What Is an Electromagnetic Field?

In electromagnetics, an electromagnetic field is a physical field produced by a charged object. A charged object in an electromagnetic field will feel the force of the electromagnetic field. The interaction between an electromagnetic field and a charged object (charge or current) can be described using Maxwell's equations and Lorentz's law of forces.

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

Features: A charged object in an electromagnetic field will feel the force
Description: Maxwell's equation and Lorentz force law
Concept: General term for the unity of dependent electric and magnetic fields

Definition of electromagnetic field

Electromagnetic field is a general term for a unified body of electric and magnetic fields that are internally linked and interdependent. The time-varying electric field generates a magnetic field, and the time-varying magnetic field generates an electric field. The two are causal to each other and form an electromagnetic field . Electromagnetic fields can be caused by charged particles that move at a variable speed, or by currents that change in strength. Regardless of the cause, the electromagnetic field always travels around at the speed of light, forming electromagnetic waves. Electromagnetic field is a medium of electromagnetic action, has energy and momentum, and is a form of existence of matter. The nature and characteristics of electromagnetic fields and the laws of their motion are determined by Maxwell's equations.

An electromagnetic field that changes over 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 have promoted the development of electrical technology.

Electromagnetic waves are a form of motion of electromagnetic fields. However, in high-frequency electrical oscillations, magnetoelectricity changes very quickly, and it is impossible for all energy to return to the original oscillating circuit, so electrical energy and magnetic energy are transmitted to space in the form of electromagnetic waves with the periodic conversion of electric and magnetic fields. The electromagnetic wave is a transverse wave. The magnetic field, the electric field, and the direction of travel of the electromagnetic wave are perpendicular to each other. The propagation of electromagnetic waves includes ground waves that propagate along the ground, and air waves that propagate from the air. The longer the ground wave, the less attenuation it has. The longer the wavelength of the electromagnetic wave, the easier it is to continue to propagate around obstacles. Air waves such as medium or short waves are propagated by repeated reflections of the ionosphere surrounding the earth and the ground (the ionosphere is between 50 and 400 kilometers above the ground). The amplitude changes periodically in the vertical direction of the propagation direction. Its intensity is inversely proportional to the square of the distance. The wave itself carries energy. The energy and power at any position is proportional to the square of the amplitude. Its speed is equal to the speed of light (300,000 kilometers per second ). Light waves are also electromagnetic waves. Radio waves also have the same characteristics as light waves. For example, when it passes through different media, it also undergoes refraction, reflection, diffraction, scattering, and absorption. The distance of the electromagnetic wave propagating in space is the wavelength of the electromagnetic wave (magnetic field) with the same intensity direction and the largest distance between the two points. The frequency of the electromagnetic wave is the frequency of the electric oscillating current. The unit used in radio broadcasting is kilohertz and the speed is c. From = c, = c / is obtained.

Electricity can generate magnetism, and magnetism can also generate electricity. The changing electric field and the changing magnetic field constitute an inseparable unified field. This is the electromagnetic field, and the changing electromagnetic field propagates in space to form electromagnetic waves. It's radio waves. In 1864, British scientist Maxwell established a complete theory of electromagnetic waves on the basis of summing up the results of previous studies on electromagnetic phenomena. He judged the existence of electromagnetic waves and deduced that electromagnetic waves have the same propagation speed as light.

In 1887, the German physicist Hertz experimentally confirmed the existence of electromagnetic waves. Since then, many experiments have been performed, not only to prove that light is an electromagnetic wave, but also to find more forms of electromagnetic waves, which are essentially the same in nature, except that the wavelength and frequency are very different. Arranging these electromagnetic waves in the order of wavelength or frequency is the electromagnetic spectrum. If the frequency of each band is arranged in order from low to high, they are power frequency electromagnetic waves, radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and r-rays.

History of electromagnetic fields

People have been exposed to electricity and magnetism for a long time, and they know that magnetic rods have north and south poles. In the 18th century, two types of charges were found: positive and negative charges. Both the charge and the magnetic pole are repulsive by the same sex, attracted by the opposite sex, the direction of the force is on the connection line between the charges or the magnetic poles, and the magnitude of the force is inversely proportional to the square of the distance between them. It is very similar to gravity in these two points. It was discovered at the end of the 18th century that electric charges could flow, and this was electricity. But for a long time, people have only discovered the phenomenon of electricity and magnetism, and have not found the connection between electricity and magnetism.
In the early 19th century, Oster discovered that current could deflect small magnetic needles. Then Ampere found

Electromagnetic field The direction and the direction of the current are perpendicular to each other, and the direction of the vertical line from the magnetic needle to the wire passing the current is perpendicular to the direction of the current. Soon after, Faraday discovered that when a magnetic rod was inserted into a guide coil, a current was generated in the guide coil. These experiments show that there is a close connection between electricity and magnetism. After the connection between electricity and magnetism was discovered, people realized that the nature of electromagnetic force is similar to gravitational force in some aspects, but different in others. For this reason, Faraday introduced the concept of force lines. It is believed that currents generate magnetic lines of force around electric wires, and electric charges generate electric lines of force in all directions. Based on this, the concept of electromagnetic fields is generated.
It is recognized that electromagnetic fields are a special form of material existence. Electric charges generate an electric field around them, and this electric field acts on other charges with force. Magnets and currents generate a magnetic field around them, and this magnetic field acts on other magnets and objects with current inside by force. The electromagnetic field also has energy and momentum, is a medium for transmitting electromagnetic force, and it permeates the entire space.
In the second half of the 19th century, Maxwell summarized the laws of macroscopic electromagnetic phenomena and introduced the concept of displacement current. The core idea of this concept is: a changing electric field can generate a magnetic field; a changing magnetic field can also generate an electric field. On this basis, he proposed a set of partial differential equations to express the basic laws of electromagnetic phenomena. This set of equations is called Maxwell's equations, and is the basic equation of classical electromagnetics. Maxwell's electromagnetic theory predicted the existence of electromagnetic waves, and its propagation speed was equal to the speed of light. This prediction was later confirmed by Hertz's experiments. So people realized that Maxwell's electromagnetic theory correctly reflected the laws of macroscopic electromagnetic phenomena, affirming that light is also an electromagnetic wave. Because the electromagnetic field can act on charged particles with a force, a charged particle in motion is subjected to both the force of the electric field and the force of the magnetic field. Force is Lorentz force. The Maxwell equations and Lorenz force describing the basic laws of electromagnetic fields form the basis of classical electrodynamics.
Inspired by Oerstedt's current-magnetism experiment and a series of other experiments, Ampere recognized that the essence of magnetic phenomena is current, and attributed various interactions involving current and magnets to interactions between currents. Basic issues of the law of element interaction. In order to overcome the difficulty that the isolated current element cannot be measured directly, Ampere carefully designed four zero indicating experiments and accompanied the meticulous theoretical analysis, and obtained the results. However, because of Ampere's concept of over-range action on electromagnetic interactions, the hypothesis that the force along the connection between two current elements was imposed in the theoretical analysis was expected to comply with Newton's third law, making the conclusion wrong. The above formula is the corrected result of abandoning the assumption that the wrong force is along the line. It should be understood from the perspective of close action that a current element generates a magnetic field, and the magnetic field exerts a force on the other current element.

Electromagnetic field

[Purpose and requirements]

Through the experiment of the magnetic field of the linear current and the magnetic field of the energized solenoid, it is recognized that there is a magnetic field around the charged conductor, and the Ampere's right-hand spiral rule is further recognized and tested.

Current magnetic field experiment equipment

[Instruments and Equipment]

Dedicated power supply (low voltage, high current for a short period of time), thick copper wire ( 3 × 30 mm), small magnetic needle (J2406 type, set of 10), cardboard (20 cm × 20 cm), square base bracket (J1 102 Type), pencil, solenoid with cardboard (15 cm x 20 cm), 2 wires, fine iron powder.

experimental method

First, the magnetic field of linear current

1. Pass a 30 mm long thick steel wire through the center of a 20 cm square cardboard.

2. Fix the thick copper wire in a vertical position (for example, use a small bracket to hold the cardboard, or use a square bracket to hold the cardboard). The two ends of the thick copper wire are additionally connected to the output end of the dedicated power supply with a wire, and then a layer of fine iron powder is evenly spread on the cardboard.

3 Turn on the dedicated power switch and tap the cardboard with a pencil at the same time, so that the iron powder on it is aligned along the magnetic field lines. (The short-time current output of the dedicated power supply is more than 30 or 40 amps, and then it will automatically disconnect at a predetermined time.)

4 Draw a sketch of what you see on white paper. Then place a small magnetic needle on each of the four different orientations of the concentric circle on the cardboard. Turn on the dedicated power again, and mark the direction of the magnetic lines of force on the concentric circles in the direction pointed by the north pole of the small magnetic needle.

5. Check that the direction of the current and magnetic field lines in the wire conforms to the right-hand spiral rule

6. Change the direction of the current and repeat the above experiment. Draw a simplified magnetic line diagram, compare it with the above picture, and check the right-hand spiral rule.

2. Magnetic field of energized solenoid

1. Connect the two ends of the solenoid with cardboard to the output end of the dedicated power supply, press the power on, and tap the cardboard with iron powder evenly while tapping, so that the iron powder above is aligned along the magnetic lines of force

2. Draw a diagram showing the arrangement of the iron powder inside and outside the solenoid.

3 Put a few small magnetic needles in different places inside and outside the solenoid, and turn on the special power again. According to the direction of the north pole of the small magnetic needle, mark the direction of the magnetic field lines on the sketch and indicate the direction of the current to see if Meets the right-hand spiral rule

[Reference]

Coil compass

Wrap two layers of kraft paper on the pencil, and then use the enameled wire to wrap the paper in a sequence of 60-70 turns. After fixing the thread ends, remove the coil together with the paper tube from the pencil, so that a spiral coil is obtained. .

Another copper sheet, zinc sheet, each small piece, a piece of foam plastic, the coil is placed on the foam plastic, the two ends of the coil are respectively connected to the copper sheet, zinc sheet. Then, place the device in a cup filled with saline (or vinegar), let it float on the surface, and immerse the zinc and copper pieces in the solution. At this point you can see that the axis of the coil tube always points in the north-south direction. No matter how you change its direction, it will return to the north-south position, like a compass.

This is because the steel sheet and zinc sheet inserted in the salt water form a chemical galvanic cell. The current generated by the galvanic cell flowing through the spiral coil will generate a magnetic field, making the two ends of the spiral coil show different magnetic poles, so the coil will indicate the direction like a compass Already.

Electromagnetic field

Introduction to electromagnetic fields

The electromagnetic field is disturbed by near and far

Electromagnetic field Propagating to form electromagnetic waves, electromagnetic fields that change 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 have promoted the development of electrical technology.

Electromagnetic induction

The phenomenon of induced electromotive force is caused by the change of magnetic flux: when a part of the conductor of a closed circuit performs a motion of cutting magnetic lines of force in a magnetic field, a current will be generated in the conductor. This phenomenon is called the law of electromagnetic induction ^[1] .

After discovering the magnetic effect of current by HC Oster in 1820, many physicists tried to find its inverse effect, and raised the question of whether magnetism can generate electricity and whether magnetism can affect electricity. In 1822, DFJ Arago and A.von Hong When measuring the strength of the geomagnetism, he accidentally found that metal has a damping effect on the oscillation of nearby magnetic needles. In 1824, Arago conducted a copper disk experiment based on this phenomenon, and found that the rotating copper disk would cause the magnetic needles hanging freely to rotate above, but the rotation of the magnetic needles was not synchronized with the copper disk and was slightly delayed. Electromagnetic damping and electromagnetic driving were the earliest electromagnetic induction phenomena to be discovered, but they were not directly explained as induced currents at that time ^[1] .

Conditions for induced current

The circuit is closed and open;

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

M. Faraday's law of electromagnetic induction shows that changes in the magnetic field generate an electric field. This electric field is different from the electric field derived from Coulomb's law. It can promote the current to flow in the closed conductor loop, that is, its loop integral can be non-zero and become an induced electromotive force. A large number of modern power equipment, generators, transformers, etc. are closely related to electromagnetic induction. Because of this role. Eddy currents and skin effects will occur in large conductors in a time-varying field. Induction heating, surface hardening, and electromagnetic shielding in electricians are all direct applications of these phenomena.

Magnetic induction is one of the most important discoveries in electromagnetics, and it reveals the interconnection between electrical and magnetic phenomena. The significance of Faraday's law of electromagnetic induction is that, on the one hand, according to the principle of electromagnetic induction, people have manufactured generators, and large-scale production and long-distance transmission of electrical energy become possible; on the other hand, the phenomenon of electromagnetic induction in electrical and electronic technology As well as electromagnetic measurement, it has a wide range of applications ^[1] .

Electromagnetic field research process

(A) the law of electromagnetic induction

Following Faraday's law of electromagnetic induction, JC Maxwell proposed the concept of displacement current. Electrical displacement results from the effect of electric field forces on the charged particles in the dielectric. Although these charged particles cannot flow freely, they will undergo small displacements on the atomic scale. Maxwell will this

Faraday The term is generalized to the electric field in a vacuum, and thinks that the electric displacement also needs to generate a magnetic field with time. Therefore, the time change of the electric flux over an area is called the displacement current, and the time derivative of the electric displacement vector D (ie t) is the displacement current density. It supplements the role of displacement current in addition to the conduction current in the Ampere Loop Law, thereby summing up a complete set of electromagnetic equations, known as Maxwell's equations, describing the distribution of electromagnetic fields.

(2) Maxwell's equation

The Maxwell equation of electromagnetic radiation shows that not only changes in the magnetic field generate an electric field, but changes in the electric field also generate a magnetic field. Under this interaction, time-varying fields generate electromagnetic radiation, which is electromagnetic waves. This electromagnetic wave propagates from the field source to the surroundings at the speed of light, and there is a corresponding time lag phenomenon in various places according to the distance from the field source. Another important feature of electromagnetic waves is that its field vector has a component that is inversely proportional to the distance from the field source to the observation point. The attenuation of these components during space propagation is much smaller than the constant field. According to Poynting's theorem, electromagnetic waves carry energy in their propagation and can be used as a carrier of information. This opens the way for radio communications, radio, television, remote sensing and other technologies.

Maxwell The phenomena of the above-mentioned phenomena different from the static field in the time-varying field of the quasi-steady electromagnetic field are significantly related to the frequency and the size of the device. According to actual needs, within the allowable approximate range, part of the process of the time-varying field can be treated as a constant field, which is called a pseudo-stable electromagnetic field or a quasi-static field. This method greatly simplifies the analysis work. It is an effective method in electrical technology and has been widely used by people.

(Three) alternating electromagnetic field and transient electromagnetic field

Time-varying electromagnetic fields can be further divided into periodic electromagnetic fields and alternating non-periodic transient electromagnetic fields. Their research has some characteristics in terms of purpose and method. Under the sinusoidal change of a single frequency, an alternating electromagnetic field can be expressed in complex numbers to simplify calculations. It is widely used in power technology and continuous wave analysis. Transient electromagnetic fields are also called pulsed electromagnetic fields. They cover a wide range of frequencies. The medium or transmission system exhibits excellent dispersion characteristics. Frequently, methods such as frequency domain or time series expansion are required for analysis.

Electromagnetism

From a scientific point of view, electromagnetic waves are a type of energy. Any object that can release energy will release electromagnetic waves.

Electricity and magnetism can be said to be integrated on both sides. Fluctuation of electricity generates magnetism, and fluctuation of magnetism generates electricity. Electromagnetic changes are just like breeze lightly bubbling water to produce water waves, so they are called electromagnetic waves, and the number of changes per second is frequency. When the frequency of an electromagnetic wave is low, it can be transmitted mainly through a tangible conductor; when the frequency is gradually increased, the electromagnetic wave will spill out of the conductor, and it can transmit energy without a medium, which is a kind of radiation. For example, the distance between the sun and the earth is very far away, but when outdoors, we can still feel the light and heat of the harmonious sunlight, which is like the principle of "electromagnetic radiation transfers energy through radiation phenomena".

Electromagnetic field classification

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

Free radiation

Non-ionizing radiation with thermal effects

Non-Ionizing Radiation without Thermal Effects

Base station electromagnetic waves are by no means free radiation waves

Just as people have been living in the air and their eyes cannot see the air, so people cannot see the ubiquitous electromagnetic waves. Electromagnetic waves are such a "friend" that humans have never met. Electromagnetic waves are a form of motion of electromagnetic fields.

In the case of high-frequency electromagnetic oscillations, the general name of the electric and magnetic waves formed by part of the energy being radiated to the surrounding space is called electromagnetic waves. In low-frequency electromagnetic oscillations, the mutual change between electricity and magnetism is relatively slow, and almost all of its energy is returned to the original circuit without being radiated out. The speed of electromagnetic waves is equal to the speed of light (3 × 10 ^ 10 cm per second).

The wavelength is between 10 and 3000 meters, and it can be divided into long wave, medium wave, medium and short wave, and short wave. The fax (television) uses a wavelength of 3 to 6 meters; the radar uses a shorter wavelength, 3 meters to a few centimeters. The electromagnetic waves include infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. All kinds of light and rays are also electromagnetic waves with different wavelengths. Among them, the wavelength of radio is the longest, and the wavelength of cosmic rays is the shortest.

Radio waves are 3,000 meters to 0.3 millimeters.

Infrared 0.3 mm to 0.75 microns.

Visible light 0.7 micrometers to 0.4 micrometers.

UV 0.4 micron to 10 nm

X-ray 10 nm to 0.1 nm

ray 0.1nm 0.001nm

Cosmic rays less than 0.001 nm

Electromagnetic field electromagnetic radiation

Broadly defined electromagnetic radiation usually refers to the electromagnetic wave spectrum. Narrow electromagnetic radiation refers to the radiation waves generated by electrical equipment, usually the part below the infrared.

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Electromagnetic field radio wave

Radio wave propagation methods: ground wave-propagation along the ground; long wave, short wave, medium and short wave can be transmitted by ground wave (radio broadcast), the propagation distance is within a few hundred kilometers; sky wave: radio waves that rely on the reflection of the ionosphere, Shortwave is applicable. Can travel thousands of kilometers away; straight-line propagation: suitable for microwave --- ultra-short waves (also known as space waves or apparent waves), with a general propagation distance of tens of kilometers ^[2] .

Long wave: The wavelength is 30,000m ~ 3000m, and the frequency is 10kH _Z ~ 100kH _Z. It propagates through ground waves and is used for ultra-long-range radio communication and navigation.

Medium wave: Wavelength 3000m ~ 200m, frequency 100kH _Z ~ 1500kH _Z , transmitted through ground wave and sky wave, used for amplitude modulation (AM) radio broadcasting, telegraph, communication.

Medium and short wave: Wavelength 200m ~ 50m, frequency 1500kH _Z ~ 6000kH _Z , transmitted through ground wave and sky wave, used for amplitude modulation (AM) radio broadcasting, telegraph, communication.

Short wave: Wavelength 50m ~ 10m, frequency 6MHZ ~ 30MHZ, propagated through skywave, used for amplitude modulation (AM) radio broadcasting, telegraph, communication ^[2] .

microwave:

Meter wave VHF: Wavelength 10m ~ 1m, frequency 30MHZ ~ 300MHZ, propagates through approximately straight line, used for FM radio broadcasting, television, navigation.

Decimeter wave UHF: Wavelength 1m ~ 0.1m, frequency 300MHZ ~ 3000MHZ, propagated through ground waves, used for television, radar, navigation.

Centimeter wave: Wavelength 10cm ~ 1cm, frequency 3000MH _Z ~ 30000MH _Z , propagated through ground waves, used for television, radar, navigation.

Millimeter wave: Wavelength 10mm ~ 1mm, frequency 30000MH _Z ~ 300,000MH _Z , propagated through ground waves, used for television, radar, navigation ^[2] .