1. D.C. Is a current that

a) changes its direction of flow

b) flows in one direction

2. A.C. Flows provided

a) a direct voltage source is applied

b) an alternating voltage source is applied

3. In an alternating current circuit

a) current flows in one direction 60 times per second

b) current flows in one direction 60 times and in the other direction 60 times per second

4. a.c.

a) can be changed into d.c.

b) cannot be changed into d.c.

Exercise 3. Finish the sentences using the words from the text:

1. An alternating current changes its direction of flow _____.

2. A direct current flows provided a direct voltage source is applied _____.

3. Transformers are used to _____.

4. The number of cycles per second is called _____.

Exercise 4. Answer the following questions:

1. What is current?

2. What types of current do you know?

3. When does a direct current flow?

4. What type of current is called an alternating current?

5. What type of current is called a direct current?

6. What is called the frequency of current?

7. What device is used to transform a.c. power from one voltage to another?

8. Is it often necessary to change a.c. into d.c.?

4. How Electrical Energy is Produced Word list

1. a purpose [´pə:pəs] мета

2. a pocket torch [´pɔkıt ´tɔ:tʃ] кишеньковий ліхтар

3. enormous [ı´nɔməs] величезний

4. to enumerate [ı´nｊu:məreıt] перелічувати

5. a primary cell [´praımərı sel] гальванічний елемент

6. a junction [´dʒʌnkʃn] з’єднання, стик

7. a voltage [´vəultıdʒ] напруга

8. a source [´sɔ:s] джерело

9. a face [´feıs] грань

10. a means [mı:nz] засіб

11. frequency [´frı:kwǝnsı] частота

12. to be suitable [sju:tǝbl] бути придатним

13. gramophone pick-ups [´pık ´ʌps] патефонні звукознімачі

14. evacuated tube [tju:b] вакуумна лампа

15. to be liberated [lıbǝ´reıtıd] виділятись

Exercises

I. Find equivalents:

1. a source of energy а. електростанція

2. purely chemical actions b. засіб керування частотою

3. generators of various sizes c. чисто хімічні реакції

4. a very small voltage d. генератори різних розмірів

5. application of pressure e. джерело енергії

6. a means of frequency control f. під дією світла

7. a power station g. застосування напруги

8. by the action of light h. дуже мала напруга

9. temperature measurement i. застосувати на практиці

10. to employ in practice j. вимірювання температури

How Electrical Energy is Produced

There are several methods of producing electricity for practical purposes. The battery of a pocket torch may be contrasted with the source of enormous energy represented by a larger power-station. Both are examples of the application of electrical energy to a particular purpose, and in general the purpose determines the nature of the method used to produce the energy.

Practical methods of producing electricity may be enumerated as follows:

Chemical, as represented by the various types of batteries or primary cells in which the electricity is produced by purely chemical actions.

Electromagnetic, forming the basis of operation of rotating generators in which the electricity is produced by conductors moving through a magnetic field. This is the method employed in practice for generators of various sizes.

Thermo-electric, in which the heating of the junction between two different metals produces a very small voltage which may be used for purposes of temperature measurement and as a source of power.

Piezo-electric, in which a very small voltage is produced across certain faces of a crystal by means of frequency control in radio oscillators or for gramophone pick-ups, but it is not suitable for power supply.

Electronic, characterized by the flow of electrons through evacuated or gas-filled tubes, and having the following forms:

1. Thermionic emission, in which the electrons are produced by the heating of special materials.

2. Photo-electric emission, in which electrons are liberated at the surface of certain substances by the action of light.