§ 32. Electric Current. Sources of Electric Current
When they talk about the use of electrical energy in the home, in industry or transport, they are referring to the work of the electric current. Electric current is brought to the consumer from the power plant through the wires. That is why, when all of a sudden the light bulbs go out or the movement of electric trains or trolley buses stops, they say that the current in the wires has disappeared.
What is electric current and what is necessary for it to arise and exist for the time we need?
The word “current” means the movement or flow of something.
What can move in the wires that connect the power plant to the consumers of electrical energy?
In order to get an electric current in a conductor, it is necessary to create an electric field in it. Under the action of this field, charged particles which can move freely in the conductor will move in the direction of the electric forces acting on them. There will be an electric current.
In order for an electric current to exist in a conductor for a long time, an electric field must be maintained in it all the time. The electric field in conductors is created and can be maintained for a long time Electric current sources.
In current sources, in the process of work to separate charged particles, there is a transformation of mechanical, internal, or some other energy into electrical energy. Thus, for example, mechanical energy is converted into electric energy in an electric machine.
It is also possible to convert internal energy into electric energy. If two wires made of different metals are soldered together and then the junction point is heated, an electric current is generated in the wires Such a current source is called a thermocouple. In it, the internal energy of the heater is converted into electrical energy. When some substances, for example selenium, copper (I) oxide, silicon, are illuminated, loss of negative electric charge is observed This phenomenon is called photoeffect. It is the basis of the construction and operation of photovoltaic cells. Thermocells and photocells are studied in the physics course of the high school.
Let’s take a closer look at the construction and operation of two current sources: a galvanic cell and a battery, which we will use in our experiments on electricity.
В galvanic cell (Fig. 47, a) chemical reactions occur, and the internal energy released during these reactions is converted into electrical energy. The cell depicted in Figure 47, b consists of a zinc vessel (housing) C. Carbon rod U is inserted into the body and has a metal lid M. The rod is placed in a mixture of manganese (IV) oxide MnO2 and crushed carbon C. The space between the zinc body and the mixture of manganese oxide and carbon is filled with jelly salt solution (ammonium chloride NH4Cl) P.
In the course of chemical reaction of zinc Zn with ammonium chloride NH4Cl the zinc vessel becomes negatively charged.
The manganese oxide carries a positive charge, and the inserted carbon rod is used to transfer the positive charge.
An electric field is created between the charged carbon rod and the zinc vessel, which are called electrodes. If a carbon rod and a zinc vessel are connected by a conductor, the free electrons will move in an orderly fashion along the entire length under the action of the electric field. An electric current is produced.
Galvanic cells are the most common sources of direct current in the world. The advantage of these electrodes is that they are easy and safe to use.
Batteries (from the Latin “rechargeable” batteries) are often used in everyday life. battery. accumulate). The simplest battery consists of two lead plates (electrodes) placed in a solution of sulfuric acid.
In order for the battery to become source of current, it must be charged. To charge the battery, a direct current is passed through the battery from some source. In the charging process, as a result of chemical reactions, one electrode becomes positively charged and the other negatively charged.
Once the battery is charged, it can be used as a separate power source. The poles of the batteries are marked with ” ” and “-“. When charging, the positive terminal of the battery is connected to the positive terminal of the power supply, and the negative terminal is connected to the negative terminal.
In addition to lead or acid batteries, iron-nickel or alkaline batteries are widely used. They use an alkaline solution and plates. one of compressed iron powder, the other of nickel peroxide. Figure 48 shows a modern battery.
Batteries have a wide and varied range of applications. They are used to power the lighting circuit in railroad cars, automobiles, and to start automobile engines. Batteries power a submarine underwater. Radio transmitters and scientific equipment on artificial satellites are also powered by batteries installed on the satellite.
In power plants, electricity is generated with the help of generators (lat. generator. maker, producer). This electric current is used in industry, transport, agriculture.
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If you simply bring a heater into thermal contact with a refrigerator, the internal energy of the heater will be transferred to the refrigerator by heat transfer without doing any work. In order to perform mechanical work, there must be an intermediate link. a so-called working body, which can be, for example, a gas in a cylinder closed by a piston. In a recurrent machine all the processes with the working body are repeated so that it periodically returns to its initial state. [3]
If we simply bring a heater into thermal contact with a refrigerator, then the internal energy of the heater will be transferred to the refrigerator by heat transfer without doing any work. [5]
We have seen that in this case the cooler receives less heat than the heater gives off. The internal energy of the heater decreases not only because it transfers the heat to the cooler, but also because the work is done. [7]
How Thermocouples Work. basic working principle RTD
From the point of view of thermodynamics, the thermocouple is quite analogous to a heat machine ( see [7]). r. Here there are two bodies with different temperatures, playing the role of a heater and a cooler, and the working body is an electron gas. Unlike an ordinary thermal machine, where part of the internal energy of the heater is converted into mechanical energy, in the thermocouple it is converted into electric current energy. [8]
In current sources in the process of work on the separation of charged particles there is a transformation of mechanical, internal or some other energy into electrical. So, for example, in an electroforming machine ( Fig. 42) mechanical energy is converted into electrical energy. This current source is called a thermocouple, which converts the internal energy of the heater into electrical energy. When some substances, such as selenium, copper oxide (I), silicon, are illuminated, the light energy is directly converted into electrical energy. is the phenomenon of photoelectric effect. Thermocells and photocells are studied in high school physics course. [9]
A refrigeration unit is a cyclically acting device that maintains a lower temperature in the refrigeration chamber than in the environment. This is done by transferring some amount of heat from a cold body to a body with a higher temperature. Such a transition does not contradict the second principle of thermodynamics ( 11.4.9.2), because this heat transfer is not the only process. A compensating process takes place ( 11.4.9.3) of transformation of mechanical energy of surrounding bodies into internal energy of heater. [10]
A refrigeration unit is a cyclic device that keeps the refrigeration chamber at a temperature lower than the ambient. It is realized by transition of some quantity of heat from a cold body to a body with a higher temperature. Such transition does not contradict the second principle of thermodynamics ( 11.4.9.2), because this transition is not the only process. There is a compensating process ( 11.4.9.3) of transformation of mechanical energy of surrounding bodies into the internal energy of heater. [11]
The transition of energy in the form of heat from a cold body to a hot body. is a process leading to a decrease in entropy. The second principle of thermodynamics forbids such a process if it is the only. But in considered case of reversed Carnot cycle one more process takes place: transformation of mechanical energy of surrounding bodies into internal energy of heater. And this process is accompanied by an increase in entropy. [12]
So, as a result of this cycle, a certain amount of heat passes from a cold body to a body with a higher temperature. The transition of energy in the form of heat from a cold body to a hot body. is a process leading to a decrease in entropy. The second beginning of thermodynamics forbids such a process, if it is the only. But in considered case of reverse Carnot cycle there is one more process: transformation of mechanical energy of surrounding bodies into internal energy of heater. And this process is accompanied by an increase in entropy. [13]
In the thermocouple, the energy of the heater turns into energy
If an electric current is passed through the boundary area between two different metals in contact, the electrons passing through this area will, depending on the direction of the current, either be accelerated by the contact field indicated above or be inhibited by it. In the first case, in the boundary layer, heat is released (electrons that have gained kinetic energy will transfer energy to metal atoms during collisions); in the second case, heat is absorbed (electrons that have lost speed will
in collisions with atoms receive energy from them, t. е. to cool the metal). If, for example, through the thermocouple (Fig. II 1.37) when an electric current is passed through, the contact temperatures will begin to change, since the field performs positive work in one of them and negative work in the other. The release or absorption of heat during the passage of current through the boundary region, due to the work of the contact electric field is called the Peltier effect; the amount of heat over time at a constant current
where is, as stated above, the potential difference due to the field the amount of electricity passed, and the thermoelectromotive force constant.
So, when electric current passes through the boundary area between two metals, the following phenomena occur:
1) a certain amount of heat is released according to the Lenz-Joule law: where the resistance of the boundary region. This heat does not depend on the direction of the current and is proportional to the square of the current strength;
2) emission or absorption of heat caused by positive or negative work of the contact electric field (Peltier effect). This heat is proportional to the first degree of current. At small values of current, the heat may be greater than
3) transfer of energy from one metal to another along with the electrons that have passed through the boundary area. The average kinetic energy of electrons in different metals (at the same temperature) may be different, so the electrons that have passed from one metal to another will carry energy
Note that the above release and absorption of heat in the contacts occurs in the case when the electric current flows in the circuit of the thermocouple, not caused by an extraneous current source, but the thermoelectromotive force itself (at ), in which case the contact with a high temperature is absorbing heat, and in contact with a low temperature. heat release. Thus, the Peltier effect is aimed at equalizing the temperature difference in the contacts. If the creation of the temperature difference between the contacts is regarded as an external effect on the thermocouple, the Peltier effect is the counteracting reaction of this element.
A thermocouple can be considered as a physical system, in which there is a direct transformation of heat into electrical energy. Let’s assume that in the circuit of the thermocouple the current is equal to the Work done by the thermoelectromotive force
However, the heat absorbed (by the Peltier effect) in contact with high temperature, the heat released in contact with low temperature; the difference between them is converted into electrical energy. Therefore, the coefficient of efficiency of the thermocouple
Thus, in full agreement with the second sign of thermodynamics, in the thermocouple there is heat extraction from the body with a high temperature, transfer of some heat to the body with a low temperature and transformation of the difference into another type of energy. However, in case of metal thermocouple considerable part of heat passes from hot contact to cold contact by means of heat conduction, therefore, the amount of heat transformed into electric energy even in case of great contact temperature difference is only a very small part (fractions of percent) of total amount of heat transferred from hot contact to cold contact. In the case of a thermocouple of metals, a considerable part of the heat is transferred from the hot contact to the cold contact by conduction. Ф. In the last article of Ioffe’s paper, of semiconductor materials with small thermal conductivity it is possible to approach efficiency factor of thermoelectric heat machine to its ideal value.
Above only metal conductors (conductors of the first kind) at which occurrence of a contact potential difference and passage of an electric current is not accompanied by any chemical changes were considered. However, the contact potential difference is also found in the system of any conductors, including, for example, electrolytes (conductors of the second kind), in which the excitation of the potential difference and the passage of current is accompanied by chemical reactions (galvanic cells, accumulators). Unlike metallic conductors, in a system containing electrolytes, special forces of “chemical” origin act on the charges (electrons, ions). Due to the presence of these extrinsic forces in a closed system of conductors containing electrolytes, there is a continuous one-way transfer of charges, t. е. there is an electric current.
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Thematic control “Electric current. Sources of current
Vytoptova Tatiana Alexandrovna, KGKOU “Evening (part-time) high school”, c. Shipunovo, Altai Territory, teacher of physics. Thematic control “Electric current. Current sources.”. 8th Grade Physics. Annotation to the test The test serves for the current test of the 8th grade physics knowledge of the students. It consists of multiple-choice questions, each of which covers material from two to three lessons. The test contains from three to ten questions, arranged in ascending order of difficulty. Each question has two to five answers, of which one (rarely two) are correct and the rest are incomplete, inaccurate or incorrect. The test is accompanied by a control card (see Appendix 1). slide 2). The following literature was used for this test: Postnikova A.В. Test of students’ knowledge of physics: 7-8 grades. Didact. The material is. Teacher’s Guide.
POWER, REVOLUTION, RESISTANCE 8th grade 2 1 3 4 Electric current. Current sources Test
Option 1 Electric current is called The movement of electrons along a conductor. The ordered motion of electrons in a conductor. Movement of electric charges in a conductor. The ordered motion of electric charges in a conductor. II. What energy transformations take place in galvanic cells? Electrical energy turns into chemical energy. Mechanical energy converts to electrical energy. Internal energy turns into electrical energy. Chemical energy is converted into electrical energy. III. What metal are the electrodes made of in Volta’s element?? Lead. From coal and manganese peroxide. From zinc and copper. Zinc and carbon. IV. What solution is used in the Volta element? Aqueous sulfuric acid solution. Glue made from flour and ammonia solution. Aqueous solution of table salt.
In picture 1. Here is a section of a dry element. VI. What is marked with number 1? VII. Which is represented by the number 2?? V. Which electrode in the Volta element is positively charged and which is negatively charged?? Zinc. positively, copper. negative. Zinc. negative, copper. positive. Coal. Positive, zinc. negative. Charcoal. negative, zinc. positively. Charcoal rod. Zinc case. Bag of coal and manganese peroxide. Clay cake made of flour and ammonia solution. A resin layer. Figure.1
Option 2 I. What process occurs inside a power supply when it works? A current source creates electric charges that move along conductors. A current source generates electric current. Performs the work of separating particles that have charges. This causes one electrode to be positively charged and the other negatively charged. II. What are the electrodes in dry cells made of?? Of lead. Carbon and manganese peroxide. Of zinc and copper. Of zinc and charcoal. III. Which solution is used in dry cells? Aqueous sulfuric acid solution. Claymore made of flour and ammonia solution. Aqueous solution of table salt.
III. Which solution is used in dry cells? Aqueous sulfuric acid solution. Claster of flour and ammonia solution. Aqueous solution of table salt. IV. Which electrode in the dry cell is positive and which one is negative? Zinc. positive, copper. negative. Zinc is negative, copper is positive. Coal is positive, zinc is negative. Coal. negative, zinc. positive. In diagram 1. Volt’s element is shown. V. Determine from the signs of the charges of which metals electrodes A and B are made? A. zinc, B. copper. A. carbon, B. zinc. A. copper, B. zinc. Fig.1
In Figure 1. Volt’s element is shown. VI. Which electrode has a surplus of electrons and which one has a deficit of? A. excess, B. deficiency. A. deficiency, B. excess. VII. If you close the circuit, which charged particles and in which direction will move through the wires? Electrons from A to B. Electrons from B to A. Ions from A to B. Ions from B to A. Figure.1
Option 3 In order to get an electric current in a conductor, it is necessary to Create an electric field in it. Separate the electric charges in it. Create electric charges in it. II. What energy conversions occur when charging batteries? Electrical energy is converted into mechanical energy. Mechanical energy turns into electrical energy. Internal energy is converted into electrical energy. Chemical energy is converted into electrical energy. Electrical energy turns into chemical energy. III. Which metal is used to make the electrodes in the simplest acid batteries?? zinc. From coal and manganese peroxide. Zinc and copper. Of zinc and charcoal. Two lead plates.
IV. Which solution is used in acid batteries? Aqueous sulfuric acid solution. Aqueous solution of table salt. Glue made of flour and ammonia solution. V. When charging batteries, connect the positive pole of the battery to the pole of the current source and the negative pole to the pole of the current source. positive negative. negative positive. VI. Figure 1. A section of a dry cell is shown. Which pole has a positive charge and which one a negative charge? A is positive, B is negative. A. negative, B. positive. VII. What is filled in the cloth bag of the dry cell? Manganese oxide with carbon powder. Carbon powder. Flour mixed with ammonia. Figs.1
Option 4 I. In a thermocouple, the energy of the heater is converted into energy. II. In photocells energy is converted into energy. III. What energy transformations occur when batteries are discharged? Electrical energy turns into chemical energy. Chemical energy is converted into other kinds of energy by the work that a current does. Internal energy is converted into electrical energy. IV. Which chemical current sources are used in automobiles? Dry cells. Volt cells. Batteries. Chemical into electrical. Light into chemical. Light into electrical energy. Internal into electrical energy.
Figure 1 shows a section of a dry cell. V. Which in this figure is indicated by the number 3? VI. What is marked with number 4? VII. Which is represented by the number 5? Carbon rod. Zinc case. A linen sack filled with manganese oxide and charcoal. Flour paste on ammonia solution. A layer of resin. Fig.1
Electrical phenomena
In my paper “Electrical Phenomena,” I will conduct and explain three experiments described in Peryshkin’s textbook, A.В. Physics. Grade 8 (Fig. 57, Fig. 43, Fig. 33).
Objective: to broaden students’ horizons, improve their erudition, develop an interest in experimental physics, develop the ability to demonstrate and explain experiments, and learn to work independently.
Experiment Magnetic Action of Current
Objective of the experiment: to prove that there is a magnetic field around a coil of current.
Tools and materials: a strip magnet, a coil on flexible wire, a tripod, a current source, a key, three wires.
Fix a coil on flexible wires in a tripod. Connect the ends of the coil to the poles of the current source. I close the key and bring a strip magnet to the coil, it will be attracted or repelled by the magnet.
Conclusion of the experiment: There is a magnetic field around the coil with the current; like a magnetic arrow it has two poles, the north and the south. The magnetic field can be detected by means of a permanent strip magnet. The magnetic effect of a current is always observed, no matter what kind of conductor the current is, solid, liquid, or gaseous.
The phenomenon of the interaction between a coil of current and a magnet is used in the device of a device called a galvanometer.
Sultana’s experiment
Objective: to learn how to operate an electroforming machine, Prove that similarly charged bodies repel each other, and differently charged bodies attract each other. Bodies become electrified by friction.
Apparatus and materials: electroforming machine, two homemade sultans, 2 wires, an insulating pointer.
Connect two sultans with long wires to different inductors of the electroforming machine. One sultan will charge positively, the other negatively. You can see from the light papers that the charges of the same name repel. I’ll use a pointer to bring the sultans closer together. They will be mutually attracted to each other, i.e. к. charged with different signs.
Conclusion: bodies having electric charges of the same sign will mutually repel, and bodies having charges of the opposite sign will mutually attract. Electrification of bodies occurs when they come into contact.
The principle of photocopiers is based on the phenomenon of electrification of bodies upon contact.
There are more and more ways of obtaining the main communal resource. electricity. No one is surprised any more by its geothermal power plants and wind turbines and solar panels. The newest way, until recently, was only in dreams. Russian scientists have figured out how to intelligently use the prospects of ebonite sticks in the national economy by developing a converter which will allow the application of static electricity to generate electricity on an industrial scale.
What is electrostatics, everyone knows by experience. In a dry room, electrical discharges are sometimes very annoying. How to overcome this effect is known. But how to make use of it, no one could say until recently. The Research Institute of Electrification of Agriculture found a way out.
By trial and error, using the latest electronics, our scientists managed to construct a converter under laboratory conditions. The work of the Russian invention can be shown as follows. To create a static field a Chizhevsky chandelier is used. Then, through a special remover, the energy is transmitted to the converter. It converts electrostatics into direct current.
From 60 kilovolts of static electricity they get 90 volts of direct current. In practice, it is possible to remove a voltage much higher. It is enough to place the current collectors in such areas of the planet, where the air is highly charged. For example, at the South Pole with its dry climate or in the mountains, where harvesting energy from the air is just as promising as in Antarctica.
The wind drives the clouds in the mountains. On one side you can put wind generators, on the other side. current collectors, which will collect static electricity and convert it into a current.
Thermocouple experiment
Objective: to test the operation of a current source called a thermocouple.
Tools and materials: Two wires made of different metals and soldered together (thermocouple), tripod, spirit-lamp, microammeter, wires.
I connect the terminals of the thermocouple to the microammeter with wires. I bring the litmeter to one of the junctions. The reading of the microammeter begins to increase. Bringing a spirit to another junction, the reading of the microampere meter starts decreasing. So, one end is the positive pole and the other is the negative pole.
Conclusion: If two wires made of different metals are soldered together and then the junction point is heated, an electric current will arise in the wires. Such a current source is called a thermocouple, which converts the internal energy of the heater into electrical energy.
If you solder two dissimilar conductors, you get a device called a thermocouple. It was created in 1621 by the German physicist Seebeck. If you connect it to a galvanometer and heat the junction, the arrow of the device will register the presence of current in the circuit. Many scientists have tried to generate electricity with thermocouples, but due to the very low efficiency, there has been little success in this area to this day. But the thermocouple. This is the name given to several thermocouples that have been joined together structurally. very sensitive to the slightest difference in temperature of its junctions.
In 1830, Italian scientists L. Nobile and M. The Melonis have created a battery of 32 miniature thermocouples. It made the hand of the galvanometer deflect under the action of the thermal radiation of a person standing ten meters away from the device. Such a device could have been part of a burglar alarm system, but the novelty hardly attracted any attention at the time. In 1869, the English astronomer Lord Ross began using a thermocouple to measure the temperature of various parts of Jupiter’s surface. He focused the thermal radiation of the planet on a thermocouple using a telescope. The sensitivity of his device far exceeded the thermal sensitivity of the rattlesnake! And just half a century later, inventors began to think about using the thermocouple for military purposes. In 1910, patents were issued on its basis to create devices that could register the radiation of people, airplanes and ships. It was even proposed to use thermocouples for homing aircraft torpedoes. The Germans were apparently the first to use thermocouples for military purposes, creating in 1914 in Ostend on the North Sea coast the first thermocouples. They detected British ships by thermal radiation in the dark and in fog at distances of more than 10 km. And in the early 1970s, the Soviet Union created the Arrow missile, which is self-directed to helicopters by their thermal radiation. Similar rockets were used in the Vietnam War. Ultrasensitive thermocouples are not, of course, only used in military applications.
Electric current. Sources of electric current.
I want to dedicate the first entry to electric current. The material I took from the 8th grade physics textbook. Author A.В. Peryshkin. DROFA Publishing House, Moscow 2006.
§ 32 Electric current. Sources of Electric Current.
When people talk about the use of electricity in daily life, production, or transportation, they are referring to the work of electric current. Electric current is carried to the consumer from a power plant through wires. Therefore, when house lights go out unexpectedly or electric trains or trolley buses stop moving, they say that the current in the wires has disappeared.
What is an electric current and what is necessary for its occurrence and existence for the time we need??
The word “current” means the movement or flow of something.
What can move in the wires connecting the power plant to electrical consumers?
We already know that in bodies there are electrons, the motion of which explains various electrical phenomena. § 31).Electrons possess a negative electric charge. Larger particles of matter, such as ions, can also be electrically charged. So you can have all kinds of charged particles moving around in a conductor.
In order to get an electric current in a conductor, it is necessary to create an electric field in it. Under the action of this field, the charged particles which can move freely in this conductor will move in the direction of the electric forces acting on them. An electric current is produced.
In order for an electric current to exist in conductors for a long time, it is necessary to maintain an electric field in it all this time. electric field in conductors is created and can be maintained for a long time This is why the absorbed or released charge on the contacts of two conductors (Fig.
In current sources, the work of separating charged particles converts mechanical, internal, or some other energy into electrical energy. Thus, for example, mechanical energy is converted into electrical energy in an electroforming machine.
It is also possible to convert internal energy into electric energy. If two wires made of different metals are soldered together and then the junction is heated, an electric current is generated in the wires
Such a current source is called a thermocouple. It turns the internal energy of the heater into electric energy. When some substances, like selenium, copper (I) oxide, silicon, are lit, the negative electric charge is lost
This phenomenon is called the photoelectric effect. This is the basis for the construction and action of photocells. Thermocells and photocells are studied in the physics course of high school.
Let’s take a closer look at the structure and operation of two current sources, the galvanic cell and the battery, which we will use in our experiments on electricity.
В galvanic cell Chemical reactions take place and the internal energy released during these reactions is converted into electrical energy.
The cell shown in Figure 46 consists of a zinc vessel (housing) C. Carbon rod U is inserted into the shell and has a metal lid M. The rod is placed in a mixture of manganese (IV) oxide MnO2 and crushed carbon C. The space between the zinc case and a mixture of MnO2 and C is filled with a gelatinous salt solution (ammonium chloride NH4Cl) P.
During chemical reaction of zinc Zn with ammonium chloride NH4Cl the zinc vessel becomes negatively charged.
The manganese oxide carries a positive charge, and the carbon rod inserted into it is used to transfer the positive charge.
An electric field is generated between the charged carbon rod and the zinc vessel, which are called electrodes. If a carbon rod and a zinc vessel are joined by a conductor, the free electrons will move in an orderly fashion along the entire length under the action of the electric field. An electric current is produced.
Galvanic cells are the most common source of direct current in the world. Their advantage is that they are easy and safe to use.
Batteries that can be recharged repeatedly are often used in the home rechargeable batteries (from Latin. the words battery. accumulate). The simplest battery consists of two lead plates (electrodes) placed in a solution of sulfuric acid.
In order for a battery to become source of current, it must be charged. Accumulators are charged by passing a direct current through the battery from some source. In the charging process, as a result of chemical reactions, one electrode becomes positively charged and the other negatively charged. Once the battery is charged, it can be used as an independent source of electricity. The poles of a battery are marked with a ” ” and a “-“. When charging, the positive pole of the battery is connected to the positive pole of the power supply, the negative pole to the negative pole.
In addition to lead, or acid, rechargeable batteries are widely used iron-nickel, or alkaline, rechargeable batteries. They use an alkaline solution, and the plates consist of one of compressed iron powder, the other of nickel peroxide. Figure 47 shows a battery of three such batteries.
Batteries have a wide and varied application. They are used to light railroad cars, automobiles, to start a car engine. Battery batteries power a submarine underwater. Radio transmitters and scientific equipment on artificial satellites are also powered by batteries installed on the satellite.
In power plants, electricity is generated by generators (lat. words generator. creator, producer). This electric current is used in industry, transportation, and agriculture.
2. What must be created in a conductor for a current to arise and exist in it?
Peltier effect.
If an electric current is passed through a circuit (thermocouple) consisting of two different conductors, heat is released on one of their contacts and cooled on the other. is absorbed (Fig.
4.40). This process is called the Peltier phenomenon and is thermodynamically reversible: in one direction of the current the contact is heated, while in the opposite direction. is cooled. Note that when an electric current flows through the contacts between two substances in the circuit in Fig. 4.40 the following processes occur:
- Joule heat release Q. PRt, which does not depend on the direction of the current / and is proportional to the product of the contact resistance R and the square of the current;
- heat release or absorption Qn = Pit, caused by the Peltier effect (positive or negative work of the contact electric field), which is proportional to the first degree of the current. Therefore, the absorbed or emitted at the contacts of the two conductors (Fig. 4.40) the heat power in the Peltier effect is proportional to the current 1 and depends on the nature of the contacting materials:
where Q1B. Peltier heat flux power absorbed in the junction; n,1B. relative Peltier coefficient for conductors А и В;
energy transfer from one conductor to another along with the energy transferred through the boundary region N = q/e = Itle by electronami (q. charge of N electrons) having average kinetic energies of electrons gA и gv in substances А и В (at the same temperature). The value of this energy will be
Peltier heat release or absorption in the contacts also occurs when the circuit (Fig. 4.40) flows an electric current caused not by an extraneous current source, but by the TEMF itself (at TA TV). In this case, as noted above, there is heat absorption in contact with a higher temperature, while in contact with a lower temperature. heat generation. This means that the Peltier effect is aimed at equalizing the temperature difference in the contacts. If creating a temperature difference between the contacts is regarded as an external action on the thermocouple, the Peltier effect is the counteracting reaction of this element.
Peltier effect. The phenomenon on which heat extraction by thermoelectric cooling is based. The relative Peltier coefficient is positive when heat is absorbed by passing current from А к В.
Peltier coefficients, like Seebeck coefficients, have the property of summation, which is expressed by the following equation:
The relationship between relative and absolute Peltier coefficients, by analogy with formula (4.4), is expressed by the relation
Thomson effect. According to the experiments of William Thomson (Lord Kelvin), if a metal rod is heated at one point (e.g., at the middle of the rod in Fig.
4.41) and simultaneously pass an electric current through it, there is a temperature difference at its ends, equidistant from the heating point.
Experiments have shown that at the end of the rod, where the current is directed toward the place of heating, the temperature decreases, while at the other end, where the current is directed away from the place of heating. rising. This phenomenon is called Thomson effect. This effect refers to the thermodynamically reversible absorption (or release) of heat in a homogeneous conductor with current in the presence of a temperature gradient and for a conductor of the type А is characterized by the relation
where 4. the thermal power absorbed per unit length of the conductor; xA. Thomson coefficient for a conductor A ATYx. temperature gradient.
Thomson coefficient xA is positive, if at the same direction of current / and temperature gradient AT1Ax heat is absorbed. Since the Thomson coefficient characterizes the properties of only one material, it is the only
Fig. 4.41. Schematic observation of the Thomson effect by the thermoelectric coefficient, which can be measured on a homogeneous conductor.
It should also be noted that the thermocouple (the circuit in Fig. 4.39 or 4.40) at 7″, TV can be considered as a physical system in which there is a direct transformation of heat into electrical energy. Let’s assume that in the circuit of the thermocell the work done by the thermocouple u lv in time / at current strength /, is equal to
At TA TV due to the Peltier effect the work ]UL 5 and1Tls] is the heat (1 ?, absorbed in the contact А with high temperature, and ]UV = 51VTvc1. heat Ov, emitted in contact with low temperature, so the difference between them is converted into electrical energy. In this regard, the efficiency of the thermocouple can be reduced to the well-known thermodynamics relation:
Thus, in full agreement with the second law of thermodynamics, in the thermocouple takes place the production of heat А From a body with a high temperature, the transfer of a certain amount of heat в to a body with a low temperature and the transformation of the difference into another form of energy (electricity). Note that in the case of a thermocouple made of good conductors of electricity (metals), much of the heat is transferred from the hot contact to the cold contact by thermal conduction. Therefore, the amount of heat converted into electrical energy, even at a large temperature difference between the contacts is only a very small fraction (fractions of a percent) of the total amount of heat transferred from the hot contact to the cold contact. When using semiconductor materials with low thermal conductivity (and sufficiently high electrical conductivity) it is possible to bring the efficiency of a thermoelectric heat machine closer to its ideal value.
