Electromagnetic induction refers to the phenomenon of induced electromotive force due to the change of magnetic flux. The discovery of electromagnetic induction is one of the greatest achievements in electromagnetism. It not only reveals the internal relationship between electricity and magnetism, but also lays the experimental foundation for the mutual transformation between electricity and magnetism, and opens the way for human beings to obtain huge and cheap electric energy, which has important practical significance. The discovery of electromagnetic induction marks the arrival of a major industrial and technological revolution. Facts have proved that the wide application of electromagnetic induction in electrician, electronic technology, electrification and automation has played an important role in promoting the development of social productive forces and science and technology.
If the closed circuit is n turns coil, the instantaneous electromotive force can be expressed as ε = n * δ φ/δ t (δ t→ 0). Where n is the number of turns of the coil, Δ φ is the change of magnetic flux in Wb (Weber), and Δ T is the time required for the change in s (seconds). ε is the induced electromotive force generated, and the unit is V (volts for short). Electromagnetic induction, commonly known as magnetic power generation, is used in generators of many units.
Basic introduction Chinese name: electromagnetic induction mbth: electromagnetic induction alias: Faraday principle proposer: Michael Faraday proposer: 183 1 year Applied subject: the scope of application of physics: basic concepts of electromagnetic physics, magnetic flux, phenomena, energy conversion, induced electromotive force, related knowledge, calculation formulas, laws, discoveries, descriptions, conditions and important experiments. Recorder, car speedometer, molten metal, motor, transformer, meaning, basic concept 18365433 As long as the magnetic flux passing through the closed circuit changes, an induced current will be generated in the closed circuit. This phenomenon of using magnetic field to generate current is called electromagnetic induction, and the generated current is called induced current. Electromagnetic induction has two conditions (both are indispensable). I shut down the circuit. L the magnetic flux through the closed circuit changes. There are two ways to change the magnetic flux, as shown in figure 1. One method is to make the conductor in the closed circuit cut the magnetic induction line in the magnetic field; Another method is to let the magnetic field move in the conductor. Figure 1 Method for generating magnetic flux The magnetic flux is set in a uniform magnetic field with a plane perpendicular to the direction of the magnetic field, and the magnetic induction intensity of the magnetic field is b, and the area of the plane is S. (1) Definition: In a uniform magnetic field, the product of the magnetic induction intensity b and the area s perpendicular to the direction of the magnetic field is called the magnetic flux passing through the plane, which is referred to as magnetic flux for short. Electromagnetic induction (2) Definition formula: φ = bs When the plane is not perpendicular to the magnetic field direction: φ = bs ⊥ = bs cos θ (θ is the dihedral angle of two planes) (3) Physical meaning The number of magnetic induction lines vertically passing through a plane indicates the magnetic flux passing through the plane. (4) Unit: In the international system of units, the unit of magnetic flux is Weber, abbreviated as Wei, and the symbol is Wb. 1WB =1t1m2 =1v S. (5) Scalar: Magnetic flux is scalar, but there are positive and negative points. Phenomenon (1) Electromagnetic induction phenomenon: Some conductors in the closed circuit cut magnetic induction lines, and induced current is generated in the circuit. (2) Induced current: the current generated in electromagnetic induction. The induction cooker is the condition for applying the electromagnetic induction diagram (3) to produce electromagnetic induction phenomenon: ① There are two different expressions: a. A part of the conductor in the closed circuit moves relative to the magnetic field; B, the magnetic field passing through the closed circuit changes; ② Comparison and unification of the two expressions; A. The root causes of induced current are different in the two cases. When a part of the conductor in a closed circuit moves relative to the magnetic field, the free electrons in the conductor move with the conductor, and a component of Lorentz force makes the free electrons move directionally, forming a current. When the magnetic field passing through the closed circuit changes, according to the electromagnetic field theory, an electric field is generated around the changed magnetic field, which makes the free electrons in the conductor move directionally and form a current. The current generated in this case is called induced current or induced current. B. the unification of the two expressions can be unified as the change of magnetic flux through a closed circuit. (3) Conditions of electromagnetic induction No matter what method is used, as long as the magnetic flux passing through the closed loop changes, there will be current in the closed loop. Conditions: a. Closed circuit; B. a part of a conductor; C. The conservation law of kinetic energy conversion energy for cutting magnetic induction lines is a universal law in nature, which is also applicable to electromagnetic induction. Definition of induced electromotive force (1): The electromotive force generated by electromagnetic induction is called induced electromotive force. The direction is from low potential to high potential. (2) Conditions for generating induced electromotive force: the magnetic flux passing through the loop changes. (3) Physical meaning: Induced electromotive force is a physical quantity that reflects the essence of electromagnetic induction. (4) Direction adjustment: The direction of induced current in the internal circuit is the direction of induced electromotive force. Electromagnetic induction (5) Back electromotive force: When the motor rotates, the induced electromotive force will also be generated in the coil, which will always weaken the effect of the electromotive force of the power supply. This electromotive force is called back electromotive force. The electromagnetic induction part of related knowledge involves three aspects: First, the law of electromagnetic induction phenomenon. Electromagnetic induction studies the characteristics and laws of other forms of energy converted into electric energy, and its core is Faraday's law of electromagnetic induction and Lenz's law. Lenz's law of electromagnetic induction electromagnetic induction lamp is expressed as follows: the magnetic field of induced current always hinders the change of magnetic flux that causes induced current. That is to say, in order to obtain induced current (electric energy), we must overcome the ampere force generated by induced current to do work, and we need to do external work to convert other forms of energy into electric energy. Faraday's law of electromagnetic induction reflects the ability of external work. The greater the change rate of magnetic flux, the greater the induced electromotive force and the greater the ability of the outside world to do work. The second is the knowledge of circuits and mechanics. This paper mainly discusses the characteristics and laws of electric energy transmission and distribution in the circuit, and uses electrical appliances to convert it into other forms of energy. In practical application, the three laws of circuit (ohm's law, resistance law and Joule's law) and the concepts of Newton's law, momentum law, momentum conservation law, kinetic energy law and energy conservation law in mechanics are often used. The third is the right hand rule. Lay your right hand flat so that your thumb is perpendicular to the other four fingers, and all of them are in the same plane as your palm. Put your right hand in a magnetic field. If the magnetic line of force enters the palm vertically (when the magnetic induction line is straight, it means that the palm faces the N pole) and the thumb points to the direction of the wire movement, then the direction pointed by the four fingers is the direction of the induced current in the wire. In electromagnetism, the right-hand rule mainly judges the direction that has nothing to do with force. In order to make it easier to remember and distinguish it from the left-handed rule, it can be recorded as: left force and right electricity (that is, the left-handed rule determines the direction of judgment and the right-handed rule determines the direction of current). Or left force right, left force right. Calculation formula 1. [calculation formula of induced electromotive force]1) e = n Δ φ/Δ t (universal formula) {Faraday's law of electromagnetic induction, e: induced electromotive force (v), n: number of turns of induction coil, Δ φ/Δ t: magnetic flux change rate}. 2)E=BLVsinA (cutting magnetic induction line) V and L in E=BLV may not be parallel to the magnetic induction line, but they may not be perpendicular to the magnetic induction line, where sinA is the angle between V or L and the magnetic induction line. {L: effective length (m)}, which is generally used to find the instantaneous induced electromotive force, but the average electromotive force can also be found. 3)Em=nBSω (maximum induced electromotive force of alternator) {Em: peak value of induced electromotive force}.
4) e = b (l 2) ω/2 (one end of the conductor is fixed and cut by ω) {ω: angular velocity (rad/s), v: speed (m/s), (l 2) refers to the square of l}. Hand-held electromagnetic induction II. Magnetic flux φ = BS {φ: magnetic flux (Wb), b: magnetic induction intensity of uniform magnetic field (t), s: facing area (m2)} Calculation formula △φ = φ 1-φ 2, △φ = b△ s = blv △ T.3. The positive and negative poles of induced electromotive force can be determined by the direction of induced current. 4. Self-induced electromotive force Efrom = nΔ φ/Δ t = lΔ i/Δ t {l: self-inductance coefficient (h) (the coil with iron core is larger than the coil without iron core), Δ i: changing current, Δ t: time spent, Δ i/Δ t: rate of change (speed of change) of self-induced current}. △ Pay special attention to that φ, △φ and△φ /△ t are not necessarily related, and E has nothing to do with resistance. E = n △φ/△ T. The unit of electromotive force is V, the unit of magnetic flux is Weber Wb, and the unit of time is second. After S.H.C. Oster discovered the magnetic effect of current in 1820, many physicists tried to find its inverse effect, and raised the question of whether magnetism can generate electricity and whether magnetism can act on electricity. 1822, when D.F.J arago and A.von humboldt were measuring geomagnetic intensity, they happened to find that metal had damping effect on the oscillation of nearby magnetic needles. 1824, arago made a copper disk experiment according to this phenomenon, and found that the rotating copper disk would drive the magnetic needle hanging freely above to rotate, but the rotation of the magnetic needle was out of sync with the copper disk, which was slightly lagging behind. Electromagnetic damping and electromagnetic driving were the earliest electromagnetic induction phenomena, but they were not directly expressed as induced current, so they could not be explained at that time. Michael faraday 183 1 August, M Faraday wound two coils on both sides of the soft iron ring, one of which was a closed loop. One magnetic needle is placed in parallel near the lower end of the wire, and the other is connected to the battery pack and connected to the switch to form a closed loop with power supply. It is found that when the switch is closed, the magnetic needle deflects. When the switch is turned off, the magnetic needle deflects in the opposite direction, indicating that there is induced current in the coil without battery pack. Faraday immediately realized that this was an unstable transient effect. Then he did dozens of experiments and classified the induced current into five categories: changing current, changing magnetic field, moving constant current, moving magnet and moving conductor in magnetic field, and formally named these phenomena as electromagnetic induction. Furthermore, Faraday found that under the same conditions, the induced current generated in the loops of different metal conductors is directly proportional to the conductivity of the conductors, from which he realized that the induced current is generated by the induced electromotive force that has nothing to do with the properties of the conductors. Even if there is no loop and no induced current, the induced electromotive force still exists. 1862, the British physicist Maxwell published a paper on the lines of physical force, which introduced the concept of displacement current and pointed out that changing the electric field can also produce a magnetic field. 1864, Maxwell deduced the electromagnetic theory of the system, and predicted the existence of electromagnetic waves in his paper "electromagnetic field dynamics theory". 1873, Maxwell comprehensively summarized a series of discoveries and experimental results of Coulomb, Gauss, Ohm, Ampere, Biot, Savart and Faraday before the middle of 19 century. Through scientific assumptions and reasonable logical thinking, the electric field theory system is established completely for the first time, and the electric field theory appears in a concise, symmetrical and perfect mathematical form. 1888, German physicist Hertz verified the existence of electromagnetic waves through experiments. After Faraday demonstrated electromagnetic induction, Lenz's law for determining the direction of induced current and Faraday's law for describing the quantitative law of electromagnetic induction were given. According to the different causes, induced electromotive force can be divided into dynamic electromotive force and induced electromotive force. The former comes from Lorentz force, and the latter comes from rotating electric field generated by changing magnetic field. Faraday's law was originally an experimental law based on observation. Later, it was formalized, and the finite version of its partial derivative was listed as the modern Havishay version of Maxwell equations together with other electromagnetic laws. Faraday's law of electromagnetic induction is based on Faraday's experiment in 183 1 year. Joseph henry discovered this effect at about the same time, but Faraday published it earlier. See Maxwell's original work on electromotive force. Lenz's law, discovered by Russian scientist Heinrich Lengci in 1834, provides the direction of induced electromotive force and the direction of current that produces induced electromotive force. Describe the phenomenon of induced electromotive force caused by magnetic flux change. When a part of the conductor of a closed circuit cuts the magnetic induction line in a magnetic field, a current will be generated in the conductor. This phenomenon is called electromagnetic induction. When a part of the conductor of a closed circuit cuts the magnetic induction line in a magnetic field, a current will be generated in the conductor. This phenomenon is called electromagnetic induction. The generated current is called induced current. This is an electromagnetic induction phenomenon defined in junior high school physics textbooks for the convenience of students' understanding, and it can't comprehensively summarize the electromagnetic induction phenomenon: the closed coil area is unchanged, and the magnetic flux will change when the magnetic field strength is changed, and the electromagnetic induction phenomenon will also occur. So the accurate definition is: the phenomenon of induced electromotive force caused by the change of magnetic flux. Conditions of electromagnetic induction law 1. The circuit closes and cycles. 2. The magnetic flux through the closed circuit changes. Electromagnetic induction 3. A part of the circuit cuts the magnetic induction line in the magnetic field (the action of cutting the magnetic induction line is to ensure that the magnetic flux of the closed circuit changes) (only partial cutting is invalid) (there is no induced current without a condition). 4. Microscopic explanation of induced current: When a part of the circuit cuts the magnetic induction line, it is equivalent to the movement of free electrons in a part of the circuit in the magnetic field, so the free electrons will move directionally in the conductor under the action of Lorentz force. If a part of the circuit is in a closed loop, an induced current will be formed. If it is not a closed loop, the charge will accumulate at both ends to produce induced electromotive force. 5. The reason why electromagnetic induction emphasizes the "part of the conductor" of the closed circuit is because when the whole closed circuit cuts the magnetic induction line, the induced currents on the left and right sides are counterclockwise and clockwise respectively, which cancels out the current for the whole circuit. 6. Energy relationship in electromagnetic induction: electromagnetic induction is an energy conversion process, for example, gravitational potential energy and kinetic energy can be converted into electrical energy and thermal energy. An important experiment is to wind a set of conductor coils connected with galvanometer on a hollow paper tube. When the magnetic bar is wound around the coil, the galvanometer pointer deflects, and when the magnetic bar is pulled out of the coil, the galvanometer pointer deflects in the opposite direction. The faster the magnetic bar is pulled out or the coil is pulled out, the larger the deflection angle of the galvanometer is. But when the magnetic bar is at rest, the pointer of the galvanometer will not. Electromagnetic induction for the coil, the moving magnetic bar means that the surrounding magnetic field has changed, which makes the coil induce current. Faraday finally realized his dream for many years-to generate electricity by magnetic motion! The discovery of Oster and Faraday profoundly reveals a set of extremely wonderful physical symmetries: moving electricity produces magnetism, and moving magnetism produces electricity. The relative movement between the magnetic bar and the coil can induce current not only in the coil, but also in the other coil. When the coil is connected to the power supply through the switch K, induced current will appear in the coil 2 during the closing or opening of the switch K. If the DC power supply connected to the coil 1 is changed to AC power supply, that is, AC power is supplied to the coil 1, induced current will also appear in the coil. This is also because the current change of the coil 1 causes the magnetic field around the coil 2 to change. In order to make the audience hear the actors' voices clearly in the theater, it is often necessary to amplify the voices. The equipment used to amplify sound mainly includes microphone, loudspeaker and speaker. A microphone is a device that converts sound into electrical signals. Fig. 2 is a schematic structural diagram of a moving-coil microphone made by electromagnetic induction. When sound waves vibrate the metal diaphragm, the coil connected to the diaphragm (called voice coil) vibrates together, and the voice coil vibrates in the magnetic field of the permanent magnet, in which an induced current (electrical signal) is generated, and the magnitude and direction of the induced current change, the amplitude and frequency of which are determined by sound waves. The signal current is amplified by the speaker and transmitted to the speaker, and the amplified sound is emitted from the speaker. The working principle of microphone-electromagnetic induction recorder The recorder is mainly composed of built-in microphone, magnetic tape, recording and playback head, amplifier circuit, speaker and transmission mechanism. This is a schematic diagram of the recording and playback principle of the tape recorder. When recording, the sound causes the microphone to generate an induced current-an audio current that varies with the sound. The audio current enters the coil of the recording head after being amplified by the amplifier circuit, and a magnetic field varying with the audio current is generated at the gap of the magnetic head. The magnetic tape moves close to the gap of the magnetic head, the magnetic powder layer on the magnetic tape is magnetized, and the magnetic signal of sound is recorded on the magnetic tape. Playback is the reverse process of recording. When playing, the tape passes through the gap near the playback head. The changing magnetic field on the magnetic tape generates an induced current in the coil of the playback head. The change of induced current is the same as the recorded magnetic signal, so an audio current is generated in the coil. This current is amplified by the amplifier circuit and sent to the speaker, which restores the audio current to sound. In a tape recorder, the recording and playback functions are accomplished by sharing a magnetic head, and the magnetic head is connected to the microphone when recording; When playing, the magnetic head is connected to the speaker. The speedometer in the cab of a car is an instrument to indicate the speed of the car. It uses the principle of electromagnetic induction to make the swing angle of the pointer on the dial proportional to the driving speed of the car. The speedometer is mainly composed of transmission shaft, magnet, speedometer, spring hairspring, pointer shaft and pointer. Wherein the permanent magnet is connected with the driving shaft. The watch case is equipped with a dial with a scale of kilometers per hour. The direction of magnetic induction line of automobile speedometer-electromagnetic induction permanent magnet is as shown in figure 1. Some magnetic induction lines will pass through the speed disk, and the distribution of magnetic induction lines on the speed disk is uneven. The closer to the magnetic pole, the more magnetic induction lines. When the driving shaft drives the permanent magnet to rotate, the magnetic induction lines passing through all parts of the speedometer will change in turn, and the number of magnetic induction lines will gradually increase along the front of the magnet rotation and gradually decrease at the rear. According to Faraday's electromagnetic induction principle, when the number of magnetic induction lines passing through a conductor changes, an induced current will be generated inside the conductor. According to Lenz's law, induced current also produces a magnetic field, and the direction of its magnetic induction line is to hinder (not prevent) the change of the original magnetic field. According to Lenz's law, along the front edge of magnet rotation, the magnetic induction line generated by induced current is opposite to that generated by magnet, so they are mutually exclusive. On the contrary, the direction of the magnetic induction line generated by the rear induction current is the same as that of the magnetic induction line generated by the magnet, so they attract each other. Because of this attraction, speed dial is rotated by a magnet, and the shaft and pointer also rotate together. In order to make the pointer stay at different positions according to different vehicle speeds, a spring hairspring is installed on the pointer shaft, and the other end of the hairspring is fixed on the frame of the iron shell. When the speedometer rotates to a certain angle, the hairspring is twisted to produce the opposite torque. When it is equal to the torque driven by the permanent magnet, the speedometer stays at that position and is in a balanced state. At this time, the pointer on the pointer shaft indicates the corresponding speed value. The rotating speed of the permanent magnet is directly proportional to the driving speed of the car. When the vehicle speed increases, the current induced in the speedometer will increase in proportion to the corresponding torque to drive the speedometer to rotate, which makes the pointer rotate at a larger angle, so the speed indicated by the pointer varies with the vehicle speed. When the car stops running, the magnet stops rotating, and the spring hairspring resets the pointer shaft to make the pointer at "0". The eddy current produced by the alternating magnetic field of melting metal will produce thermal effect. Compared with fuel heating, this heating method has many advantages, including: high heating efficiency, which can reach 50 ~ 90%; Fast heating speed; Different heating depths can be obtained by alternating current with different frequencies, because the distribution of eddy current in metal is not uniform. The closer to the metal surface, the stronger the current and the higher the frequency, which is the so-called "skin effect". In industry, induction heating is divided into four types according to frequency: power frequency (50hz); Intermediate frequency (0.5 ~ 8 khz); Superaudio (20 ~ 60 khz); High frequency (60 ~ 600 kHz). Power frequency alternating current is directly provided by distribution transformer; The intermediate frequency alternating current is generated by a three-phase motor-driven intermediate frequency generator or a silicon controlled inverter; Superaudio and high-frequency alternating current are generated by high-power tube oscillators. Using eddy current to heat and melt metal-The purpose of electromagnetic induction coreless induction melting furnace is to melt cast iron, steel, alloy steel, copper, aluminum and other nonferrous metals. The frequency of alternating current should be selected according to the quality of metal that can be accommodated in the crucible to obtain the best effect. For example: 5 kg 20 kHz, 100 kg 2.5 kHz, 5 tons 1 kHz or even 50 kHz. The melting pot is filled with the melted metal, and the high-frequency alternating current passes through the coil, which makes the melted metal generate strong eddy current, thus generating a lot of heat to melt the metal. This smelting method has the advantages of high speed and easy temperature control, and can avoid harmful impurities from mixing into the smelted metal, and is suitable for smelting special alloys and special steels. Induction heating method is also widely used in heat treatment of steel parts, such as quenching, tempering and surface carburizing. For example, gears and shafts only need surface quenching to improve hardness and wear resistance, and they can be put into air-core coils with high-frequency alternating current. The surface layer can be heated to the high temperature required for quenching in a few seconds, and its internal temperature rises slightly, and then it is quickly cooled with water or other quenching agents. Other heat treatment processes can be heated as required. The motor-generator can run in the reverse direction and become a motor. For example, take Faraday disk as an example, assuming that direct current is driven by voltage and passes through a conductive shaft arm. Then, according to the Lorentz force law, the moving charge is forced by the magnetic field B, which will make the disk rotate in the direction set by Fleming's left-hand rule. In the absence of irreversible effects (such as friction or Joule heat), the rotation speed of the disk must make d φ b/dt equal to the voltage of the driving current. The electromotive force predicted by Faraday law of transformer is also the working principle of transformer. When the current in the coil changes, the changing current produces a changing magnetic field. The second wire in the magnetic field will feel the change of the magnetic field, so its own coupling magnetic flux will also change (d φ b/dt). So the second coil will have electromotive force, which is called induced electromotive force or transformer electromotive force. If an electrical load is connected to both ends of the coil, current will flow. Faraday's experiment shows that as long as the magnetic flux passing through a closed circuit changes, there will be current. This phenomenon is called electromagnetic induction, and the generated current is called induced current. Faraday summed up the following rules according to a large number of experimental facts: the magnitude of induced electromotive force in the circuit is directly proportional to the change rate of magnetic flux passing through the circuit. The induced electromotive force is expressed by ε, that is, ε = n δ φ/δ T, which is Faraday's law of electromagnetic induction. Electromagnetic induction is one of the most important discoveries in electromagnetism, which reveals the relationship between electric and magnetic phenomena. The significance of Faraday's law of electromagnetic induction lies in that on the one hand, according to the principle of electromagnetic induction, people have made generators, which makes it possible to generate electric energy on a large scale and transmit it over a long distance; On the other hand, electromagnetic induction is widely used in electrical technology, electronic technology and electromagnetic measurement. Since then, human society has entered the era of electrification.