Pair work. Put these questions to your groupmate and ask him/her to answer them.

 

1. What is a capacitor used for?

2. What are the main parts of a capacitor?

3. What is the function of insulators?

4. What does the capacity of a capacitor depend on?

5. What is the difference between a fixed capacitor and a variable one?

6. What should be done in order to change a capacitor?

7. What is the relation between the value of capacity and the distance of plates?

8. What type of insulators have variable capacitors?

9. What should be done in case a capacitor has a trouble?

 

5. Solve these problems:

Draw a diagram of a circuit consisting of two resistors and two capacitors

connected in parallel. A battery of four cells is applied to the circuit.

Two ammeters are used: one is connected to the main line, the other - to a

parallel branch. What is the function of each element? In what way can one

increase the value of resistance in the circuit?

Suppose one of the branches stops operating. What does it result from?

 

Практическое занятие 8. Проводники и изоляторы

Lesson 8. Conductors and Insulators

1. Read and learn the words:

 

Cheap - дешевый

copper - медь

Decrease - уменьшение

Load - нагрузка

make smb (smth) do smth - заставить кого-л. (что-л.) делать что-л.

Thus - таким образом, так

Difficulty - трудность

rubber - резина

Since - так как

to decrease - уменьшать

increase - увеличение

 

Read and translate the text.

 

Conductors and Insulators

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

The most common conductors are metals. Silver and copper are the 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 of the common functions of wire conductors is to connect a voltage source to a load resistance. Since copper wire conductors have a very low resistance a minimum voltage drop is produced in them. Thus, all of the applied voltage can produce current in the load resistance.

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

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

Materials having a very high resistance are called insulators. Current 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 order to make them conduct. The higher the resistance of an insulator, the greater the applied voltage must be.

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

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

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

 

3. 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?

 

4. 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 an insulator

a) it does not conduct current.

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 decreases.

b) when the temperature increases.

 

5. Complete the sentences using while.

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 applied

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

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