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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Basic laws of electric current. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electricity for beginners Ohm's law. Voltage and current are considered the most convenient characteristics of electrical circuits. One of the main features of the use of electricity is the rapid transportation of energy from one place to another and its transfer to the consumer in the desired form. The product of the potential difference and the current strength gives power, i.e., the amount of energy given off in the circuit per unit time. As mentioned above, to measure the power in an electrical circuit, it would take 3 devices. Is it possible to do with one and calculate the power from its readings and some characteristic of the circuit, such as its resistance? Many people liked this idea, they considered it fruitful. So, what is the resistance of a wire or a circuit as a whole? Does a wire, like water pipes or pipes in a vacuum system, have a constant property that might be called resistance? For example, in pipes, the ratio of the pressure difference creating flow divided by the flow rate is usually a constant characteristic of the pipe. In the same way, the heat flow in a wire is subject to a simple relationship, which includes the temperature difference, the cross-sectional area of the wire, and its length. The discovery of such a relationship for electrical circuits was the result of a successful search. In the 1820s, the German schoolteacher Georg Ohm was the first to start looking for the above ratio. First of all, he aspired to fame and fame, which would allow him to teach at the university. That was the only reason he chose a field of study that offered particular advantages. Om was the son of a locksmith, so he knew how to draw metal wire of different thicknesses, which he needed for experiments. Since in those days it was impossible to buy a suitable wire, Om made it with his own hands. During the experiments, he tried different lengths, different thicknesses, different metals and even different temperatures. All these factors he varied in turn. In Ohm's time, batteries were still weak, giving a current of variable magnitude. In this regard, the researcher used a thermocouple as a generator, the hot junction of which was placed in a flame. In addition, he used a crude magnetic ammeter, and measured potential differences (Ohm called them "voltages") by changing the temperature or the number of thermal junctions. The doctrine of electrical circuits has just received its development. After the invention of batteries around 1800, it began to develop much faster. Various devices were designed and manufactured (quite often by hand), new laws were discovered, concepts and terms appeared, etc. All this led to a deeper understanding of electrical phenomena and factors. Updating knowledge about electricity, on the one hand, caused the emergence of a new field of physics, on the other hand, was the basis for the rapid development of electrical engineering, i.e., batteries, generators, power supply systems for lighting and electric drive, electric furnaces, electric motors, etc. were invented , other. Ohm's discoveries were of great importance both for the development of the theory of electricity and for the development of applied electrical engineering. They made it easy to predict the properties of electrical circuits for direct current, and later for alternating current. In 1826, Ohm published a book in which he outlined the theoretical conclusions and experimental results. But his hopes were not justified, the book was met with ridicule. This happened because the method of rough experimentation seemed little attractive in an era when many people were fond of philosophy. Omu had no choice but to leave his position as a teacher. He did not achieve an appointment at the university for the same reason. For 6 years, the scientist lived in poverty, without confidence in the future, experiencing a feeling of bitter disappointment. But gradually his works gained fame first outside of Germany. Om was respected abroad, his research was used. In this regard, compatriots were forced to recognize him in their homeland. In 1849 he received a professorship at the University of Munich. Ohm discovered a simple law that establishes a relationship between current and voltage for a piece of wire (for part of the circuit, for the entire circuit). In addition, he made rules that allow you to determine what will change if you take a wire of a different size. Ohm's law is formulated as follows: the current strength in a section of the circuit is directly proportional to the voltage in this section and inversely proportional to the resistance of the section. Law of Joule-Lenz. Electric current in any part of the circuit performs a certain work. For example, let's take some section of the circuit, between the ends of which there is a voltage (U). By the definition of electric voltage, the work done when moving a unit of charge between two points is equal to U. If the current strength in a given section of the circuit is i, then the charge it will pass in time t, and therefore the work of the electric current in this section will be: A = Uit. This expression is valid for direct current in any case, for any section of the circuit, which may contain conductors, electric motors, etc. Current power, i.e. work per unit time, is equal to: P \uXNUMXd A / t \uXNUMXd Ui. This formula is used in the SI system to determine the unit of voltage. Let us assume that the section of the circuit is a fixed conductor. In this case, all the work will turn into heat, which will be released in this conductor. If the conductor is homogeneous and obeys Ohm's law (this includes all metals and electrolytes), then: U = ir, where r is the resistance of the conductor. In this case: A = rt2t. This law was first empirically derived by E. Lenz and, independently of him, by Joule. It should be noted that the heating of conductors finds numerous applications in engineering. The most common and important among them are incandescent lighting lamps. Law of electromagnetic induction. In the first half of the XNUMXth century, the English physicist M. Faraday discovered the phenomenon of magnetic induction. This fact, having become the property of many researchers, gave a powerful impetus to the development of electrical and radio engineering. In the course of experiments, Faraday found out that when the number of magnetic induction lines penetrating a surface bounded by a closed loop changes, an electric current arises in it. This is the basis of perhaps the most important law of physics - the law of electromagnetic induction. The current that occurs in the circuit is called inductive. Due to the fact that the electric current occurs in the circuit only in the case of external forces acting on free charges, then with a changing magnetic flux passing over the surface of a closed circuit, these same external forces appear in it. The action of external forces in physics is called the electromotive force or induction EMF. Electromagnetic induction also appears in open conductors. In the case when the conductor crosses the magnetic field lines, a voltage appears at its ends. The reason for the appearance of such a voltage is the induction EMF. If the magnetic flux passing through the closed circuit does not change, the inductive current does not appear. Using the concept of “EMF of induction”, one can talk about the law of electromagnetic induction, i.e., the EMF of induction in a closed loop is equal in absolute value to the rate of change of the magnetic flux through the surface bounded by the loop. Lenz's rule. As we already know, an inductive current occurs in the conductor. Depending on the conditions of its appearance, it has a different direction. On this occasion, the Russian physicist Lenz formulated the following rule: the induction current that occurs in a closed circuit always has such a direction that the magnetic field it creates does not allow the magnetic flux to change. All this causes the appearance of an induction current. Induction current, like any other, has energy. This means that in the event of an induction current, electrical energy appears. According to the law of conservation and transformation of energy, the above-mentioned energy can only arise due to the amount of energy of some other type of energy. Thus, Lenz's rule fully corresponds to the law of conservation and transformation of energy. In addition to induction, the so-called self-induction can appear in the coil. Its essence is as follows. If a current appears in the coil or its strength changes, then a changing magnetic field appears. And if the magnetic flux passing through the coil changes, then an electromotive force arises in it, which is called the EMF of self-induction. According to Lenz's rule, the EMF of self-induction when the circuit is closed interferes with the current strength and does not allow it to increase. When the EMF circuit is turned off, self-induction reduces the current strength. In the case when the current strength in the coil reaches a certain value, the magnetic field stops changing and the self-induction EMF becomes zero. Author: Smirnova L.N.
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