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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 conducting circuit in one direction only. It flows provided a direct voltage source is applied to the circuit. An alternating current (a.c.) is a current that changes its direction of flow through a circuit. It flows provided an alternating voltage source is Current transformers are used for operating ammeters, wattmeters, aJapplied to the circuit. Alternating current flows in cycles. The number other measuring devices. They produce in the meters a current lower thai of cycles per second is called the frequency of the current. In a 60-cycle
fhft mMSlirpH rilTTPnt Kilt nmnnrtinnol «л i4 I.... _,. ,• • sn • alternating current circuit the current flows m one direction 60 times and in the other direction 60 times per second. It is easy to transform a.c. power from one voltage to another by a transformer. Transformers are also used to step down the voltage at the receiving point of the line to the low values that are necessary for use. When necessary a.c. can be changed into d.c. but this is seldom necessary. 4. Complete the sentences using the correct variant:
1. a) Cover the right column and read the English words. Translatij them into Russian and check your translation. b) Cover the left column and translate the Russian words back int<| English.
переменный прямой направление течение необходимый рассматривать использование
1. D.c. is a current that 2. A.c. flows provided
a) changes its direction of flow. b) flows in one direction.
a) a direct voltage source is applied. b) an alternating voltage source is applied.
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5. Complete these sentences using while. Follow the model on page 13. 1. An alternating current changes its direction of flow 2. A direct current flows provided a direct voltage source is applied 6. 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? 7. Read about frequency, answer the question that follows. Frequency The number of cycles per second is the frequency of an alternating cul rent. There are two frequencies: the standard for Europe is 50 cycles pa second while the standard for the USA is 60 cycles per second. A standard frequency has a great advantage since different systems can be interco nected. What is the advantage of a standard frequency?
Translal b) Cover the left column and translate the Russian words back in
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Translate into Russian and put down the Russian equivalents. Then translate them back into English (orally). a. definite value primary coil wire coil mutual inductance _______________________________ varying current _______________________________ one ampere per second ________________________________ Ь. 1. Coils of wire are called inductors. 2. Two coils are brought close together. ______ 3. A source of current is applied to one of the coils. _______ 4. Mutual inductance is measured in henries. 3. Which of the words are nouns and which are verbs? resistor, resist, resistance; induce, induction, inductor, inductance; conductor, conduct, conductance; compute, computer
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