| SERIES CIRCUIT AND PARALLEL CIRCUIT Compare circuits a and b. Circuit a consists of a voltage source and two resistors. The resistors are connected in series. Circuit a is a series circuit. Circuit b consists of a voltage source and two resistors, The resistors are connected in parallel. Circuit b is a parallel circuit. A parallel circuit has the main line and parallel branches. In circuit b the value of voltage in Ri equals the value of voltage in R2. The value of voltage is the same in all the elements of a parallel circuit while the value of current is different. A parallel circuit is used in order to have the same value of voltage. n circuit a the value of current in Rx equals the value of current in R 2. The value of current is the same in all the elements of a series circuit while the value of voltage is different. A series circuit is used in order to, have the same value of current. In circuit c a trouble in one element results in no current in the whole circuit. In circuit d a trouble in one branch results in no current in that branch only. A trouble^ in the main line results in no current in the whole circuit. | . . . . . . . . R1 . R2. . . R1 R2 . . - D . . |
| Familiar Types of Circuits Since you cannot see the electricity flow in circuits, you must reason where the current flows. To help you in such reasoning, make drawings or plans first. Tnen if it appears to work on paper, do the wiring. Any circuit is provided with a voltage source, a conductor, and a switch. A voltage source supplies current. A conductor is used for connecting the elements of the circuit. A switch is used for controlling the current in the circuit. There are many kinds of circuits, such as open circuits, closed circuits, short circuits, parallel circuits, and series circuits. The circuit is said to be open when a switch is at rest or not being pressed or closed. A broken wire is an example of open circuits. An open circuit results in no current in it. A closed circuit is simply a complete circuit. Pressing a pushbutton or making proper connections refers to closing circuits. Short circuits are seldom desirable. They often result in damage. In other words, damage in the circuit often results from the short circuit. The short circuit refers to the conditions in the circuit which are the cause of fires, blown fuses, etc. Fuses are used as safety devices to stop the flow if it should become too great. In everyday electric work, we very often -deal with circuits where the current is divided between two or more branches. When a circuit is divided in such a manner that part of the current goes through one branch and part through another, it is called a parallel circuit. A parallel circuit has the main line and parallel branches. In parallel circuits there is a difference between an open in the main line and an open in a branch. An open in the main line of a circuit of this type results in no current in the whole circuit while an open in a parallel branch results in no current in that branch only. This is the advantage of a parallel circuit. When electrical devices are in a line so that the current is not divided at any point, they are said to be in series. The electric bell circuit is considered to be a typical example of a series circuit. The (minus) terminal of each part of the circuit is joined to the +(plus) terminal of the next part, so that all the current does flow through each part of the circuit | .. , . . . , . . . . : , , , . . , . . . . . . , .. , . . , , . . . , . . , , . . , . |
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| MAGNETIC EFFECT OF AN ELECTRIC CURRENT 1.The invention of the voltaic cell in 1800 gave electrical experimenters a source of a constant flow of current. Seven years later the Danish scientist and experimenter, Oersted, decided to establish the relation between a flow of current and a magnetic needle. It took him at least 13 years more to find out that a compass needle is deflected when brought near a wire through which the electric current flows. At last, during a lecture he adjusted, by chance, the wire parallel to the needle. Then, both he and his class saw that when the current was turned on, the needle deflected almost at right angles towards the conductor. 2.As soon as the direction of the current was reversed, the direction the needle pointed in was reversed too. As seen in Fig. 5 the north end of the needle moves away from us when the current flows from left to right. Oersted also pointed out that provided the wire were adjusted below the needle, the deflection was reversed. 3.The above-mentioned phenomenon highly interested Ampere who repeated the experiment and added a number of valuable observations and statements. He began his research under the influence of Oersteds discovery and carried it on throughout the rest of his life. 4.Everyone knows the rule thanks to which we can always find the direction of the magnetic effect of the current. It is known as Amperes rule. Ampere established and proved that magnetic effects could be produced without any magnets by means of electricity alone. He turned his attention to the behaviour of the electric current in a single straight conductor and in a conductor that is formed into a coil, i.e. a solenoid. 5. When a wire conducting a current is formed into a coil of several turns, the amount of magnetism is greatly increased. It is not difficult to understand that the greater the number of turns of wire, the greater is the m.m.f. (that is the magnetomotive force) produced within the coil by any constant amount of current flowing through it. In addition, when doubling the current, we double the magnetism generated in the coil (see Fig. 6). A solenoid has two poles which attract and repel the poles of other magnets. While suspended, it takes up a I north and a south direction exactly like the compass needle. 6.A core of iron becomes strongly magnetized if placed within the solenoid while the current is flowing.When winding a coil of wire on an iron core, we obtain an electromagnet. That the electromagnet is a controllable and reliable magnet is perhaps known to everyone. It is, so to say, a temporary magnet provided by electricity. 7.Its behavior is very simple. The device is lifeless unless an electric current flows through the coil. However, the device comes to life provided the current flows. The iron core will act as a magnet as long as the current continues to pass along the winding | 1. 1800 . 7 13 , , , , . , . 2. , . 5 , . . 3. . . 4. , . . 5. . . 2 2 . 2 . . 6. . . . . 7. . . . , . |
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| ELECTRIC GENERATORS AND MOTORS A device for converting mechanical energy into electric energy is called a generator. The function of a motor is just the reverse, that is, it transforms electric energy into mechanical energy. The enormous energy of steam engines, gas engines, and water turbines can now be transformed into. electricity and transmitted many miles. The generator has revolutionized modern industry by furnishing cheap electricity. The essential parts of a generator are: a) the magnetic field, which is produced by permanent magnets or electromagnets; and b) a moving coil of copper wire, called the armature, wound on a drum. D. c. generators are used for electrolytic processes. Large d. c. generators are used in certain manufacturing processes, such as steel making. Generators of small capacities are used for various special purposes, such as welding, automobile generators, train lighting, communication systems, etc. | - . , . . , . . . : ) , .; ) . . , . : , , , . |
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The Generator
| The dynamo invented by Faraday in 1831 is certainly a primitive apparatus compared with the powerful, highly efficient generators and alternators that are in use today. Nevertheless, these machines operate on the same principle as the one invented by the great English scientist. When: asked what use his new invention had, Faraday asked in his turn: What is the use of anew-born child? As a matter of fact, the new-born child soon became an irreplaceable device we cannot do without. Although used to operate certain devices requiring small currents for their operation, batteries and cells are unlikely to supply light, heat and power on a large scale. , Indeed, we need electricity to light up millions of lamps, to run trains, to lift things, and to drive the machines. Batteries could not supply electricity enough to do all this work. That dynamo-electric machines are used for this purpose is a well-known fact. These are the machines by means of which mechanical energy is turned directly into electrical energy with a loss of only a few per cent. It is calculated that they produce more than 99.99 per cent of all the worlds electric power. There are two types of dynamos, namely, the generator and the alternator. The former supplies d.c. which is similar to the current from a battery and the latter, as its name implies provides a.c. To generate electricity both of them must be continuously provided with energy from some outside source of mechanical energy such as steam engines, steam turbines or water turbines, for example Both generators and alternators consist of the following principal parts: an armature and an electromagnet. The electromagnet of a d.c. generator is usually called a stator for it is in a static condition while the armature (the rotor) is rotating. Fig. 7 shows the principles the construction of an elementary d.c. generator is based upon. We see the armature, the electromagnet, the shunt winding, the commutator and the load. Alternators' may be divided into two types: 1. alternators that have a stationary armature and a rotating electromagnet; 2. alternators whose armature serves as a rotor but -this is seldom done. In order' to get a strong e.m.f., the rotors in large machines rotate at a speed of thousands of revolutions per minute (r.p.m.). 'The faster they rotate, the greater the output voltage will produce. In order to produce electricity under the most economical conditions, the generators must be as large as possible. In addition to it, they should be kept as fuily loaded as possible all the time. | 1831, - , , , , . , , , . : , , , " "? , " , .. , , . , , , , , , . .. .. . , 99.99 , , ( ). , ., , - , , , : . , , (). . 7 . , , , . . ' , , : 1. , ; 2. , .. , '' ...., (r.p.m.). , . , , . , . |
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| BRANCHES OF ELECTRICITY The study of electricity may be divided into three branch es: magnetism, electrostatics and electrodynamics. Magnetism is the property of the molecules of iron and some other substances to store energy in a field of force. Electrostatic is the study of electricity at rest. Rubbing glass with silk produces static electricity. Electrodynamics is the study c electricity in motion, or dynamic electricity. The electric current which flows through wires is a good example of the latter type of electricity. This flow of electricity through a conductor is analogous to the flow of water through a pipe. A difference of pressure at the two ends of the pipe is necessary in order to maintain a flow of water. A difference of electric pressure is necessary to maintain a flow of electricity in a conductor. Different substances differ in electrical conductivity because of the ease with which their atoms give up electrons. Electrical energy has intensity and quantity. Instruments have been devised which can be used to measure it in amperes and volts. | , 3 : , . . . . , . . . . . . , , , . |
| From the History of Electricity. 1. There are two types of electricity, namely, electricity at rest or in a static condition and electricity in motion, that is the electric current. Both of them are made up of electric charges, static charges being at rest, while electric current flows and does work. Thus, they differ in their ability to serve mankind as well as in their behaviour. 2. Let us first turn our attention to static electricity. For a long time it was the only electrical phenomenon to be observed by man. As previously mentioned at least 2.500 years ago, or so, the Greeks knew how to get electricity by rubbing substances. However, the electricity to be obtained by rubbing objects can't be used to light lamps, to boil water, to run electric trains, and so on. It is usually very high in voltage and difficult to control, besides it discharges in no time. 3. As early as 1753, Franklin made an important contribution to the science of electricity. He was the first to prove that unlike charges are produced due to rubbing dissimilar objects. To show that the changes are unlike and opposite, he decided to call the charge on the rubber-negative and that on the glass-positive. In this connection one might remember the Russian academician V.V.Petrov. He was the first to carry on experiments and observations on the electrification of metals by rubbing them one against another. As a result he was the first scientist in the world who solved that problem. 4. Who does not know that the first man to get the electric current was Volta after whom the unit of electric pressure, the volt was named? His discovery developed out of Galvani's experiments with the frog. Galvani observed that the legs of a dead frog jumped as a result of an electric charge. He tried his experiment several times and every time he obtained the same result. He thought that electricity was generated within the leg itself. 5. Volta began to carry on similar experiments and soon found that the electric source was not within the frog's leg but was the result of the contact of both dissimilar metals used during his observations. However, to carry on such experiments was not an easy thing to do. He spent the next few years trying to invent a source of continuous current. To increase the effect obtained with one pair of metals, Volta increased the number of these pairs. Thus the Voltaic pile consisted of a copper layer and a layer of zinc placed one above another with a layer of flannel moistened in salt water between them. A wire was connected to the first disc of copper and to the last disc of zinc. The year 1800 is a date to be remembered for the first time in the world's history a continuous current was generated. | . 1. , , , . , , , . , . 2. , . , . 2.500 , - , , , . , , , , , , , . - , ( ). 3. 1753, . , , . , , -, -. .. . , , . , . 4. , , , , ? . , . . , . 5. , , () , . , .. , . , , . , , . . 1800 , () . |
| 2/THE NATURE OF ELECTRICITY The ancient Greeks knew that when a piece of amber is rubbed with wool or fur it achieves the power of attracting light objects. Later on the phenomenon was studied, and the word electric, after the Greek word electron, meaning amber was used. Many scientists investigated electric phenomena, and during the nineteenth century many discoveries about the nature of electricity, and of magnetism, which is closely related to electricity, were made. It was found that if a sealing-wax rod is rubbed with a woolen cloth, and a rod of glass is rubbed with a silken cloth, an electric spark will pass between the sealing-wax rod and the glass rod when they are brought near one another. Moreover, it was found that a force of attraction operates between them. An electrified sealing-wax is repelled, however, by a wax rod, and also an electrified glass rod is repelled, by a similar glass rod. The ideas were developed that there are two kinds of electricity, which were called resinous electricity, and that opposite kinds of electricity attract one another, whereas similar kinds repel one another. | , , .. , "" . , , , .. , , , , , , , .. , . , , , ., |
| .Kinds of Currents As you, certainly, know electric charges in motion constitute a current. There are two kinds of current, namely: direct and alternating. A direct current (d.c. for short) flows through a conducting circuit in one direction only.vl.t does not change in magnitude. It is measured by the quantity of electricity passing through any section of the conductor in one second. By the way, direct current generators produce a direct current. A direct current is, of course, useful and we know the electrical systems in airplane, and in automobile to use direct current. It is also used in research work, in the telephone, in the telegraph, etc. An alternating current (a.c. for short) is a current that changes its direction of flow through a c ircuit. To alternate means to move back and forth. The current flowing first in one direction and then in another, we name it an alternating current. An alternating current flows in cycles, the number of cycles per second being called the frequency of the current. The frequency of the current is known to be measured in cycles per second. The standard frequency in Russia is 50 c.p.s. (cycles per second). From the above-mentioned, it is clear that a direct current system is useful. However, in spite of its usefulness a direct current system has one great disadvantage. Namely, there is no economical way by means of which one can increase or decrease its voltage. The alternating current does not know this disadvantage, alternating voltage increasing or decreasing with little loss owing to a transformer. Using a transformer, it is possible to convert power at low voltage into power at high voltage, power at high voltage being also transformed into power at low voltage. Power being transmitted over long distances with less loss at high voltage, it is more economical to increase the voltage for transmission and to decrease it to the voltage which is best suited for the particular use. An alternating current is widely used both in industry and in everyday life. A.c. generators produce alternating current. It was Yablochkov, the great Russian scientist, who first widely applied the alternating current in practice. His electric candle was fed by the a.c, Yablochkov's candle having given the first and most decisive stimulus to the development of the a.c. system, a number of plants began to turn out generators producing the alternating current | , , , , . , : . (d.c. ) . . , . , . , , . , , . (a.c. ) - , . . , . , . , . 50 , ., . , . , () , . , () () ,() () . () , () . , . A.c. . , , . a.c ( ), , (a.c.), , |
| ELECTROMOTIVE FORCE AND RESISTANCE As was previously stated, there is always a disorderly movement of free electrons within all substances, especially metals. Let us assume that there is a movement of electrons through the wire, say, from point A to point B. What does it mean? It means that there is an excess of electrons at point A. Unless there were a flow of electric current between A and B in any direction, it would mean that both the former and the latter were at the same potential. Of course, the greater the potential difference., the greater is the electron flow. The electromotive force (e.m.f.) is the very force that moves the electrons from one point in an electric circuit towards another. In case this e.m.f. is direct, the currcnt is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too. The e.m.f. is measurable and it is the volt that is the unit used for measuring it. One need not explain to the reader that a current is unable to flow in a circuit consisting of metallic wires alone* A source of an e.m.f. should be provided as well. The source under consideration may be a cell or a battery, a generator, a thermocouple or a photocell, etc. In addition to the electromotive force and the potential difference reference should be made here to another important factor that greatly influences electrical flow, namely, resistance. The student probably remembers that all substances offer a certain amount of opposition, that is to say resistance, to the passage of current. This resistance may be high or low depending on the type of circuit and the material employed. Take glass and rubber as an example. They offer a very high resistance and, hence, they are considered as good insulators. Nevertheless, one must not forget that all substances do allow the passage of some current provided the potential difference is high enough. Imagine two oppositely charged balls suspended far apart in the air. In spite of our having a difference of potential, no current flows. How can we explain this strange behaviour? The simple reason is that the air between the balls offers too great a resistance to current flow. However, the electrons, could certainly flow from the negatively charged ball towards the positively charged one provided we connected them by a metal wire. As a matter of fact, it is not necessary at all to connect both balls in the manner described in order to obtain a similar result. All that we have to do is to increase the charges. If the potential difference becomes great enough, the electrons will jump through the air forming an electric spark. One should mention in this connection that certain factors can greatly influence the resistance of an electric circuit. Among them we find the size of the wire, its length, and type. In short, the thinner or longer the wire, the greater is the resistance offered. Besides, could we use a silver wire, it would offer less resistance than an iron one. | 1 , . , . ? , , . , . , , . - , , . - , - , , , . , , . , . . , , .. , . , . . . . (). . . . ? . . . . . . , . , , . |
