In 1831, a man namedMichael Faraday The scientists of.Just go throughClosed circuitWhen the magnetic flux ofinduced current。This phenomenon of using magnetic field to generate current is called electromagnetic induction, and the generated current is called induced current.
Encyclopedia x Knowledgeable: Illustration of Electromagnetic Induction
There are two conditions for electromagnetic induction phenomenon:
1. Close the circuit.
2. The magnetic flux through the closed circuit changes.There are two ways to change the magnetic flux: a. As shown in Figure 1, let the conductor in the closed circuit do in the magnetic fieldCutting magnetic induction lineMovement of;b. Another way is to let the magnetic field move inside the conductor.
Figure 1 Method of generating magnetic flux
Magnetic flux
1. Definitions
Set at uniform strengthmagnetic fieldThere is a plane perpendicular to the direction of the magnetic fieldMagnetic inductionbyB, the area of the plane isS。(1) Definition: InUniform magnetic fieldMedium, magnetic inductionBThe product of the area S perpendicular to the magnetic field direction is called the plane passing throughMagnetic flux, referred to as magnetic flux.
2. Definition
Φ=BS
When the plane is not perpendicular to the magnetic field direction:
Φ=BS⊥=BScosθ（θIs the angle between the vertical line of the plane and the direction of the magnetic field)
(3) Physical meaning
The number of magnetic induction lines passing through a face vertically represents the magnetic flux passing through the face.
Magnetic flux is scalar, but it can be positive or negative,
Electromagnetic induction phenomenon
(1) Electromagnetic induction phenomenon: a part of the conductor in the closed circuit moves by cutting the magnetic induction line, and induced current is generated in the circuit.
Electromagnetic cooker is the application of electromagnetic induction pictures
a. Some conductors in the closed circuit move relative to the magnetic field
b. The magnetic field through the closed circuit changes
② Comparison and Unification of Two Expressions
a. The root causes of induced current in the two cases are different
When a part of the conductor in a closed circuit moves relative to the magnetic field, it is in the conductorfree electronMoving with the conductorLorentz forceA component of the force makes free electrons move directionally to form a current, which is sometimes calledDynamic current。
When the magnetic field passing through the closed circuit changes, according toElectromagnetic field theory, generated around the changing magnetic fieldelectric fieldThe electric field makes the free electrons in the conductor move directionally to form a current, which is calledinduced currentorInduced current。
b. Unification of two expressions
The two expressions can be unified asThe magnetic flux through the closed circuit changes.
③ Conditions for electromagnetic induction
No matter what method is used, as long as the magnetic flux through the closed circuit changes, there will be current in the closed circuit.
condition: a. Closed circuit;b. A part of conductor;c. Perform cutting magnetic induction line movement
Energy conversion
The conservation law of energy transformation isnatureThe universal law also applies to electromagnetic induction phenomena.
Induced electromotive force
(1) Definition: generated in electromagnetic induction phenomenonemf, calledInduced electromotive force。The direction is from low potential to high potential.
(2) The condition for generating induced electromotive force: the conductor cuts the magnetic induction line in the magnetic field.
(3) Physical meaning: induced electromotive force is a physical quantity reflecting the essence of electromagnetic induction phenomenon.
Electromagnetic induction type
(4) Direction regulation: the direction of induced current in the internal circuit is the direction of induced electromotive force.
(5) Back EMF: When the motor rotates,coilThe induced electromotive force will also be generated in the. This induced electromotive force always weakens the power supply electromotive force. This electromotive force is calledBack electromotive force。
Relevant knowledge
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The electromagnetic induction part involves three aspects of knowledge:
Lenz's law is expressed as: the magnetic field of induced current always hinders the change of magnetic flux that causes induced current.That is, to obtain induced current（electric energy）The induced current must be overcomeAmberiWork, external work is required to convert other forms of energy into electric energy.FaradayLaw of electromagnetic inductionIt reflects the outside worldWorkThe greater the change rate of magnetic flux, the greater the induced electromotive force, and the greater the ability of external work。
The third is the right hand rule.The right hand is flat, so that the thumb is perpendicular to the other four fingers, and is in the same plane with the palm of the hand.Put your right hand into the magnetic field. If the magnetic force line enters the palm vertically (when the magnetic induction line is a straight line, it is equivalent to that the palm faces the N pole), the thumb points to the wireDirection of movement, the direction of the four fingers is the direction of the induced current in the conductor.
In electromagnetics, the right-hand rule mainly judges the direction irrelevant to the force.In order to facilitate memory and distinguish it from the left hand rule, it can be memorized as: left force and right electricity (that is, the direction of judgment of the left hand rule,Right-handed ruleDetermine the direction of current).Or left force and right sense, left force and right power on.
Calculation formula
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1、 Calculation formula of induced electromotive force
⑴
(Universal formula) {Faraday's law of electromagnetic induction,
It can not be parallel to the magnetic induction line, but can not be perpendicular to the magnetic induction line
by
or
The included angle with the magnetic induction line.
: The effective length (m) is generally used to calculate the instantaneous induced electromotive force, but the average electromotive force can also be calculated.
2、 Magnetic fluxΦ=BS, whereΦ：Magnetic flux (Wb),B: magnetic induction of uniform magnetic field (T),S: Opposite area (mtwo）Calculation formula
，
。
3、 The positive and negative poles of induced electromotive force can be determined by the direction of induced current{Current direction inside the power supply: from negative to positive}。
4、 Self induced electromotive force
, whereL: Self inductance coefficient (H) (coilLIt is larger with iron core than without iron core),ΔI: varying current,Δt: Time spent,ΔI/Δt: Rate of change of self induction current (speed of change).
△ Special attention：Φ，△Φ，△Φ/△tNo necessary connection, E andresistanceirrelevantE=n△Φ/△t。The unit of electromotive force is V, and the unit of magnetic flux is Weber Wb,Time unitIs the second s.
Law of electromagnetic induction
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Michael Faraday
Michael Faraday It is generally believed that electromagnetic induction was discovered in 1831, although Francesco Zantedeschi's work in 1829 may have foreseen this.
1820H. C. OsterfindCurrent magnetic effectLater, manyphysical scientistHe tried to find its reverse effect and put forward the question whether magnetism can produce electricity and whether magnetism can act on electricity. In 1822D. F.J. AragoWhen measuring the geomagnetic intensity, A. von Humboldt and A. von Humboldt accidentally found that the oscillation of the metal to the nearby magnetic needleDamping action。In 1824, Arago conducted a copper disk experiment based on this phenomenon, and found that the rotating copper disk would drive the free hanging abovemagnetic needleIt rotates, but the rotation of the magnetic needle is not synchronized with the copper disk, and it lags behind slightly.Electromagnetic dampingandElectromagnetic driveIt is the earliest electromagnetic induction phenomenon, but it was not explained at that time because it was not directly manifested as induced current.
In August 1831, MFaradayTwo coils are wound on both sides of the soft iron ring, one of which is a closed loop. A 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.The experiment shows that when the switch is closed, the magnetic needle deflects;Cut off the switch, and the magnetic needle deflects in the opposite direction, which indicates that the induced current occurs in the coil without battery pack.Faraday immediately realized that this was an unsteady transient effect.Then he did dozens of experiments, and summarized the situations generating induced current into five categories: changing current, changing magnetic field, and movingConstant current, moving magnet, moving in magnetic fieldconductorAnd formally named these phenomena electromagnetic induction.Furthermore, Faraday found that under the same conditions, the induced current generated in the circuits of different metal conductors is proportional to the conductivity of the conductor. From this, he realized that the induced current is generated by the induced electromotive force unrelated to the nature of the conductor. Even if there is no circuit without induced current, the induced electromotive force still exists.
In 1862, British physicist Maxwell published a paper "On physical force lines", which introduced the concept of displacement current and pointed out that changing electric fields can also produce magnetic fields.
In 1864, Maxwell used the field theory to deduce the electromagnetic theory of the system and predicted the existence of electromagnetic waves in his paper "Dynamic Theory of Electromagnetic Field".
In 1873, Maxwell comprehensively summarized a series of discoveries and experimental results of Coulomb, Gauss, Ohm, Ampere, Biot, Savart, Faraday and others before the middle of the 19th century in his book General Theory of Electromagnetism. Through scientific assumptions and reasonable logical thinking, Maxwell established a complete theoretical system of electric field for the first timeIt is expressed in beautiful mathematical form and later sorted out by Hertz and others as the main basis of classical electrodynamicsMaxwell's equations。
In 1888, German physicist Hertz used experiments to verify the existence of electromagnetic waves.[1]
Faraday's lawIt was originally an experimental law based on observation.Later, it was formalized, and the limited version of its partial derivative was listed together with other electromagnetic lawsmaxwellThe Modern of Equationsheaviside edition.
Faraday's law of electromagnetic induction is based on Faraday's experiment in 1831.This effect was discovered by Joseph Henry at about the same time, but Faraday was published earlier.
In 1834, Russian scientistsHeinrich LenzLenz's law, discovered, providesInduced electromotive forceAnd the direction of the current generating the induced electromotive force.
describe
Law of electromagnetic induction
becauseMagnetic fluxThe induced electromotive force is generated by the change, and part of the conductor in the closed circuitmagnetic fieldWhen cutting the magnetic induction lineelectric currentThis phenomenon is called electromagnetic induction.A part of the conductor of a closed circuit is made in a magnetic fieldCutting magnetic induction line movementCurrent will be generated in the conductor.This phenomenon is calledElectromagnetic induction phenomenon。The generated current is called induced current.This isJunior high school physicsIn order to help students understand the defined electromagnetic induction phenomenon, the textbook cannot comprehensively summarize the electromagnetic induction phenomenon: the closed coil area remains unchanged, but changesmagnetic field intensityThe magnetic flux will also change, and electromagnetic induction will also occur.Therefore, the accurate definition is as follows:The phenomenon of induced electromotive force due to the change of magnetic flux。[2]
If the closed circuit is an n-turn coil, the instantaneous electromotive force can also be expressed as:ε=n*ΔΦ/Δt（Δt→0）。WherenIs the number of coil turns,ΔΦIs the change of magnetic flux, in Wb (Weber),ΔtIs the time taken for the change, in seconds.εIs the induced electromotive force generated, in V（Volts, V for short).Electromagnetic induction, commonly known as magnetoelectricity, is mostly applied toalternator。
Qualified
1. The circuit is closed and flowing.
electromagnetic induction
2. The magnetic flux through the closed circuit changes.
3. Part of the circuit is made in the magnetic fieldCutting magnetic induction line movement(The movement of the cutting magnetic induction line is to ensure that the magnetic flux of the closed circuit changes) (only partial cutting, and all cutting is invalid) (if a condition is missing, there will be no induced current).
4. Micro explanation of induced current generation: when a part of the circuit moves to cut the magnetic induction line, it is equivalent to that the free electrons in a part of the circuit do not move along the direction of the magnetic induction line 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, induced current will be formed. If it is not a closed loop,Charge will accumulate at both ends to generate induced electromotive force.
5. The reason why the "part conductor" of the closed circuit is emphasized in the electromagnetic induction phenomenon is that when the whole closed circuit cuts the magnetic induction line, the induced currents generated on the left and right sides are counterclockwise and clockwise respectively, and the current is offset for the whole circuit.
6. Energy relation in electromagnetic induction:Electromagnetic induction is a process of energy conversion, for example, you canGravitational potential energy, kinetic energy, etc. into electric energy,heat energyEtc.
Important Experiments
electromagnetic induction
Wind a group of conductor coils connected with the galvanometer on a hollow paper cylinder. When the magnetic rod is inserted into the coil, the galvanometer pointer deflects. When the magnetic rod is pulled out of the coil, the galvanometer pointer deflects in the opposite direction. The faster the magnetic rod is inserted into or pulled out of the coil, the deflected galvanometerangleThe larger it is, but when the magnetic rod does not move, the pointer of the galvanometer will not deflect.
For the coil, the moving magnetic rod means that the magnetic field around it has changed, so that the coil can induce current. Faraday finally realized his dream for many years - to generate electricity with the movement of the magnet.OsterandFaradayThe discovery ofSymmetry: Moving electricity generates magnetism, and moving magnetism generates electricity.
Not only the relative movement of the magnetic rod and the coil can make the coil produce induced current, the current in one coil changes, but also can make the other coil produce induced current.
Connect the coil with the power supply through the switch k. When the switch k is turned on or off, the coil 2 will have induced current.If the DC power supply connected to the coil 1 is changed to the transformer power supply, that is, the AC current is supplied to the coil 1, which also causes the induction current of the coil.This is also because the current change of coil 1 causes the magnetic field around coil 2 to change.
Science and technology application
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Dynamic microphone
Figure 2 Operating Principle of Microphone Electromagnetic Induction
In the theater, in order to enable the audience to hear the actors' voice clearly, it is often necessary to amplify the sound. The device for amplifying the sound mainly includes microphone, loudspeaker and loudspeaker.The microphone isvoiceA device that converts electrical signals.Figure 2 is the structural schematic diagram of the dynamic microphone, which is made of electromagnetic induction. When sound waves cause the metal diaphragm to vibrate, the coils connected to the diaphragm (called voice coils) vibrate together, and the voice coilsPermanent magnetIn the magnetic field, which produces induced current（electrical signal）The magnitude and direction of the induced current change, and the amplitude and frequency of the change are determined by the sound wave. The signal current is amplified by the loudspeaker and transmitted to the loudspeakerspeakerA amplified sound is emitted in the.
Tape recorder
magnetic taperecorderIt is mainly composed of internal microphonemagnetic tape, recording and playback head, amplification circuit, loudspeaker, transmission mechanism and other parts are the schematic diagram of recording and playback principle of the recorder.During recording, the sound causes the microphone to produce an induced current that changes with the sound - audio current. After being amplified by the amplification circuit, the audio current enters the coil of the recording head and generates a magnetic field that changes with the audio current at the gap of the head.The magnetic tape moves close to the gap of the magnetic head, and the magnetic particle layer on the magnetic tape ismagnetizationThe magnetic signal of sound is recorded on the tape.
Playback is the reverse process of recording. During playback, the tape passes through the gap of the playback head. The changing magnetic field on the tape causes the induced current in the playback head coil. The change of the induced current is the same as the recorded magnetic signal, so the audio current generated in the coil is amplified by the amplification circuit and sent to the speaker,The speaker converts the audio current into sound.
In the recorder, the two functions of recording and playing are completed by sharing a magnetic head, which is connected to the microphone when recording;The head is connected to the loudspeaker during playback.
Vehicle speedometer
Vehicle speedometer - electromagnetic induction
The speedometer in the driver's cab indicates the driving speed of the carmeter。It uses the principle of electromagnetic induction to make the pointer on the dialPendulum angleIt is proportional to the driving speed of the car.The speedometer is mainly composed of drive shaft, magnet and speed disk,SpringIt is composed of hairspring, pointer shaft and pointer.amongPermanent magnetIt is connected to the drive shaft.staywatch caseThere is a dial with a scale of km/h.
The direction of magnetic induction line of permanent magnet is shown in Figure 1.Some of the magnetic induction lines will pass through the speed disk, and the distribution of the magnetic induction lines on the speed disk is uneven, the closer the magnetic induction lines aremagnetic poleWhere the number of magnetic induction lines is more.When the drive shaft drives the permanent magnet to rotate, the magnetic induction lines passing through each part of the speed disk will change in turn. Along the front of the magnet rotation, the number of magnetic induction lines will gradually increase, and then gradually decrease.According to Faraday's electromagnetic induction principleconductorWhen the number of magnetic induction lines of is changed, the induced current will be generated inside the conductor.It is also known from Lenz's law that the induced current also generates 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, the magnetic induction line generated by the induced current is opposite to the magnetic induction line generated by the magnet in front of the rotation of the magnet, so they are mutually exclusive;The direction of the magnetic induction line generated by the reverse induced current is the same as that generated by the magnet, so they attract each other.Because of this attraction, the speed disk is driven by the magnet to rotate, 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 speed disk rotates over a certain angle, the hairspring is twisted to produce the oppositemoment, when it is connected withPermanent magnetWhen the torque driving the speed disk is equal, the speed disk will stay at that positionEquilibrium state。At this time, the pointer on the pointer shaft will indicate the corresponding speed value.
The rotation speed of the permanent magnet is proportional to the driving speed of the car.When the car is movingspeedWhen it increases, the current induced in the speed dial and the corresponding torque driving the speed dial to rotate will increase proportionally, making the pointer rotate at a greater angle. Therefore, the speed values indicated by different pointers will be different accordingly.When the car stops running, the magnet stops rotating, and the spring hairspring resets the pointer shaft, so that the pointer points to "0".
cupola metal
Use eddy current to heat and melt metal ------ electromagnetic induction
The eddy current induced by the AC magnetic field in the metal can produce a thermal effect. This heating method has many advantages compared with fuel heating. In addition to the textbook, the heating efficiency is high, reaching 50~90%;Fast heating speed;Different heating depths can be obtained by alternating current of different frequencies, because the eddy current is not evenly distributed in the metal. The closer to the metal surface layer, the stronger the current is, the higher the frequency is. This phenomenon is called“Skin effect”。In industry, induction heating is divided into four types according to frequency: power frequency (50 Hz);Intermediate frequency (0.5~8 kHz);Ultra audio (20~60 kHz);High frequency (60~600 kHz).Power frequency AC is directly controlled bydistribution transformer Provide;intermediate frequencyAlternating currentfromThree phase motorDrive intermediate frequency generator or use silicon controlled rectifierinverterGeneration;Ultra audio and high-frequency AC are produced by high-power electronic tubesoscillatorGenerate.
The purpose of coreless induction furnace is to melt cast iron, steel, alloy steel, copper, aluminum, etcNon ferrous metals。The AC frequency used should be selected according to the metal mass that the crucible can hold to achieve the best effect.For example: 20 kHz for 5kg, 2.5 kHz for 100kg and 1 kHz or even 50 kHz for 5t.
The smelting pot is filled with the smelted metal, so that high-frequency alternating current passes through the coil, and strong eddy current will be generated in the smelted metal, thus generating a lot of heat to melt the metal. This smelting method is fast, easy to control the temperature, and can prevent harmful impurities from mixing into the smelted metal. It is suitable for smelting special alloys and special steels.
The induction heating method is also widely used for the heat treatment of steel parts, such as quenching, tempering, surface carburizing, etc. For example, gears, shafts, etc., only need to be surface quenched to improve hardness and wear resistance, which can be put into a hollow coil with high-frequency AC, and the surface layer can rise to the high temperature required for quenching in a few seconds, the color is bright red, while the internal temperature increases little,Then it can be cooled quickly with water or other quenchants. For other heat treatment processes, medium frequency or power frequency can be selected according to the required heating depth.
Motor
DC motor
The generator can operate "in reverse" and become a motor.For example, using Faraday disc, let a DC currentVoltageDrive through the conductive shaft arm.Then according to Lorentz's law of forcechargeUnder the force of magnetic field BLeft-handed ruleTurn the disc in the set direction.When there is no irreversible effect (such as friction orJoule heat）The rotation of the discrateIt is necessary to make dΦB/dtA voltage equal to the drive current.
transformer
transformer
The electromotive force predicted by Faraday's law is alsotransformerThe principle of operation.When the current in the coil changes, the changing current generates a changing magnetic field.In magnetic fieldScope of actionThe second wire in will feel the change of magnetic field, so the coupling magnetic flux will also change (dΦB/dt）。Therefore, there will be electromotive force in the second coil, which is called induction electromotive force or transformer electromotive force.If an electrical load is connected at both ends of the coil, current will flow.
significance
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Electromagnetic induction refers to the phenomenon of induced electromotive force due to the change of magnetic flux.Electromagnetic induction phenomenonThe discovery of iselectromagneticsOne of the greatest achievements in the field.It not only reveals the internal relationship between electricity and magnetism, but also lays an 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 is of great practical significance.The significance of Faraday's law of electromagnetic induction is that, on the one hand, according to the principle of electromagnetic induction, people have created generators,electric energyLarge scale production and long-distance transportation become possible;On the other hand, it marks the arrival of a major industrial and technological revolution.It has been proved that electromagnetic induction is very important in electrical engineeringelectronic technique、Electrification、automationThe extensive application of the technology has played an important role in promoting the development of social productivity and science and technology.
Related research
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Electromagnetic coil gunEMC (Electromagnetic Coilgun) for short is a weapon system that uses the principle of electromagnetic induction to launch projectiles with high speed.Different from the traditional powder gun, the electromagnetic coil gun does not need to use the traditional powder cartridge, but uses the electromagnetic force to propel the shell, thus greatly improving the launch speed and range.Due to the advantages of electromagnetic coil gun, all countries in the world are conducting relevant technical research.
In September 2023, the Chinese research team successfully designed a new type of solar cell with high efficiency and low cost through the synthesis and regulation of nano materials.Through the design of this structure, the photoelectric conversion efficiency of the solar cell has been greatly improved, and the controllability, scalability and stability have been significantly improved.In addition, due to the use of new materials, the production cost of the battery is also greatly reduced, making the popularization of solar cells more feasible.
