electromagnetic field

[diàn cí chǎng]

A physical field generated by a charged object

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stay electromagnetics Electromagnetic field is a kind of physical field generated by charged objects. Charged objects in the electromagnetic field will feel the electromagnetic field Force 。 The interaction between electromagnetic field and charged object (charge or current) can be used Maxwell equation and Lorentz force Laws.

Chinese name: electromagnetic field
Foreign name: electromagnetic field
Essence: A physical field generated by a charged object

Features: Charged objects of electromagnetic field will feel the force
Description: Maxwell equation and Lorentz force law
Concept: The general term of the dependent electric and magnetic fields

catalog

eight electromagnetic radiation
nine radio waves

definition

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Electromagnetic field is the general name of the unity of electric field and magnetic field with internal connection and interdependence. Time varying electric field generates magnetic field, and time varying magnetic field generates electric field. The two are mutually causal and form electromagnetic field 。 The electromagnetic field can be determined by Variable speed movement It is caused by charged particles of, or by current with varying intensity. No matter what the reason, the electromagnetic field always spreads around at the speed of light, forming electromagnetic waves. The electromagnetic field is Electromagnetic action The medium of energy and momentum is a kind of matter Existing form 。 The nature, characteristics and movement law of electromagnetic field are determined by Maxwell equations.

Electromagnetic field changing with time. The time-varying electromagnetic field is significantly different from the static electric and magnetic fields, and there are some effects due to time-varying. These effects have important applications and promote the development of electrical technology.

Electromagnetic wave is a kind of motion form of electromagnetic field. However, in high frequency Electric oscillation Medium, the magnetoelectric interaction is very fast, and the energy cannot be completely returned to the original Oscillating circuit , then electric energy Magnetic energy With the periodic transformation of electric and magnetic fields, they spread out to space in the form of electromagnetic waves. Electromagnetic wave is transverse wave The magnetic field, electric field and travel direction of electromagnetic wave are perpendicular to each other. The propagation of electromagnetic waves has edges Ground propagation Of Surface wave And air waves propagating from the air. The longer the wavelength of the ground wave, the less its attenuation. The longer the wavelength of electromagnetic wave is, the easier it is to bypass obstacles and continue to propagate. The medium wave or short wave and other air waves are around the earth the ionosphere It is propagated by repeated reflection with the ground (the ionosphere is between 50 and 400 km above the ground). amplitude Make along the vertical direction of the propagation direction Periodic change Its intensity is inversely proportional to the square of the distance. The wave itself carries energy. The energy and power at any position are proportional to the square of the amplitude. Its speed is equal to the speed of light (300000 kilometers per second). Light waves are also electromagnetic waves, and radio waves have the same characteristics as light waves. For example, when they pass through different media, they will also be refracted, reflected diffraction , scattering and absorption, etc. For an electromagnetic wave propagating in space, the distance between two points with the same direction of the nearest electric field (magnetic field) intensity and the largest value is the wavelength λ of the electromagnetic wave. The frequency of electromagnetic wave γ is electricity Oscillating current Frequency of, radio broadcast The unit used in is Kilohertz The speed is c. According to λ γ=c, calculate λ=c/γ.

Electricity can generate magnetism, and magnetism can also bring electricity. The changing electric field and the changing magnetic field form an inseparable unified field, which is electromagnetic field. The propagation of the changing electromagnetic field in space forms electromagnetic waves, so electromagnetic waves are also often called electric waves. In 1864, British scientist Maxwell summarized previous studies Electromagnetic phenomenon On the basis of the results achieved, a complete Electromagnetic wave theory 。 He concluded that electromagnetic wave It is deduced that electromagnetic wave and light have the same propagation velocity 。

1887 German physicist hertz The existence of electromagnetic waves is confirmed by experiments. Later, people carried out many experiments, which not only proved that light is an electromagnetic wave, but also found more forms of electromagnetic waves. Their essence is identical, but the wavelength and frequency are very different. Arranging these electromagnetic waves in the order of wavelength or frequency is Electromagnetic spectrum 。 If the frequencies of each wave band are arranged from low to high, they are power frequency electromagnetic wave, radio wave, microwave, infrared ray, visible light, ultraviolet ray, X-ray and R ray 。

Development history

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People have long been exposed to the phenomenon of electricity and magnetism, and know that magnetic bar yes North and South Poles 。 In the 18th century, two kinds of electric charges were found: positive charge and negative charge 。 Both electric charges and magnetic poles repel each other and attract each other, Force Direction of the force is on connecting line between electric charges or magnetic poles, and magnitude of the force is inversely proportional to square of the distance between them. On these two points and Universal gravitation Very similar. At the end of the 18th century, it was discovered that electric charges could flow, which was called current. But for a long time, people only found the phenomenon of electricity and magnetism, and did not find the connection between electricity and magnetism.

Faraday

In the early 19th century, Oster It was found that the current could deflect the small magnetic needle. Then Ampere found that the direction of the force and the direction of the current were perpendicular to each other Through current Of the conductor Vertical line The direction is perpendicular to the current direction. Shortly afterwards, Faraday found that when the magnetic rod was inserted into the coil, current would be generated in the coil. These experiments show that there is a close relationship between electricity and magnetism. After the discovery of the connection between electricity and magnetism, people realized that Electromagnetic force In some respects, it is similar to gravity, but in other respects, it is different. For this reason, Faraday introduced Force line According to the concept of power line On this basis, the concept of electromagnetic field was generated.

It is recognized that electromagnetic field is a special form of material existence. The charge generates an electric field around it, which acts on other charges with force. magnet And the current generate a magnetic field around it, and this magnetic field acts on other magnets and objects with current inside by force. Electromagnetic field also has energy and momentum, which is the medium to transmit electromagnetic force, and it permeates the whole space.

In the second half of the 19th century, Maxwell summarized the laws of macro electromagnetic phenomena and introduced Displacement current The concept of. The core idea of this concept is: changing Electric field energy Generating magnetic field; Changing magnetic fields can also produce electric fields. On this basis, he proposed a group of partial differential equation To express the basic laws of electromagnetic phenomena. This set of equations is called Maxwell's equations , Yes Classical electromagnetism Of basic equation 。 Maxwell's Electromagnetic theory It predicted the existence of electromagnetic waves, whose propagation speed is equal to the speed of light, which was later confirmed by Hertz's experiment. So people realized that Maxwell's electromagnetic theory correctly reflected the law of macro electromagnetic phenomena, and affirmed that light is also an electromagnetic wave. Since the electromagnetic field can act on charged particles with force, a moving charged particle is subject to both the force of the electric field and the force of the magnetic field, Lorentz The force of the electromagnetic field on the moving charge is reduced to a formula, which is called Lorentz force. Maxwell equations and Lorentz force, which describe the basic laws of electromagnetic field, form the basis of classical electrodynamics.

Inspired by Oster's current magnetic effect experiment and a series of other experiments, Ampere realized that the essence of magnetic phenomenon is current, and attributed various interactions involving current and magnets to the interaction between currents. He proposed the basic problem of finding the interaction law of current elements. In order to overcome the failure of isolated current elements direct measurement Difficulties, ampere Four zero indicating experiments were carefully designed and accompanied by meticulous theoretical analysis , the results are obtained. However, due to the electromagnetic effect of ampere Action at a distance In theoretical analysis, the assumption that the force between two current elements is along the line was imposed, and it is expected to comply with Newton's Third Law , so that the conclusion is wrong. The above formula is the result of discarding the wrong assumption that the force is along the line and correcting it. It should be understood from the point of view of close action that the current element generates a magnetic field, and the magnetic field affects the other Current element Apply force.

Current magnetic field

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[Purpose and Requirements]

Current magnetic field experiment equipment

The magnetic field and Energized solenoid The experiment of magnetic field, recognize the existence of magnetic field around the charged conductor, and further understand and test the ampere Right hand spiral rule 。

[Instruments and equipment]

Special power supply (low voltage, short time large current ）, thick copper wire（ φ 3 × 30 mm) Small magnetic needle (J2406 type, 10 in a group), Cardboard (20cm × 20cm), Square seat support (J1 102 type), pencil, with cardboard (15cm × 20cm) solenoid , 2 wires, fine iron powder.

[Experimental method]

1、 Magnetic field of linear current

1. Place the 30mm long Crude steel The line passes through the center of the 20 cm square cardboard.

2. Place the thick copper wire along the vertical Fix the position well (for example, use a small bracket to fix the cardboard, or use a square bracket to hold the cardboard). Both ends of the thick copper wire are additionally connected to the output end of the special power supply with wires, and then a layer of fine iron powder is evenly spread on the cardboard.

3. Turn on the switch of the special power supply and tap the cardboard with a pencil at the same time to make the iron powder on the cardboard edge Magnetic line of force Arrange. (The short-term current output of the special power supply is more than 30 or 40 amperes, and then it will be automatically disconnected at a predetermined time.)

4. Use white paper to draw a sketch of the figure you see. Then, on the upper edge of the cardboard Concentric circle Put one small in each of the four different directions of magnetic needle 。 Turn on the special power supply again and press down magnetic needle The direction of the north pole marks the direction of the magnetic line of force on the concentric circle.

5. Check whether the current direction and magnetic force line direction in the wire comply with the right-hand spiral rule

6. Change the direction of current to repeat the above experiment, draw a magnetic force line diagram again, compare with the above figure, and check the right-hand spiral rule.

II Energized solenoid Magnetic field of

1. Connect the two ends of the solenoid with cardboard to the output end of the special power supply, press to turn on the power supply, and gently tap the cardboard evenly sprinkled with iron powder, so that the iron powder on it is arranged along the magnetic force line

2. Draw the indication solenoid Schematic diagram of iron powder arrangement shape inside and outside both ends.

3. On Spiral Put several small pieces in different places inside the pipe and outside the two ends magnetic needle , switch on the special power supply again, mark the direction of the magnetic force line on the diagram according to the direction of the north pole of the small magnetic needle, and indicate the direction of the current to see if it conforms to the right-hand spiral rule

Reference

Coil compass

Use on a pencil Kraft paper Wrap two layers and then use Enameled wire Tightly wind 60-70 turns in turn on the paper. After fixing the thread ends Paper drum Take them off the pencil together, and you will get a Helical coil 。

Take another Copper sheet 1 small piece of zinc sheet, Foam plastic One coil is placed on the foam plastic, and the two ends of the coil are respectively connected with the copper sheet and zinc sheet. Then, put the device into a cup containing salt water (or vinegar), let it float on the liquid surface, and immerse the zinc sheet and copper sheet in the solution. At this time, you can see that the axis of the coil tube always points to the north-south direction. No matter how you change its direction, it will return to its north-south position, just like a compass.

This is due to the Steel sheet The zinc sheet forms a chemical galvanic cell, and the current generated by the galvanic cell will flow through the spiral coil to generate a magnetic field, which will make the two ends of the spiral coil display different magnetic poles, so the coil will indicate the direction like a compass.

electromagnetic wave

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brief introduction

electromagnetic field

Electromagnetic field is formed by the propagation of near and far disturbances electromagnetic wave , the electromagnetic field changing with time. Time-varying electromagnetic field It is significantly different from the static electric field and magnetic field, and there are some effects due to time variation. These effects have important applications and promote the development of electrical technology.

electromagnetic induction

because Magnetic flux Change generation Induced electromotive force Phenomenon of: Closed circuit A part of the conductor is made in the magnetic field Cutting magnetic line of force The current will be generated in the conductor when it moves. This phenomenon is called Law of electromagnetic induction^[1] 。

1820 H C. Oster found Current magnetic effect Later, many physicists tried to find its reverse effect, and put forward the question whether magnetism can produce electricity and whether magnetism can act on electricity, D F. J. Arago and A. von Humboldt are measuring Geomagnetic intensity When the metal pair is found nearby magnetic needle The oscillations of are Damping action 。 In 1824, Aragoro did something based on this phenomenon Copper wire In the experiment, it was found that the rotating copper disk would drive the free hanging magnetic needle to rotate, but the rotation of the magnetic needle was not synchronized with the copper disk, and was slightly delayed. Electromagnetic damping and Electromagnetic drive It was the first discovery Electromagnetic induction phenomenon , but it is not directly shown as induced current , which could not be explained at that time^[1] 。

Conditions for generating induced current

① The circuit is closed and open;

② The magnetic flux passing through the closed circuit changes; (If a condition is missing, there will be no induced current)^[1] .

M. Faraday Law of electromagnetic induction It shows that the change of magnetic field will produce electric field. This electric field is related to Coulomb's law Different from the electric field of, it can push the current to flow in the closed conductor circuit, that is, its loop integral can not be zero, which becomes the induced electromotive force. Modern widely used power equipment And generators, transformers, etc Induction There are close connections. Because of this role. Eddy current and Skin effect 。 Electrician induction heating 、 Surface hardening 、 electromagnetic shielding And so on are direct applications of these phenomena.

magnetism Induction phenomenon yes electromagnetics One of the most important discoveries in Magnetic phenomenon The interrelationship between. Faraday Law of electromagnetic induction On the one hand, according to the principle of electromagnetic induction, people have created generators mass production And long-distance transportation is possible; On the other hand, the electromagnetic induction phenomenon in electrical technology, electronic technology and Electromagnetic measurement And so on^[1] 。

Research process

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（1） Law of electromagnetic induction

Faraday

Following Faraday's law of electromagnetic induction, J. C. Maxwell put forward the concept of displacement current. The electric displacement comes from dielectric In charged particle Under electric field Electric field force Role of. Although these charged particles cannot free flow , but a small displacement on the atomic scale will occur. Maxwell extended the term to electric field in vacuum, and believed that; Electric displacement Magnetic field will also be generated as time changes, so it is called flux The time change rate of is Displacement current , while the time derivative of the electric displacement vector D (i.e. д D/д t) is the displacement current density. It is in Ampere's loop law, except Conducting current Added Displacement current So as to summarize the complete electromagnetic Equations , that is, the famous Maxwell's equations , describes the distribution and variation of electromagnetic field.

(II) Maxwell equation

maxwell

electromagnetic radiation Maxwell equation shows that not only the change of magnetic field will produce electric field, but also the change of electric field will produce magnetic field. Under this interaction, time-varying fields generate electromagnetic radiation, which is called electromagnetic wave. This kind of electromagnetic wave propagates from the field source at the speed of light to the surroundings, and there is corresponding time in space according to the distance from the field source Hysteresis phenomenon 。 Another important feature of electromagnetic wave is that its field vector contains components that are inversely proportional to the distance between the field source and the observation point. The attenuation of these components in space propagation is much smaller than that of the constant field. according to Poynting theorem Electromagnetic wave carries energy in its propagation and can be used as a carrier of information. This is radio communication , radio, television, remote sensing and other technologies have widened the road. Quasi steady electromagnetic field The above phenomena in the time-varying field are different from those in the static field, and their significance is closely related to the frequency and the size of the equipment. According to actual needs, within the allowable approximate range, part of the process of time-varying field can be treated as a constant field, which is called quasi stable electromagnetic field or quasi-static field. This method greatly simplifies the analysis work. It is an effective method in electrotechnics and has been widely used by people.

(III) Alternating electromagnetic field And transient electromagnetic field

Time varying electromagnetic field can be further divided into periodic alternating electromagnetic field and Aperiodicity Variable transient electromagnetic field. The research on them has some characteristics in purpose and method. Under the sinusoidal change of single frequency, the alternating electromagnetic field can be represented by a complex number Simplification Calculation, in power technology and Continuous wave There are many applications in analysis. Transient electromagnetic field, also known as pulse electromagnetic field, covers a wide range of frequencies, and the medium or transmission system shows dispersion characteristics, which often requires frequency domain , or time series expansion.

Electricity and magnetism

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From a scientific point of view, electromagnetic wave is a kind of energy. Any object that can release energy will release electromagnetic wave.

Electricity and magnetism It can be said that there are two sides of one body. The changing electricity will produce magnetism, and the changing magnetism will produce electricity. The change of electromagnetic wave is just like the water wave generated by the breeze blowing on the water surface, so it is called electromagnetic wave, and the frequency of its change per second is the frequency. When electromagnetic wave frequency is low Conductor Talent transmission; When the frequency increases gradually, the electromagnetic wave will spill out of the conductor, and can transfer energy without medium. This is a kind of radiation. For example, the distance between the sun and the earth is very far, but when we are outdoors, we can still feel the light and heat of the warm sunshine, which is similar to the principle of "electromagnetic radiation transmits energy through radiation phenomena".

classification

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Electromagnetic radiation is a way to transfer energy. There are three types of radiation:

Ionizing radiation

yes heat effect Nonionizing radiation

Non ionizing radiation without thermal effect

Base platform Electromagnetic waves are by no means free radiation waves

Just as people have been living in the air but can't see the air with their eyes, people can't see the ubiquitous electromagnetic waves. Electromagnetic wave is such a "friend" that mankind has never met. Electromagnetic wave is a form of motion of electromagnetic field.

At high frequencies Electromagnetic oscillation In the case of, part of the energy propagates to the surrounding space in the form of radiation, forming radio waves and Magnetic wave It is called electromagnetic wave. In low-frequency electromagnetic oscillation, the mutual change between electricity and magnetism is relatively slow, and almost all of its energy returns to the original circuit without energy radiation. The speed of electromagnetic waves is equal to the speed of light (3 × 10 ^ 10 cm per second).

Wavelength between 10 and 3000 meters, min long wave 、 Medium wave , medium and short wave, short wave, etc. The wavelength used for fax (TV) is 3-6 meters; The wavelength used for radar is shorter, 3 meters to several centimeters. Electromagnetic wave includes infrared ray visible light 、 ultraviolet rays 、 X-ray 、 Gamma ray Etc. All kinds of light and ray are also electromagnetic waves with different wavelengths. The wavelength of radio is the longest, cosmic rays The wavelength of is the shortest.

radio waves 3000m~0.3mm.

Infrared 0.3 mm~0.75 μ m.

Visible light 0.7 μ m~0.4 μ m.

Ultraviolet 0.4 μ m-10 nm

X-ray 10 nm~0.1 nm

Gamma ray 0.1nm~0.001 nm

Cosmic ray is less than 0.001 nm

electromagnetic radiation

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Generalized electromagnetic radiation Usually refers to Electromagnetic wave spectrum for. Electromagnetic radiation in a narrow sense refers to the radiation waves generated by electrical equipment, usually infrared The following section.

How to prevent radiation - use Computer radiation Eraser

The computer radiation eliminator makes the computer and accessory equipment Of alternating current , which can dynamically discover and track computer motherboards CPU , hard disk, display, keyboard, mouse and the radiation generated by the equipment connected to the computer Smart chip The module absorbs, converts and eliminates, effectively eliminating the hidden killer that affects our health - computer radiation from the root!

radio waves

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Radiowave Communication mode ： Ground wave - propagation along the ground; Long wave, short wave, medium and short wave available Ground wave propagation (Radio broadcasting), with the transmission distance within hundreds of kilometers; Skywave : radio waves that rely on the reflection of the ionosphere to propagate, short wave is applicable. It can spread thousands of kilometers away; Rectilinear propagation : Suitable for microwave--- ultrashort wave (Also called Space wave Or visual wave), with a general propagation distance of tens of kilometers^[2] 。

Long wave: wavelength 30000m~3000m, frequency 10 kH Z ~100kH Z , transmitted by ground waves, for ultra long distance radio communication and navigation.

Medium wave: wavelength 3000m~200m, frequency 100kH Z ~1500kH Z , through ground waves and Skywave propagation , for amplitude modulation (AM) Radio broadcasting, telegraph and communication.

Medium and short wave: wavelength 200m~50m, frequency 1500kH Z ~6000kH Z , spread by ground wave and sky wave, used for AM radio broadcasting, telegraph and communication.

Short wave: wavelength: 50m~10m, frequency: 6MH Z ~30MH Z , transmitted by skywave, used for AM radio broadcasting, telegraphy and communication^[2] 。

Microwave:

Mibo VHF : Wavelength: 10m~1m, frequency: 30MH Z ~300MH Z Through approximate straight line propagation, it is used for FM radio broadcasting, television and navigation.

Decimeter wave UHF : Wavelength 1m~0.1m, frequency 300MH Z ~3000MH Z , transmitted by ground waves, used for television, radar and navigation.

Centimeter wave : Wavelength 10cm~1cm, frequency 3000MH Z ~30000MH Z , transmitted by ground waves, used for television, radar and navigation.

Millimeter wave : Wavelength: 10mm~1mm, frequency: 30000MH Z ~300000MH Z , spread by ground waves, used for TV, radar, navigation^[2] 。

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