ЕА 2 курсу 1 частина

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A current being produced means that electrical energy has been created. It has been created at the expense of mechanical work, for in moving the wire across the field, a force F had to be exerted for a distance S. The faster the wire moves, and the stronger the field through which it moves, the greater is the required force and the greater is the induced e. m. f. and the resultant electron current. Provided the wire stops moving in mid-field the e. m. f. drops to zero. These are the essential principles of the electric generator.

EXERCISES

I. Make a written translation of the second passage of the text beginning with: "The relative motion of the coil..."

II. Find in the text synonyms for the following words and give some examples of their use:

to show, movement, to stop, strength, to connect, different, to demand, in fact, quick,

III. Make adjectives from the following verbs, adding the suffix - able and translate them:

to accept, to rely, to measure, to reason, to operate, to apply, to move, to suit, to appreciate, to consider, to notice, to allow, to obtain, to attain, to vary.

IV. Give antonyms for the following words:

upward, internal, outside, slow, open, to move, the same, to increase, like, right,

V. Translate the following sentences, paying attention to the meanings of the words in italics; give your own examples:

1. Whether one plate of a 1-farad capacitor is grounded or not, the potential difference between the plates will be one volt when one plate has a positive charge of one coulomb and the other plate has a negative charge of one coulomb. 2. It should be pointed out that it makes no difference in the above treatment whether the wire moves through a stationary magnetic field or whether the field moves across a stationary conductor. 3. With a metallic sphere, whether solid or hollow, the static charge spreads uniformly over the surface. 4. Amplifiers are also classified as to whether they are tuned or untuned, i. е., whether they amplify a narrow or a wide band of frequencies, respectively.

VI. Translate the following sentences, paying attention to the different meanings of should and would:

1. Some instruments, if actuated by an alternating current, would tend to oscillate between a certain direct reading and the equal reversed reading. 2.

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The calculation of power used is not easily measured by ammeters and voltmeters, either a wattmeter or the oscillograph should be used. 3. A one microfarad condenser in series with two ohms would have a time constant of 0.000.002 second, that is, the current would rise instantaneously upon closing the switch, to some value (depending upon the voltage used in charging) and in 0.000.002 second would have fallen to 37 per cent of this value, and in a correspondingly short time would have dropped to practically zero. 4. The charging of a condenser connected to a source of continuous e. m. f. would take place instantaneously, if there were no resistance in the circuit. 5. A current of an ampere would have to flow only one millionth of a second to charge the condenser to one volt potential difference or one microampere flowing for one second would charge it to the same extent. 6. It is necessary that the voltmeter should be connected in parallel with the battery and the control resistance. 7.It should be noticed that every atom of matter is charged with minute particles of negative electricity which are called electrons. 8. The screen grid should be at a lower potential than that of the plate. 9. Such a coil arrangement would tend to make the circuit unstable since there would be the possibility of feedback of energy from the plate current to the grid circuit through the tube interelement capacity. 10. In order to limit the amount of current flowing in the filament plate circuit it is necessary that the potentiometer should be of a high resistance. 11. The grid must be at a higher potential than that of the filament or the electrons would be drawn to it. 12. Should the temperature of the filament increase, the magnitude of the electron flow will increase. 13. Ventilating ducts in generators are necessary lest the winding should get overheated. 14. The resistance of the machine was not increased lest the voltage should increase. 15. Care should be taken that proper values are applied for efficient generation.

VII. State the kind of subordinate clauses in the following sentences and translate the sentences:

1.The number of electrons emitted from the filament in a unit time depends upon the substance it is made of and upon its temperature. 2. When a straight spring is pulled to one side and released, the kinetic energy it gains upon straightening keeps it moving and it tends to the other side. 3. In the type of rectifier we are going to discuss the internal resistance of the tube varies with the power demand upon it. 4. In the synchrotron the protons continue to go around the chamber, gaining energy each time they pass the accelerating section. 5. The number of times per second the current reverses

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itself is known to be the frequency and is determined by the speed of the armature and the number of field poles. 6. As the amplitude of the alternating current approaches that of the continuous current the negative resistance it encounters decreases in value. 7.We know copper is very nearly as good a conductor as silver.

VIII. Retell the text.

XI. Supplementary reading.

TEXT 4

HYSTERESIS

When a specimen of iron is carried round a magnetic cycle, a certain dissipation of energy takes place. To build up a magnetic field requires the expenditure of a certain amount of energy, and this energy is not all returned when the magnetic field is destroyed, if iron is the medium. This lost energy is dissipated in the iron in the form of heat. If the iron is now magnetized in the reverse direction, the same process is repeated, with the result that when the iron is brought back to its initial state of magnetization, an amount of energy has been expended in taking the iron round its magnetic cycle. This effect of the dissipation of energy, due to the lagging of the flux behind the magnetizing force, is called hysteresis, and the closed curve is called a hysteresis loop. It should be noted that as the value of H is raised and lowered, the curve progresses in the direction indicated by the arrows.

It can be proved that the energy dissipated in taking the iron round a magnetic cycle is proportional to the area of the hysteresis loop, so that it is desirable to employ iron having a When subjected to alternating magnetization, the loop is traversed once for every cycle of the current, and so the energy wasted per second is proportional to the frequency. This energy wasted per second constitutes a loss, measured in watts, and is known as the hysteresis loss. In addition to being proportional to the frequency it also depends, in a somewhat complicated manner, upon the maximum flux density attained. This is usually expressed by saying that the hysteresis loss is proportional to Bx where x has a value approximately equal to 1.6 at moderate flux densities, but which may reach a value as high as 4 for extremely high flux densities.

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UNIT 9

LENZ'S LAW

Lenz's law might have been predicted from the principle of the conservation of energy. When you move a magnet toward a coil and thus induce a current in its windings, the induced current heats the wire. In order to supply the energy to do this, you must do work in overcoming an opposing force. If the force did not oppose the motion, you would create energy. Thus the magnetic field of the induced current is seen to oppose the change.

Lenz's law and the right hand rule can be used to determine the direction of an induced current. The north pole of a magnet being moved closer to a coil, the induced current causes a field which opposes the motion, a north pole being produced on the nearer face. To cause this north pole, magnetic lines must emerge from the face of the coil. Now grasp the coil with your right hand, so that your fingers point in the direction of the induced magnetic field. Your thumb will point in the direction of the current, that is, counterclockwise.

The Induced Current Opposes the Change. A magnet pole being moved toward one face of a coil, the current induced in the coil produces a magnetic field. Moreover, this field always opposes the change of magnetic flux that is occurring. For example, move the north pole of a magnet closer to one face of a coil. The induced current will be counterclockwise and will oppose the change of flux through the coil. Remove the bar magnet, and the induced current in the coil will be clockwise, again opposing the change. This rule is expressed by Lenz's law, as follows:

Whenever a current is induced, its magnetic field opposes the change of flux.

EXERCISES

I.Pick up synonyms out of the following list of words and use some of them in sentences:

to predict, movement, force, thus, for instance, reason, to eliminate, cause, strength, to supply, to foretell, motion, to provide, variation, to use, to determine, close, in this way, change, near, to define, to apply, to show, to occur, for example, to remove, to point, to take place.

II.Pick up antonyms out of the following list of words:

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right, north, clockwise, upward, to insert, difficult, downward, counterclockwise, left, south, to remove, to cool, near, easy, to heat, far.

III.Translate the following sentences; observe the different meanings of the verb to do:

1. When the molecule is placed in the electric field, the electrons try to move and do so for an instant. 2. If only a few of the insulator's molecules do release one electron each, the insulator at once completely breaks down and becomes a conductor. 3. If by some means we can change the current in a coil without changing the flux rapidly, then the current may rise and fall as suddenly as it does in a purely resistive circuit. 4. The electrons, the motion of which constitutes the current, do not actually pass from one plate of the condenser to the other through the dielectric. 5. An important question for the radio engineer to consider has to do with the shape of current which flows in a circuit connected to an alternator. 6. The emission or evaporation of electrons takes place at lower temperatures than does that of atoms.

IV.Translate the following sentences with an emphatic inversion,beginning with predicatives expressed by participles:

1. Linking the two local circuits is the transmission circuit which contains two wires, and the large windings of the two induction coils. 2. Moving around the nucleus, and at a considerable distance from it are the rest of the electrons required to make the atom neutral. 3. Included for comparison are the L-cathode emission densities at the same temperatures and field strength. 4. Superimposed upon the dominant thermionic emission is a small amount of emission caused by the increasing positive potential of the anode. 5. Rotating with the lenses is a pair of double ended reflecting prisms, each of which directs the light beam from the corresponding lens forward a photocell assembly located at the front of the instrument.

V.Translate Into English:

Було виявлено, що прямий полосний магніт, що рухається в котушці із дроту, створює в ній електричний струм. Це взаємне пересування магніту й котушки й створює цей струм. Якби цей відносний рух припинився, струм перестав би текти. Чим швидше рухається катушка, тим більше наведена е.р. с.

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UNIT 10

SELF-INDUCTION

An emf can be induced by varying the number of magnetic lines threading through a circuit, the induced current always opposing the change that is occurring, no matter what causes the change of magnetic flux. It may be due to the motion of a magnet or to the change of current in a nearby electrical circuit as in the transformer. The change of magnetic flux may also be due to a change of current in the coil itself, this effect being known as self-induction.

Suppose several hundred feet of wire, in a single loop, to be connected in series with an incandescent lamp, a 115-volt direct-current source, and a switch. The switch being closed, the current in the circuit will increase, in a few millionths of a second, to a steady value determined by Ohm's law. Now let this wire be wound onto an iron rod to form a coil. When the switch is again closed, the current will increase to the same final value as before, but the time required will be several hundredths of a second. In the coil there are hundreds of turns of wire, side by side. The current in each turn causes magnetic lines that thread through the other turns. An increase of current in any loop varies the flux through all the others, the change of flux of magnetic lines generating an emf. This induced emf opposes the change of current.

Self-induction is known to oppose not only the increase of current in a coil but the decrease also. The circuit being opened, the current will not stop instantly. The forward induced emf will cause a spark to appear at the switch.

In order to demonstrate self-induction, connect a large electromagnet and an incandescent lamp, in parallel with each other, through a resistor to a direct-current source. When you close the switch, at first the increasing current through the coils of the electromagnet increases the flux, thus generating an opposing emf. Self-induction impedes the current through the coil. Most of the current flows through the lamp, which glows brightly. The current having become constant, most of the current flows through the coils, and the lamp becomes dim. The switch being opened, the flux through the coils will decrease rapidly. The induced emf will make the lamp glow brightly for an instant.

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EXERCISES

I. Find in the text Infinitives, Gerunds and Participles, state their forms and functions and analyse them.

II. Give synonyms for the following words:

motion, to oppose, to occur, to be due to, to call, several, to connect, steady, to determine, to require, value, to vary, to decrease, to stop, constant, rapidly.

III. Find in the text antonyms for:

never, increase, to open, to start, different, backward, to separate, slowly, bright, before, unparallel

IV. State the kind of the subordinate clauses and translate them properly:

1. The relative motion of the coil and magnet is what produces the current. 2. What Ohm discovered was that the ratio of the potential difference between the ends of a metallic conductor and the current flowing through the metallic conductor is a constant. 3. When the temperature is reduced, scattering by the impurities is predominant. 4. As the temperature of the diode is increased, the lattice atoms are in more rapid motion. 5. Once a molecule has formed, it will move about and behave as a unit particle under various physical conditions. 6. To avoid any trouble the operator should always check up whether the devices are in order. 7. That the coefficients are positive follows from the problem statement. 8. It is very important that the programmer should understand some of the control logic on the interface card. 9. The phenomenon we are going to observe is of great practical importance. 10. The operator could not state the exact moment that phenomenon occurred.

V. Answer the following questions:

1. How can an e. m. f. be induced? 2. What does the induced current always oppose? 3. What is the change of magnetic flux due to? 4. What do we call self-induction? 6. When will the current in the circuite increase to a steady value determined by Ohm's law? 6. How many turns are thrtr in a coil? 7. What does the current in each turn cause?8. What generates an emf? 9. What does self-induction oppose? 10. How is the opposing emf generated? 11. When does the lamp glow brightly? 12. When does the lamp become dim? 13. What happens when the switch is open? 14. What will make the lamp glow brightly for an instant?

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VI. Be ready to speak about self-induction making use of your answers to the questions.

VII.Translate into English the summary of the text:

Е. р. с. може бути наведена шляхом зміни числа магнітних силових ліній, що проходять через ланцюг. Наведений струм завжди протидіє змінам, що відбуваються. Показано, як наводиться е.р. с. і як вона протидіє зміні струму, як його збільшенню, так і зменшенню. Пояснюється, як відбувається (виникає) самоіндукція.

VIII.Supplementary reading.

TEXT 5

ELECTROMAGNETIC INDUCTION

If a piece of wire is looped round a coil and when the current to the coil is switched either on or off, a current will flow through the loop of wire. This latter current is called an induced current, and it will occur only when the magnetic field round the coil is changing.

Another way of inducing a current is to move a length of wire across a magnetic field so that it cuts the magnetic lines of force. This is the principle of the dynamo, which consists essentially of a coil of wire rotating in a magnetic field. In this way the lines of force of the magnet are cut by the rotating coil and a current is therefore developed in the coil. Thus the mechanical energy which is used to rotate the coil in the magnetic field is converted into electrical energy within the coil. The dynamo, or generator as larger machines are usu ally called, is one of the most economical methods of producing electricity on a large scale, and this is the method adopted in power stations.

The exact reverse of a dynamo is the electric motor, in which the coil of wire is supplied with a current which is broken at regular intervals by a device known as a commutator; the resulting magnetic forces cause the coil to rotate so that electrical energy is transformed into mechanical energy.

In general, any movement of an electrically charged particle, or any electric current, creates a magnetic force, and conversely any movement of a magnetic pole creates an electric force.

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THE ELECTROMAGNETIC FIELD

The example of an induced current can be extended to the circuit illustrated in the figure presented by the teacher, in which an electric cell supplies a stream of electrons to a coil of wire, A, through a switch S.

This switch is so arranged that it reverses the connections of the cell to the coil A when it is rotated. Now, if the switch is continuously rotated by some mechanical means, the current in the coil A will constantly and regularly change in direction. The field created by the constantly changing current is called an electromagnetic field, and it will surround the coil A.

If a second and similar coil of wire, B, is placed within this field, a constantly changing current will be induced in this coil, so that if an instrument for measuring current is placed across its ends, the needle will swing backwards and forwards, indicating first a negative and then a positive current, depending on the position of the switch S. Such a current, which regularly changes direction, is called an alternating current. However, if a switch was used that gradually increased, decreased, and reversed the current in coil A and readings were taken on the ammeter at very frequent intervals, a graph of these readings would show that the current changes from positive to negative in a smooth and regular manner.

This is a typical wave-form. It is characteristic of the way in which energy is radiated from one piece of matter (coil A) to another (coil B) by an electromagnetic field in space.

In general, any oscillating current, or any vibrating electric charge, will produce an electromagnetic field through which energy will be transmitted in the form of waves at a finite speed. As it is not easy to visualize how energy can be transmitted by these electromagnetic waves, an analogy may help. Consider a long glass trough filled with water, in which at one end a pencil is inserted and vigorously vibrated. The vibration of the pencil will cause waves to run along the surface of the water which have the appearance illustrated in the figure presented by the teacher,— if we are able to assume that the trough is sufficiently long to be able to avoid the complications of waves reflected from the ends.

If the water is examined carefully, it will be seen that no water actually moves from one end of the trough to the other; each particle of water is simply moving up or down. If a cork with a nail sticking out of it is floated in the water in the middle of the trough, it will bob up and down in the waves. The nail sticking into the cork can be made to hit a bell on its

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upstroke; this illustrates that the mechanical energy of the moving pencil could be transferred by a wave-motion to the cork, as mechanical energy in ringing the bell.

In the trough the waves are supported in the medium of the water; in the case of electromagnetic waves, no medium is required. In the example the two coils, A and B, can both be housed in a container from which all the air had been evacuated Although it is not easy to visualize, electromagnetic waves need no supporting medium— they can travel through empty space.

UNIT 11

CONDENSERS AND DIELECTRIC MATERIALS

The dielectric of a condenser is one of the three essential parts. It may be found in solid, liquid, or gaseous form or in combinations of these forms in a given condenser.

The simplest form of a condenser consists of two electrodes or plates separated by air, this representing a condenser having a gaseous dielectric. If this imaginary condenser had the air between the plates replaced by a nonconducting liquid, such as transformer oil, and if the distance between the plates were the same as in the first case, the capacitance would be found to have increased several times on account of the oil having a higher value of dielectric constant than air which is usually taken as 1.

The space between the plates being occupied by a solid insulator, a condenser would result, which would be practical, as far as the possibility of constructing it is concerned. It would be found, in this case too, that the capacitance of the condenser was several times larger than when air was the dielectric.

The mechanical construction of either air or liquid dielectric condensers requires the use of a certain amount of solid dielectric for holding the two sets of plates.

There are a great many dielectric or insulating materials available from which one may choose. A material which is very good from the electric standpoint is often found to be poor mechanically or vice versa, air being the gas generally used as a dielectric. Compressed air has been used in some high-voltage condensers, compressed nitrogen and carbon oxide being also in use.

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