Magnetism: the property that a substance can attract metals such as iron, cobalt and nickel.
Magnet: a magnetic object.
In the pre-Qin period, our ancestors had accumulated a lot of knowledge in this field. When exploring iron ore, they often encountered magnetite, that is, magnetite (mainly composed of ferroferric oxide). These findings have long been recorded. These discoveries were first recorded in several articles in Guanzi: "There are magnets on the mountain and gold and copper under it." There are similar records in other ancient books, such as Shan Hai Jing. The iron-absorbing characteristics of magnets have long been discovered. The main chapter of the nine volumes of Lu's Spring and Autumn Annals is: "Kindly absorb iron, or absorb it." At that time, people called "magnetism" "kindness". They think that magnets attract iron is the attraction of loving mothers to their children. And think: "Stone is the mother of iron, but there are two kinds of stones: kind and unkind. Kindness can attract his children, but not kindness. " Before the Han Dynasty, people wrote magnets as "Guanyin Stone", which means love stone.
Since magnets can attract iron, can they attract other metals? Our ancestors made many attempts and found that magnets can not only attract metals such as gold, silver and copper, but also attract bricks and tiles. During the Western Han Dynasty, people realized that magnets could only attract iron, but not other objects.
When two magnets are put together and close to each other, sometimes they attract each other and sometimes they repel each other. Now people all know that magnets have two poles, one is called N pole and the other is called S pole. Like-sex repulsion, opposites attract. People didn't know this truth at that time, but they could still perceive this phenomenon.
In the Western Han Dynasty, there was an alchemist named Luan Da. He used this characteristic of magnet to make something similar to two chess pieces. By adjusting the mutual position of the polarities of two chess pieces, sometimes the two chess pieces attract each other and sometimes repel each other. Luan Da called it "playing chess". He dedicated the novel to Emperor Wu of the Han Dynasty and gave a live demonstration. Emperor Wu of the Han Dynasty was pleasantly surprised. He was named "General Five Blessingg". Luan made great use of the properties of magnets to make novel things to deceive Emperor Wu of Han Dynasty.
The earth is also a big magnet, and its poles are close to the geographical south pole and the geographical north pole respectively. Therefore, when the magnets on the earth's surface can rotate freely, they will represent north and south because of the nature that magnets repel each other with the same polarity and attract each other with the opposite polarity. The ancients did not understand this truth, but they knew this phenomenon very well.
Shen Kuo was the first to describe the magnetic declination. He described his exploration of magnetism and magnetic declination in Meng Qian Bi Tan. China's ancient ancestors made compasses and compasses with magnetism. A typical example of compass used in navigation is Zheng He's voyage to the West. After the compass was introduced to Europe through Arabs, it promoted the development of European navigation technology and provided favorable help for the opening of new routes.
Static charge will produce electrostatic field; A static magnetic dipole will produce a static magnetic field. The moving charge is called current, which will produce electric and magnetic fields.
The so-called electromagnetic field is a general term for electric field and magnetic field. An electric field will be built around the fixed (electrostatic) charge and the polarized substance. When your body is close to the TV or computer screen, you will feel your hair stand upright. Because of the existence of (electrostatic) electric field; The magnetic field comes from the movement of electric charge. The greater the current, the stronger the magnetic field. The unit of magnetic field strength is a/m, and the magnetic field we generally say actually refers to the magnetic field density, and the unit is t/ Tesla or g/ gauss.
The so-called "field" generally refers to an area in space, and all objects that enter will feel the force. For example, we live in the gravity field of the earth, and we also live in the geomagnetic field. When lightning strikes, we are shrouded in a powerful electric field.
◎ Electric field: We often find the existence of electric field in our life, such as the crackle of taking off our sweater in winter and the electric shock of touching the door handle. These are all static phenomena caused by friction. In the use of electricity, as long as there is voltage, an electric field will be generated around wires or electrical equipment. The electric field is usually in kilovolts per meter (KV/M). For example, when lightning strikes, the surface DC electric field intensity is about 30 kV/m ~ 150 kV/m, and the 60 Hz electric field intensity under the transmission line is below 3 kV/m ~ 5 kV/m.
◎ Magnetic field: Put the magnet under the cardboard, sprinkle iron powder on the cardboard, and you will find several circles of stripes connected between the North Pole and the South Pole. This is the magnetic field. In power use, as long as there is current passing through, a magnetic field will be generated around the conductor, and the unit of the magnetic field is expressed by Tesla (T) or Gauss (G) or Milligauss (mG).
earth's magnetic field
The magnetism of the earth is one of the physical properties inside the earth. The earth is a big magnet, and a magnetic field is formed around it, that is, the space to show the magnetic force, which is the geomagnetic field. It is very similar to the magnetic field of a magnetic dipole placed in the center of the earth, which is the most basic feature of the geomagnetic field. The magnetic poles of the earth's magnetic field are geographically opposite to the north-south direction and do not coincide with the north and south poles of the earth. The angle between them is about 1 1 degree, which is called magnetic declination. In addition, the magnetic pole position of the earth's magnetic field is not fixed, but it changes periodically ... The intensity of the earth's magnetic field is very weak, which is another characteristic of the earth's magnetic field. At the strongest poles, the intensity is less than 10-4(T), and the average intensity is about 0.6x 10-4(T), which changes less with the place or time, so it is commonly used (γ
History of electromagnetic field theory
People have been exposed to the phenomena of electricity and magnetism for a long time, and they know that magnetic rods have North Pole and South Pole. /kloc-in the 0/8th century, it was found that there are two kinds of charges: positive charge and negative charge. 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. Similar to gravity in these two points. 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.
In the early19th century, Oster discovered that current could deflect a small magnetic needle. Then Ampere found that the direction of force and the direction of current, as well as the vertical direction from the magnetic needle to the wire passing through the current, were perpendicular to each other. Soon after, Faraday found that when a magnetic rod is inserted into a coil, a current will be generated in the coil. These experiments show that there is a close relationship between electricity and magnetism. 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.
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.
/kloc-In the second half of the 9th century, Maxwell summarized 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. On this basis, he put forward a set of partial differential equations to express the basic laws of electromagnetic phenomena. This set of equations, called Maxwell equations, is the basic equation of classical electromagnetism. Maxwell's electromagnetic theory predicted the existence of electromagnetic waves, and its propagation speed was equal to the speed of light, which was later confirmed by Hertz's experiments. So people realized that Maxwell's electromagnetic theory correctly reflected the law of macroscopic electromagnetic phenomena, and affirmed that light is also an electromagnetic wave. 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.
Comparison between electromagnetic field and ordinary radiation
Radiation is a way of energy transfer. According to the strength of energy, radiation can be divided into three types:
Free radiation: the strongest energy can destroy biological cell molecules, such as α, β and γ rays.
Non-ionizing radiation (with thermal effect): the energy is weak, and it will not destroy biological cell molecules, but it will produce temperature, such as microwave and light.
Non-ionizing radiation (no thermal effect): the energy is the weakest, it will not destroy biological cells and molecules, and it will not produce temperature, such as radio waves and electromagnetic fields.
Will the electromagnetic field be attenuated? Can we try to separate it?
The intensity of electromagnetic field will decrease rapidly with the distance from the source. The electric field is easily isolated by the metal shell and the reinforced concrete building. Power equipment, such as transformers, has almost no electric field outside because it is a metal shell. It is difficult to isolate magnetic fields, but the magnetic fields generated by currents in opposite directions and the same size can cancel each other out. Therefore, the magnetic field generated by three-phase power lines is much smaller than that generated by single-phase power lines. The transmission lines of our company are all three-phase lines, and the magnetic field generated after mutual cancellation is very low.
The size of the electromagnetic field generated by household appliances.
Household appliances use low voltage, that is, 1 10 volts or 220 volts, so the electric field intensity is very small. As for the size of the magnetic field, it is very different from power consumption, brand and distance.
The following table shows the magnetic field data published by NRPB National Radiation Protection Bureau:
distance
wiring
3 cm
1 m
TV
25~500 (milligauss)
0. 1~ 1.5 (milligauss)
microwave oven
750~2000 (milligauss)
2.5~6 milligauss
hair dryer
60~2000 (milligauss)
0. 1~3 (milligauss)
refrigerator
5~ 17 (milligauss)
& lt0. 1 (milligauss)
Electric beard knife
150~ 15000 (milligauss)
0. 1~3 (milligauss)
washing machine
8~500 (milligauss)
0. 1~ 1.5 (milligauss)
vacuum cleaner
2000~8000 (milligauss)
1.3~20 (milligauss)
desk lamp
400~4000 (milligauss)
0.2~2.5 (milligauss)
The magnitude of electromagnetic field under overhead transmission line
The magnitude of electric field is related to voltage and distance, and the magnitude of magnetic field is related to current and distance. Because the current varies with the load, it is not a constant value, so the value of the magnetic field also varies within a range.
At present, the magnetic field limitation standards of foreign advanced countries
The following table lists the limit standards of power frequency magnetic field widely used in various countries, among which IRPA standard is the most stringent.
Recommended value of 50/60 Hz magnetic field limit in advanced countries
country
Limit value (milligauss)
professional
ordinary people
International Radiation Protection Association
(IRPA)
the whole day
5,000
1,000
hour
50,000
10,000
Riben
Continuous exposure
50,000
2,000
Short time exposure
100,000
10,000
Su Lian
8 hours
18,000
-
1 hour
75,000
-
British National Radiation Protection Agency
(NRPB)
20,000
20,000
American Federation of Government Workers and Health Scholars
(ACGIH)
10,000
-
Germany
50,000
50,000
Aozhou
Like IRPA
Like IRPA
Is electromagnetic field harmful to human body?
In recent years, scientists generally believe that extremely low frequency electromagnetic fields (that is, electromagnetic fields generated by general power lines and power equipment) will neither break molecular bonds or chemical bonds, nor adversely affect human health due to a small amount of heat.
Although some epidemiological studies suspect that there is a "statistical correlation" between a few cancers and electromagnetic fields (note), some careful and responsible scientists point out that there are still many problems in the design and interpretation of these studies.
Since 1979 began its research in this field, more than 1000 papers and reports have been published abroad. Due to the low percentage of cases and various carcinogenic factors, it is difficult to exclude other factors. Some results obtained show a little correlation, while others deny it.
1989 10 The U.S. Department of Labor (DOL) requested the Radiation Research and Policy Coordination Committee (CIRRPC) to assist in evaluating past reports on the effects of exposure to electromagnetic fields. CIRRPC entrusted Oak Ridge Affiliated University (ORAU) to set up a team of 1 1 person, including outstanding American scientists. From September of 199 1 year to May of 1992, this paper analyzes and sorts out about 1000 papers in recent years. The conclusion of the assessment is that there is no convincing evidence to support that exposure to extremely low frequency magnetic fields of household appliances, power lines and display screens will cause health hazards.
How do authorities evaluate the relationship between electromagnetic fields and health?
☆ International Non-ionizing Radiation Protection Association (IRPA): Although there are epidemiological studies that exposure to 50/60 Hz electromagnetic field is related to cancer, it cannot be confirmed, and some people think that there is no correlation.
World Health Organization (WHO):
Exposure to extremely low frequency electromagnetic fields will not produce physiological effects.
Congressional Office of Technology Assessment:
All the research is still controversial. Many experiments have found that there is no difference between contact with electromagnetic field and non-contact, so we can't confirm the existence of danger.
Southern California Power Company (SCE):
From 1960 to 1988, the Southern California Electric Power Company conducted an occupational epidemiological survey on 3,6221employees who had served in the company for more than one year, and announced the survey results in March 1993. The conclusion is that employees are more likely to be exposed to magnetic fields than the general public on average, but there is no possibility of leukemia or brain tumor.
☆ Swedish National Electricity Security Bureau (NESB):
1994 published a pamphlet on electromagnetic field information, explaining that whether the magnetic field has an effect on human body has not been confirmed, so there are not enough reference standards to set the limit value, so the limit value of magnetic field intensity will not be set in the short term.
Contrary to the statement that electromagnetic field may have an impact on human body, some 60 Hz magnetic field therapeutic devices with the intensity as high as 550,000 milligauss are popular in the market at present, which are used to treat various chronic diseases and claim to have been proved by medical clinic. As for its long-term effects, no one has done this research.