Current situation of electromagnetism (2)

5. Is electricity related to magnetism?

For a long time, people have always thought that electricity and magnetism are two unrelated things; But there are some similarities between them. Charge and magnetic pole repel each other and opposites attract. The direction of the force is on the connection line of charges or magnetic poles, and the magnitude of the force is inversely proportional to the square of the distance between them. At the end of 18, it was found that charge could flow, which is current. But the connection between electricity and magnetism has never been discovered.

During the period of 1777, Cullen did a torsion balance experiment in the process of studying the method of improving the magnetic needle in the navigation compass, which can measure electrostatic force or magnetic force. This seems to imply that there is a close difference and connection between electricity and magnetism. On the basis of a large number of experiments, in 1789, Coulomb summed up the law of interaction between two magnetic poles similar to the interaction between two point charges. Coulomb enriches the measurement methods in the study of electricity and magnetism, and extends Newton's mechanical principles to electricity and magnetism. It paves the way for the development of electromagnetism and the establishment of electromagnetic field theory. But Coulomb suggested that electricity and magnetism are essentially different.

Before 1820, Cullen, Ampere, Thomas Young and Biot all thought that electricity and magnetism were two concepts at first, and there was no connection between them. However, Oster has always believed that electricity, magnetism, light and heat are interrelated in essence. Franklin, in particular, once found that Leiden bottle discharge can magnetize steel needles, which strengthened his view. 1820, Danish Oster first discovered the magnetic effect of current. This experiment initiated electromagnetism, which linked electricity and magnetism, and caused a great shock in the scientific community.

Two weeks later, Frenchman Ampere put forward the relationship between the direction of magnetic needle rotation and the direction of current-the famous "right hand rule". 1820, Biot and Savart jointly established Biot-Savart law, which is a basic law of magnetostatics and accurately describes the magnetic field generated by the current of current-carrying wires. The two men turned to believe that there is a close relationship between electromagnetism and supported Oster's point of view.

Inspired by a series of experiments such as Oster's current magnetic effect experiment, Ampere 182 1 1 put forward the "molecular current hypothesis", which holds that the magnetic field is generated by moving current, and points out that the essence of magnetic phenomenon is current, thus solving the mystery for thousands of years. Ampere attributed all kinds of interactions involving current and magnet to the interaction between currents, and put forward the basic problem of finding the interaction law of current elements. 1822, Ampere revolutionized the formula "Ampere's Law" of the acting force of magnetic field on moving charge, and summed up the motion law of current element in electromagnetic field in current-carrying circuit by using superb mathematical skills. Ampere's law is an electromagnetic law and a very important law in physics. The concept of "current" was also created by Ampere. To put it bluntly, ampere has played a great role in electromagnetism, and "Newton in electricity" deserves its name.

183 1 Faraday discovered the phenomenon of electromagnetic induction-when a magnet passes through a closed circuit, there will be current in the circuit, from which Faraday's law of electromagnetic induction is obtained; Then the method of generating alternating current is obtained. His discovery laid the foundation of electromagnetism and was the pioneer of Maxwell. Joseph henry, former president of the American Academy of Sciences, discovered Faraday's law of electromagnetic induction in the independent research of 1830, which was earlier than Faraday's, but he did not disclose this discovery. 1875- 1876, Roland, former president of the American Academy of Sciences, made an experiment on the magnetic effect of a charged rotating disk, which revealed for the first time that the moving charge could produce a magnetic field.

So far, after years of debate and hard experiments, the conclusion that electromagnetism is necessarily related has been confirmed, and the two can be transformed into each other. Oster and Ampere proved that electricity produces magnetism; Faraday, Henry and Roland confirmed that magnetic energy can generate electricity.

Sixth, the electromagnetic theory is put forward.

After the connection between electricity and magnetism was discovered, people realized that the essence of electromagnetic force was similar to gravity in some aspects, but different in others. For this reason, Faraday introduced the concept of electric field lines, thinking that current produces magnetic field lines around wires, and charge produces electric field lines in all directions, and on this basis, the concept of electromagnetic field was produced. 183 1 year, Faraday experimented with iron powder, which vividly proved the existence of magnetic field lines. He pointed out that this line of force is not geometric, but exists objectively and has physical properties.

Weber made many contributions to the establishment of absolute measurement of electrical units. 1849, he proposed the absolute unit of current intensity and electromagnetic force, and Gauss proposed the absolute unit of magnetic quantity with the help of Weber. From 1846 to 1878, Weber's research on electrodynamics (electromagnetics) measurement methods has important basic significance. He invented many electromagnetic instruments to quantitatively measure current intensity, magnetic field intensity and electric power.

1855- 1856, Maxwell introduced the concepts of "electric field" and "magnetic field" into Faraday's magnetic field lines. Maxwell summed up the law of macroscopic electromagnetic phenomena and introduced the concept of displacement current. The core idea of this concept is: changing electric field can produce magnetic field; Changing the magnetic field can also produce an electric field. He predicted the existence of electromagnetic waves in 1865. 1873, Maxwell completed the unified electromagnetic theory in his monograph on electricity and magnetism.

In Germany at that time, people still clung to Newton's traditional concept of physics. Faraday's and Maxwell's theories give a new description of the material world, but they violate the tradition, so they have no place in the heart of Europe such as Germany, and even are regarded as whimsy. This situation continued until Hertz discovered the suspected and long-awaited electromagnetic wave.

From 1885 to 1889, Hertz first comprehensively verified the correctness of Maxwell's theory through experiments. Radio waves are generated in the laboratory, which proves that radio radiation has the characteristics of waves, and the existence of electromagnetic waves is confirmed for the first time, and the wavelength and speed are measured. Hertz also confirmed through experiments that electromagnetic waves are shear waves and have similar characteristics to light; He pointed out that the vibration of radio waves and their characteristics of reflection and refraction are the same as those of light waves and heat waves. As a result, he undoubtedly affirmed that both light and heat are electromagnetic radiation.

Because electromagnetic field can act on charged particles with a force, a moving charged particle is acted by both electric field and magnetic field. Lorentz attributed the force of electromagnetic field on moving charge 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.

Now people realize that electromagnetic field is a special form of matter. The electric charge generates an electric field around it, which exerts a force on other charges. Magnets and current generate a magnetic field around them, which acts on other magnets and objects with current inside. Electromagnetic field also has energy and momentum, and it is the medium to transmit electromagnetic force, which permeates the whole space. People realize that Maxwell's electromagnetic theory correctly reflects the macro-electromagnetic phenomena's law and affirms that light is also an electromagnetic wave. Electricity, magnetism and light were unified, and the second big synthesis of physics was realized.

Seven, the use of electromagnetic waves

The electric field (or magnetic field) of electromagnetic wave changes with time and has periodicity. The distance traveled in an oscillation period is called wavelength. The reciprocal of the oscillation period, that is, the number of vibrations (changes) per second, is called frequency. The whole electromagnetic spectrum includes the collection of waves, light and rays from electric waves to cosmic rays. Paragraphs with different frequencies are named as radio waves (3KHz-3000GHz), infrared rays, visible rays, ultraviolet rays, X rays, gamma rays and cosmic rays, that is, the wavelength is getting shorter and shorter and the frequency is getting higher and higher.

Electromagnetic waves are shear waves, which can be used for detection, location and communication. The most commonly used is the lowest frequency radio wave. Infrared ray is used in remote control, thermal imager, infrared guidance, fire warming (heat radiation), and phenomena related to thermal effect are all. Visible light is the basis for most creatures to observe things; Ultraviolet rays are used for medical disinfection, banknote inspection, distance measurement, engineering flaw detection, etc. X-rays are used for medical perspective CT photography, engineering flaw detection and physical measurement of crystal structure; Gamma rays are used in medicine and make atoms jump to produce new rays.

Radio waves with frequencies between 3KHz and 3000GHz are mainly used in communication and other fields. Radio broadcasting (commonly used radios) and television are conducted by electromagnetic waves. According to different broadcasting characteristics and different services, the whole radio spectrum is divided into 9 sections: very low frequency (VLF), low frequency (LF), intermediate frequency (MF), high frequency (HF), very high frequency (VHF), ultra-high frequency (UHF), ultra-high frequency (SHF), extremely high frequency (EHF) and to.

Radio spectrum band division table

Ultra-low frequency (SLF) wavelength is 10000km( 1000m) to 1000km, and the corresponding frequency range is 30Hz ~ 300Hz, which is widely used in many aspects of military and civilian. Civil use is mainly used for medical treatment, engineering exploration, geophysical exploration, earthquake research, etc. Militarily, it is mainly used for remote control and underwater communication of underwater weapons. Among many applications, submarine underwater communication is the most prominent one, which can solve the problem of long-distance and deep communication between shore command post and submarine at sea. The ultra-low frequency submarine communication system is huge, complex and high-tech, and only a few developed countries such as the United States and Russia have mastered the ultra-low frequency submarine communication technology.

Ultra-high frequency (SHF) has a wavelength range of 10cm ~ 1cm, and centimeter wave has a frequency range of 3GHZ~30GHZ, which is widely used in satellite communication and broadcasting, cellular telephone and paging dispatching systems and 3G-4G wireless range. The wavelength range of extremely high frequency (EHF) is 1mm ~ 10mm, and the frequency range of millimeter wave is 30 ~ 300 GHz. It is mainly used in weather radar, space communication, radio astronomy, waveguide communication and 5G mobile communication system. These two frequencies are the radio bands that are most closely related to us at present.

1753, 17 In February, The Scotsman published an article in which the author put forward a bold idea of using current to communicate, which was regarded as an enlightenment of electromagnetic communication. Later, an unknown Swede, Chape brothers in France, Russian diplomat Schilling, young British Cook, Weber and Gauss all made efforts in electromagnetic telegraph. 1793, the Chape brothers in France set up a 230-kilometer-long support line between Paris and Lille to relay information. 1833, Weber and Gauss set up two copper wires over the city of G? ttingen and built the first electromagnetic telegraph, which realized the telegraph communication between the Institute of Physics of the University of G? ttingen and the Observatory with a distance of about 1.5 km.

In fact, in 1820, Ampere first proposed to use electromagnetic phenomena to transmit telegraph signals. At the end of 19, Italian Guillermo Marconi and Russian popov conducted a radio communication experiment at 1895. In India, Jagdish Chandra Bos rang the bell with radio waves, causing an explosion. In 190 1 year, nikola tesla, an American electrical pioneer of Serbian origin, said that he invented the wireless telegraph in 1893. So many people will be unconvinced by Marconi's father of radio. No wonder the information was underdeveloped at that time, and it was normal to engage in similar experiments in different places; It's just that Marconi's awareness of intellectual property rights is stronger (he was the first to get the patent right) and his publicity is more in place. 1913 April 14, when the Titanic hit an iceberg, Marconi Company with convenient communication rescued and received 700 survivors. Nowadays, mobile phones, broadcasting, weather forecasting, aerospace and so on are all inseparable from radio communication.

Eight, after the order

Like all cognitive processes, human's understanding of electromagnetic motion forms is gradually deepening from special to general and from phenomenon to essence. People's understanding of electromagnetic phenomena is a process from special aspects such as static electricity, magnetostatics and quasi-steady current to general motion.

In the early stage of the development of electromagnetism, people realized that there was a force between charged bodies and between magnetic poles, but the concept of "field" introduced as a means to describe this force was not generally accepted as an objective existence. Now people have clearly realized that electromagnetic field is a form of matter, which can interact with all charged substances to produce various electromagnetic phenomena.

The motion of the electromagnetic field itself obeys the law of fluctuation, which is called electromagnetic wave. In the information age, the application of electromagnetic fields and electromagnetic waves is everywhere. Electromagnetic field theory describes the objective physical laws with exquisite mathematical language, and reveals the objective existence of fields and waves through the solution of mathematical equations.

We should thank all the scientists who have contributed to electromagnetic waves and related theories-Gilbert, Franklin and Aepinus (1724- 1802, German physicist, for the first time trying to systematically apply mathematics to electromagnetic theory. His experiments led to the design of parallel plate capacitors and the discovery of electrical characteristics of mineral tourmaline. And explored its pyroelectricity), Coulomb, Cavendish, Ampere, Gauss, Volt, Ohm, Biot, Savart, Oster, Faraday, Henry, Joule, Roland, Kirchhoff, Maxwell, Havisha, Lorenz, Hertz, Kelvin, Marconi, Tesla, popov. Their fruitful work has made great contributions to the progress of electromagnetic theory and related technologies, and constantly promoted the progress and development of human society.