

5. In what case do insulators conduct current?

6. How does resistance change when the temperature decreases?

to step down frequency due to

\x. Put down the Russian for:

iron core ____________

closed core ____________

input voltage ____________

output voltage ____________

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