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III. Выберите из правого столбика правильные окончания предложений.
IV. Переведите предложения, обратив внимание на сравнительную степень имен прилагательных. 1. The internal resistance of the armature of a d.c. motor is usually less than one ohm. 2. The starting resistance can be gradually reduced allowing a further increase in speed and back-e.m.f.
V. Выразите согласие или несогласие с данными утверждениями (парная работа). 1. Since the induced voltage opposes the applied terminal voltage it is called the back-e.m.f. 2. This back-e.m.f. does not depend on the speed, direction of rotation and the field strength. 3. What actually moves the armature current through the armature coils is the internal voltage drop. 4. The internal resistance of the armature of a d.c. motor is very low. 5. The starting resistance must keep the current flow within safe limits.
VI. Расскажите о пусковом сопротивлении, употребив данные слова и словосочетания: the starting (temporary) resistance, that (которое) current flow, within safe limits, as, the motor, this resistance, gradually, allowing, a further increase in speed and back-e.m.f., at normal speed, out of the circuit, completely, is, keeps, speeds up, is shorted.
U N I T 4
Text 1 Electricity from Magnetism
You will recall that electricity can be generated by moving a wire through a magnetic field. As long as there is a relative motion between the conductor and the magnetic field, electricity is generated. If there is no relative motion between the conductor and the magnetic field, electricity is not generated. The generated electricity is actually a voltage, called an "induced voltage", and the method of generating the voltage by cutting a magnetic field with a conductor is called "induction". You also know that this induced voltage will cause a current to flow if the ends of the conductor are connected through a closed circuit. You know that the amount of voltage induced in the wire cutting through the magnetic field depends on a number of factors. First, if the speed of the relative cutting action between the conductor and the magnetic field increases, the induced e.m.f. increases. Second, if the strength of the magnetic field increases, the induced e.m.f. also increases. Third, if the number of turns cutting through the magnetic field is increased, the induced e.m.f. is increased again. The polarity of this induced e.m.f. will be such that the resulting current flow will build up a field to react with the field of the magnet, and to oppose the movement of the coil. This phenomenon illustrates a principle known as "Lenz’s Law", which states that in all cases of electromagnetic induction, the direction of the induced e.m.f. is such that magnetic field it sets up opposes the motion which produces it. You also know that the direction of the generated current flow is determined by the direction of the relative motion between the magnetic field and the cutting conductor.
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I. Составьте из данных слов предложения. 1. A wire, is generated, electricity, through, by moving, a magnetic field. 2. Is called, an induced voltage, the electricity, generated. 3. A number of factors, of, depends on, the amount, the induced voltage. 4. The speed, if, of the cutting action, increases, the e.m.f. increase, induced. 5. The number of turns, if, cutting, through the magnetic field, is increased, again, is increased, the induced e.m.f. 6. If, of the magnetic field, the strength, increases, also, the induced e.m.f., increases.
II. Ответив одним предложением на вопросы, вы выразите основную мысль первого и третьего абзаца текста.
1. How is electricity generated? 2. On what factors does the amount of the induced voltage depend?
III. Ответьте на вопрос What is this text about?, употребив следующие слова и словосочетания:
the generation of electricity by moving a wire through a magnetic field, the induction, and the direction of the generated current flow.
IV. Письменно изложите содержание текста на английском языке (4-5 предложений). Text 2 Circuit Breakers Circuit breakers are magnetic-type overload relays which are designed to protect circuits from overloads and other abnormal conditions – such as voltage failure and reversed current. Since circuit breakers are basically relays, they have many of the advantages of relays. They are vast acting, may be remotely controlled, and can be adjusted to operate on different values of current. One type of circuit breaker, which is very often encountered and which has characteristics common to all types, is open-type circuit breaker. This circuit breaker has three sets of contacts – the main contact and two auxiliary contacts. The main contact (A) is made of thin strips of copper pressed very close together, and curved into the form of an arch. When the circuit breaker is closed, the ends of the arch of copper strips connect with two fixed contacts. The arch is compressed as the breaker closes, causing the ends of the copper strips to wipe the faces of the fixed contacts, and to ensure good connection. The auxiliary contact (C) called an "arcing contact", has removable carbon tips mounted on long copper springs. The auxiliary contact (B) consists of a heavy copper spring with a removable copper tip. The triggering mechanism in the circuit breaker is a magnetic coil, which may be either in shunt or in series with the circuit. When the current through the coil exceeds a certain value, the coil moves a triggering bar, which permits the contacts to pull open, either by their own weight or as a result of spring action. When the breaker opens, the main contact opens first, while the others are still making contact. The current, therefore, still has a path through the auxiliary contacts (B) and (C), and the main contact (A) opens without arcing. The auxiliary contact (B) opens next, with a very small arc. The last to open is the auxiliary contact (C), where the most severe arcing occurs. But the carbon tips are able to withstand the heat caused by the arcing, and are not burnt away as would be the copper.
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