Importance of Inductance in a. C. Circuits.— Inductance is a property of a circuit, just as is resistance, and is therefore possessed by d.C.

as well as a.c. circuits. In d.c. circuits, however, its effects are not apparent when the current is steady, and are only noticeable when the current is started or stopped, or when it changes in value. The reason for this is that the induced e.m.f. resulting from inductance is due to the rate at which the current, and therefore the flux, is changing. If the current does not change, there is no induced e.m.f. In a.c. circuits, on the other hand, the current is always changing, and therefore the e.m.f. of self-induction is always present. This modifies the value of the current and has far reaching effects.

UNIT 7 CHARGED BODY DIALOGUE

Demonstrator: What body is considered to be negatively charged and what body is considered to be a positively charged one?

Student: From the electric viewpoint a negatively charged body is one having more than its normal number of electrons and a positively charged body is one having less than its normal number of electrons. Demonstrator: Is the normal atom charged or not?

Student: The normal atom is not charged. This means that it does not exert any attractive or repulsive force on the other atoms. Demonstrator: What do you know about the structure of the atom?

Student: The structure of the atom is as follows. In the centre part of the atom are grouped some positive and negative particles, called protons and neutrons respectively. The nucleus is surrounded with a cloud of electrons. So an atom having an equal number of positive and negative particles shows no electrical charge, the negatives and the positives just neutralizing each other.

Demonstrator: Why is the nucleus said to be positively charged? Student: It is said to be positively charged because the center part of the atom always consists of protons and neutrons only. Demonstrator: What determines the nature of the atom? Student: The number and arrangement of the outer electrons as well as protons and neutrons determine what the atom is whether hydrogen, oxygen, copper, gold, etc.

Demonstrator: How do different elements differ?

Student: All elements we know differ only in the number and arrangement of protons and electrons.

Demonstrator: What do you know about tungsten?

Student: Tungsten has very many electrons placed in certain well- known complex arrangements.

Demonstrator: What happens if one electron is removed from the atom by some means or other?

Student: In this case the balance between positive and negative charges is destroyed; an excess of positive charge exists on the atom, the atom is positively charged. The particle formed after one electron had been removed from the atom is called an ion.

Demonstrator: That's right.

EXERCISES

1. Give derivatives from the following words and translate them:

to attract, to repel, certain, remark, equal, to close, to exceed, to lose, proper.

2. Give synonyms for the following words and use some of them in sentences:

viewpoint, shape, substance, degree, amount, to consist of, sufficient, whereas, complex, peculiar, almost, define, tightly, because of.

3. Make up sentences, using the following word groups: for the most part, in question, in excess of, as a whole.

4. Translate the following sentences, paying attention to the different meanings of but:

1. It seems that the electron is nothing but electricity. 2. If but a few of the atoms of a body have had an electron removed the body has a small charge. 3. One can easily note how voltage increases during the first quarter-turn but then decreases during the next quarter turn. 4.The operator saw all the tubes but one function in the proper way. 5. We expect the current cycle and the voltage cycle to finish in step if they start in step. But in many a. c. circuits the current cycle does not get started as soon as the voltage cycle.

5. Translate the following sentences, paying attention to the tense- form in passive:

1. The readings of every indicating instruments are obtained in a minimum of time. 2. The nature of the material determines the ease with which electrons are allowed to pass. 3. The space surrounding a charged body, in which another charged body is acted upon by a force tending to move it, constitutes an electric field. 4. The action of some instruments cannot be much relied upon because of their being not sensitive enough. 5. The choice of the method is highly influenced by the results required. 6. The space surrounding a charged body is called an electric field of force. This electric field may be thought of as consisting of a number of lines of force representing the directions in which the electric force acts. 7. An e. m. f. induced in a stationary electric circuit, by a change in the number of magnetic lines, linking with it, is reffered to as a "statically induced" e. m.

6. 8. A number of lines of magnetic intensity are spoken of collectively, as "magnetic flux" and the number per square centimetre of cross section as the "flux density".

6. Translate the following sentences:

1.In case there is no current in a conductor, there can be no electric field within it. 2. Unless the opposing force just balanced the impressed force, a steady state would not be obtained. 3. If each atom in an orange measured 1 inch in diameter, the orange would be as large as the Earth. 4. If the two charged plates of a condenser had been "short-circulated" by a wire a momentary current would have been established through the wire. 5. If the coil were wound on an iron frame, and provided there were a soft iron core in a centre, the field would, of course, be much stronger. 6. It is necessary that the power apparatus should be operated at maximum efficiency. 7. The transmission of information by radio waves requires that some means be provided to control the radio waves by the desired intelligence. 8. Power apparatus is usually operated at maximum efficiency lest the generating apparatus should burn up. 9. Sometimes the electron acts as though it were a wave. 10. Should the resistance on the lead increase, the efficiency will be improved. 11. If a magnet is brought near any substance traversed by an electric current, this substance will generally be acted upon by a force tending to move it.

7. Translate the following sentences, paying attention to the meanings of the modal verbs with Perfect Infinitives:

1. The experiment must have been done in a wrong way because of the data obtained being in contradiction with Lenz's law. 2. The voltage may have been top high, the insulation being broken down. 3. The friction between the wax and the flannel must have rubbed off some electrons of the flannel molecules and left them on the surface of the wax. 4. The operator should

have examined the trouble caused due to the conductor being overheated. 5. This magnetized body may have exerted some attractive or repulsive force on the other one. 6. The alpha grains must have formed during the 30-sec cooling required to reach a temperature below which diffusion was negligible. 7. To get better results another method ought to have been applied. 8. It must have been known for centuries that a colored glass is cooler at the bottom of a pot than a colorless glass.

VIII. Be ready to speak about a charged body. UNIT 8 INDUCED ELECTRIC CURRENTS

The discovery of induced electric currents goes back more than one hundred years to 1831 and the experiment of Michael Faraday. A straight bar magnet plunged into a coil of wire was found to produce an electric current. The N pole of the magnet being plunged into the coil, a galvano­meter needle deflects to the right. It being withdrawn, the needle deflects to the left, indicating a current in the opposite direction. If the S pole be moved down into the coil, the needle deflects to the left and as it is withdrawn, the deflection is to the right.

The relative motion of the coil and magnet is what produces the current and it makes no difference whether the coil alone moves, whether the magnet alone moves, or whether they both move. In either case, if the relative motion ceased, the current would stop. A "somewhat old-fashioned" way of describing the action is to say that only when a wire is cutting the line of force is there an induced e. m. f. A somewhat more acceptable statement at the present time is, in effect, that only when the total magnetic flux linking a closed electrical circuit is changing is there an induced e. m. f.

A flexible wire connected to an ammeter, and held in the hands, is moved in various ways across the pole of a magnet. In case a straight section of the wire were held over the N pole and moved to the right, an electron current would flow in the one direction. Were the wire moved in the opposite direction, the induced e. m. f. and current would reverse direction. Should the wire be moved vertically upward or downward, parallel to the magnetic induction, no current will flow. In other words, there is an induced e. m. f. only when the total number of lines of induction through the closed circuit is changing.

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

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

2. 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,

3. 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.

4. Give antonyms for the following words:

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

5. 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. e., whether they amplify a narrow or a wide band of frequencies, respectively.

6. 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.

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.

7. 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 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.

8. Retell the text.

8. 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.