Types of Current

Current is a flow of electricity through a circuit. Let us consider two main types of current: direct and alternating. A direct current (d.c.) flows through a circuit that conducts in one direction only. It can flow if a direct voltage source is applied to the circuit.

An alternating current (a.c.) is a current which changes its direction of flow through a circuit. It can flow if an alternating voltage source is applied to the circuit. Alternating current flows in cycles. The number of cycle per second is called the frequency of the current. In an alternating current circuit that has the frequency of 60 Hertz the current flows in one direction 60 times and in the other direction 60 times per second.

When this necessary a.c. can be changed into d.c. although it doesn’t occur very often.

The number of cycles per second is the frequency of an alternating current. There are two frequencies: the standard for Europe is 50 cycles per second while the standard for the USA is 60 cycles per second. A standard frequency has a great advantage since different systems can be interconnected.

Упражнение III. Закончите предложения.

1. D.c. is a current that a) changes its direction of flow.

b) flows in one direction.

2. A.c. flows if a) a direct voltage source is applied.

b) an alternating voltage source is applied.

3. In an alternating current a) current flows in one direction 60 times

circuit 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.

Упражнение IV. Ответьте на вопросы по тексту.

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. It is often necessary to change a.c. into d.c.?

Unit III Electron tubes

Познакомьтесь с новыми словами

1. plate [pleit] – 1) пластина, плата; 2) анод

2. to emit [i’mit] – испускать, излучать

emission – излучение, эмиссия

emitter – излучатель, эмиттер

3. to collect [kƏ’lekt] – собирать

collection – 1) собирание; 2) собрание, коллекция

collector – собиратель, коллектор

4. to heat [hi:t] – нагревать

heating – нагревание, накал

heater – нагреватель

5. direct [di’rekt] – прямой

indirect – непрямой, косвенный

direction – направление

6. case [keis] – случай

7. power [’pauƏ] – 1) мощность; сила; 2) власть

8. to supply [sƏ’plai] – 1) питание, подводить эл. энергию; 2) снабжать, поставлять

supply – 1) (электро) питание; 2) запас

9. voltage supply = power supply = voltage source = power source – источник питания

10. to separate [’sepƏreit] – разделять, отделять

separate [’seprit] – отдельный

11. to repel [ri’pel] – отталкивать

12. to attract [Ə’trækt] – притягивать

13. toward(s) [tƏ’wo:dz] – к, по направлению к

14. thus [ð۸s] – таким образом

15. that is – то есть

16. to detect [di’tekt] – 1) детектировать, демодулировать; 2) обнаруживать

detection – 1) детектирование; 2) обнаружение

detector – 1) детектор; 2) устройство обнаружения

17. to switch [swit∫] – переключать

to switch on – включать; to switch off – выключать

switch – переключатель, ключ

18. to consist of [kƏn’sist] – состоять из

19. the same [seim] – тот же самый

20. to control [kƏn’troul] – 1) управлять, регулировать; 2) контролировать, проверять

21. grid [grid] – сетка

control grid – управляющая сетка

22. to serve [sƏ:v] – служить

23. amount [Ə’maunt] – 1) количество; 2) величина

24. value [’vælju:] – величина, значение

25. to depend on [di’pend] – зависеть от

dependence – зависимость

26. to result in [ri’z۸lt] – приводить к

to result from – исходить из

27. back [bæk] – 1) назад, обратно; 2) спина

28. to pass [pa:s] – 1) проходить; 2) пропускать

passage [’pæsidʒ] – 1) прохождение; 2) проход

29. wire [’waiƏ] – 1) проволока; 2) провод

to wire – делать проводку, монтировать

wireless – 1) беспроволочный; 2) радио

30. because of – из-за

31. property [’propƏti] – свойство