Under certain conditions, alternating electric and magnetic fields can generate each other. They form an electromagnetic field, which is not their totality at all. This is a single whole in which these two fields cannot exist without each other.
From the history
The experiment of the Danish scientist Hans Christian Oersted, carried out in 1821, showed that electricity generates a magnetic field. In turn, a changing magnetic field can generate electric current. This has been proven English physicist Michael Faraday, who discovered the phenomenon in 1831 electromagnetic induction. He is also the author of the term “electromagnetic field”.
At that time, Newton's concept of long-range action was accepted in physics. It was believed that all bodies act on each other through the void at an infinitely high speed (almost instantly) and at any distance. It was assumed that electric charges interact In a similar way. Faraday believed that emptiness does not exist in nature, and interaction occurs with terminal speed through some material environment. This medium for electric charges is electromagnetic field. And it travels at a speed equal to the speed of light.
Maxwell's theory
Combining the results previous studies, English physicist James Clerk Maxwell created in 1864 theory electromagnetic field . According to it, a changing magnetic field generates a changing electric field, and an alternating electric field generates an alternating magnetic field. Of course, first one of the fields is created by a source of charges or currents. But in the future, these fields can already exist independently of such sources, causing each other to appear. That is, electric and magnetic fields are components of a single electromagnetic field. And every change in one of them causes the appearance of another. This hypothesis forms the basis of Maxwell's theory. The electric field generated by the magnetic field is a vortex. His power lines closed.
This theory is phenomenological. This means that it is created based on assumptions and observations, and does not consider the cause of electric and magnetic fields.
Properties of the electromagnetic field
An electromagnetic field is a combination of electric and magnetic fields, therefore at each point in its space it is described by two basic quantities: tension electric field E and magnetic field induction IN .
Since the electromagnetic field is the process of converting an electric field into a magnetic field, and then magnetic into electric, its state is constantly changing. Propagating in space and time, it forms electromagnetic waves. Depending on the frequency and length, these waves are divided into radio waves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma radiation.
The vectors of intensity and induction of the electromagnetic field are mutually perpendicular, and the plane in which they lie is perpendicular to the direction of propagation of the wave.
In the theory of long-range action, the speed of propagation of electromagnetic waves was considered infinitely large. However, Maxwell proved that this was not the case. In a substance, electromagnetic waves propagate at a finite speed, which depends on the dielectric and magnetic permeability of the substance. Therefore, Maxwell's Theory is called the theory of short-range action.
Maxwell's theory was confirmed experimentally in 1888. German physicist Heinrich Rudolf Hertz. He proved that electromagnetic waves exist. Moreover, he measured the speed of propagation of electromagnetic waves in a vacuum, which turned out to be equal speed Sveta.
IN integral form this law looks like this:
Gauss's law for magnetic field
The flux of magnetic induction through a closed surface is zero.
The physical meaning of this law is that in nature there is no magnetic charges. The poles of a magnet cannot be separated. The magnetic field lines are closed.
Faraday's Law of Induction
A change in magnetic induction causes the appearance of a vortex electric field.
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Magnetic field circulation theorem
This theorem describes the sources of the magnetic field, as well as the fields themselves created by them.
Electric current and changes in electrical induction generate a vortex magnetic field.
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E– electric field strength;
N– magnetic field strength;
IN- magnetic induction. This vector quantity, showing the force with which the magnetic field acts on a charge of magnitude q moving with speed v;
D – electrical induction, or electrical displacement. Represents a vector quantity, equal to the amount tension vector and polarization vector. Polarization is caused by the displacement of electric charges under the influence of an external electric field relative to their position when there is no such field.
Δ - operator Nabla. The action of this operator on specific field called the rotor of this field.
Δ x E = rot E
ρ - density of external electric charge;
j- current density - a value showing the strength of the current flowing through a unit area;
With– speed of light in vacuum.
The study of the electromagnetic field is a science called electrodynamics. She considers its interaction with bodies that have an electric charge. This interaction is called electromagnetic. Classical electrodynamics only describes continuous properties electromagnetic field using Maxwell's equations. Modern quantum electrodynamics believes that the electromagnetic field also has discrete (discontinuous) properties. And so electromagnetic interaction occurs with the help of indivisible particles-quanta that have no mass and charge. The electromagnetic field quantum is called photon .
Electromagnetic field around us
An electromagnetic field is formed around any conductor with alternating current. Sources of electromagnetic fields are power lines, electric motors, transformers, urban electric transport, railway transport, electrical and electronic household appliances - televisions, computers, refrigerators, irons, vacuum cleaners, radiotelephones, mobile phones, electric shavers - in a word, everything related to the consumption or transmission of electricity. Powerful sources electromagnetic fields - television transmitters, antennas of cellular telephone stations, radar stations, microwave ovens, etc. And since there are quite a lot of such devices around us, electromagnetic magnetic fields surround us everywhere. These fields affect environment and man. This is not to say that this influence is always negative. Electric and magnetic fields have existed around humans for a long time, but the power of their radiation a few decades ago was hundreds of times lower than today.
Before a certain level electromagnetic radiation can be safe for humans. Thus, in medicine, low-intensity electromagnetic radiation is used to heal tissues, eliminate inflammatory processes, and have an analgesic effect. UHF devices relieve spasms of the smooth muscles of the intestines and stomach, improve metabolic processes in the body's cells, reducing capillary tone, and lower blood pressure.
But strong electromagnetic fields cause disruptions in the functioning of the human cardiovascular, immune, endocrine and nervous systems, and can cause insomnia, headaches, and stress. The danger is that their impact is almost invisible to humans, and disturbances occur gradually.
How can we protect ourselves from the electromagnetic radiation surrounding us? It is impossible to do this completely, so you need to try to minimize its impact. First of all, you need to arrange household appliances in such a way that they are located away from the places where we are most often. For example, don't sit too close to the TV. After all, the further the distance from the source of the electromagnetic field, the weaker it becomes. Very often we leave the device plugged in. But the electromagnetic field disappears only when the device is disconnected from the electrical network.
Natural electromagnetic fields also affect human health – cosmic radiation, Earth's magnetic field.
An electromagnetic field is a type of matter that arises around moving charges. For example, around a conductor carrying current. The electromagnetic field consists of two components: electric and magnetic field. They cannot exist independently of each other. One thing begets another. When the electric field changes, a magnetic field immediately appears.
Electromagnetic wave propagation speed V=C/EM
Where e And m respectively magnetic and the dielectric constant environment in which the wave propagates.
An electromagnetic wave in a vacuum travels at the speed of light, that is, 300,000 km/s. Since the dielectric and magnetic permeability of a vacuum are considered equal to 1.
When the electric field changes, a magnetic field appears. Since the electric field that caused it is not constant (that is, it changes over time), the magnetic field will also be variable.
A changing magnetic field in turn generates an electric field, and so on. Thus, for the subsequent field (it does not matter whether it is electric or magnetic), the source will be the previous field, and not the original source, that is, a conductor with current.
Thus, even after turning off the current in the conductor, the electromagnetic field will continue to exist and spread in space.
An electromagnetic wave propagates in space in all directions from its source. You can imagine turning on a light bulb, the rays of light from it spread in all directions.
An electromagnetic wave, when propagating, transfers energy in space. The stronger the current in the conductor that causes the field, the greater the energy transferred by the wave. Also, the energy depends on the frequency of the emitted waves; if it increases by 2,3,4 times, the wave energy will increase by 4,9,16 times, respectively. That is, the energy of wave propagation is proportional to the square of the frequency.
The best conditions for wave propagation are created when the length of the conductor is equal to the wavelength.
The magnetic and electric lines of force will fly mutually perpendicular. Magnetic lines of force surround a current-carrying conductor and are always closed.
Electrical lines of force go from one charge to another.
An electromagnetic wave is always transverse wave. That is, the lines of force, both magnetic and electric, lie in a plane perpendicular to the direction of propagation.
Electromagnetic field strength is a strength characteristic of the field. Also, tension is a vector quantity, that is, it has a beginning and a direction.
The field strength is directed tangentially to the lines of force.
Since the electric and magnetic field strengths are perpendicular to each other, there is a rule by which the direction of wave propagation can be determined. When the screw rotates along the shortest path from the electric field strength vector to the magnetic field strength vector forward movement The screw will indicate the direction of wave propagation.
Electromagnetic fields and radiation surround us everywhere. It is enough to flip a switch and the light comes on, turn on the computer and you are on the Internet, dial a number on your mobile phone and you can communicate with distant continents. In fact it is electrical devices created modern world as we know it. However, in Lately The question is increasingly being raised that electromagnetic fields (EMFs) generated by electrical equipment are harmful. Is it so? Let's try to figure it out.
Let's start with a definition. Electromagnetic fields, as is known from school course physicists represent a special Key Feature Such fields are the ability to interact in a certain way with bodies and particles that have an electrical charge. As the name suggests, electromagnetic fields are a combination of magnetic and electric fields, and in in this case they are so closely interconnected that they are considered a single whole. Features of interaction with charged objects are explained using
Electromagnetic fields were first expressed mathematically in theory by Maxwell in 1864. Actually, it was he who revealed the indivisibility of magnetic and electric fields. One of the consequences of the theory was the fact that any disturbance (change) of the electromagnetic field causes the appearance of electromagnetic waves propagating in a vacuum. Calculations have shown that light (all parts of the spectrum: infrared, visible, ultraviolet) is precisely an electromagnetic wave. In general, when classifying radiation by wavelength, they distinguish between X-rays, radio, etc.
The appearance of Maxwell's theory was preceded by Faraday's work (in 1831) on research in a conductor moving or located in a periodically changing magnetic field. Even earlier, in 1819, H. Oersted noticed that if a compass is placed next to a current-carrying conductor, its needle deviates from the natural one, which suggested a direct connection between magnetic and electric fields.
All this indicates that any electrical device is a generator of electromagnetic waves. This property especially pronounced for some specific devices and high-current circuits. Both the first and the second are now present in almost every home. Since EMF propagates not only in conductive materials, but also in dielectrics (for example, vacuum), a person is constantly in the zone of their action.
If earlier, when there was only “Ilyich’s lamp” in the room, the question did not bother anyone. Now everything is different: the electromagnetic field is measured using special devices to measure field strength. Both components of the EMF are recorded in a certain frequency range (depending on the sensitivity of the device). The SanPiN document indicates the PDN (permissible norm). At enterprises and in large companies EMF PDN checks are carried out periodically. It is worth noting that there are still no final results of studies on the effects of EMF on living organisms. Therefore, for example, when working with computer technology It is recommended to organize 15-minute breaks after every hour - just in case... Everything is explained quite simply: there is an EMF around the conductor, which means there is also an EMF. The equipment is completely safe when the power cord is unplugged from the outlet.
Obviously, few people will decide to completely abandon the use of electrical equipment. However, you can further protect yourself by connecting home appliances to a grounded network, which allows the potential not to accumulate on the housing, but to “drain” into the ground loop. Various extension cords, especially those wound in rings, enhance EMF due to mutual induction. And, of course, you should avoid placing several switched on devices close together.
What is an electromagnetic field, how it affects human health and why it should be measured - you will learn from this article. Continuing to introduce you to the assortment of our store, we will tell you about useful devices - indicators of electromagnetic field strength (EMF). They can be used both in enterprises and at home.
What is an electromagnetic field?
The modern world is unthinkable without household appliances, mobile phones, electricity, trams and trolleybuses, televisions and computers. We are accustomed to them and do not think at all about the fact that any electrical device creates an electromagnetic field around itself. It is invisible, but affects any living organisms, including humans.
Electromagnetic field - special shape matter arising from the interaction of moving particles with electric charges. The electric and magnetic fields are interrelated with each other and can generate one another - which is why, as a rule, they are spoken of together as one, electromagnetic field.
The main sources of electromagnetic fields include:
- power lines;
— transformer substations;
— electrical wiring, telecommunications, television and Internet cables;
— cell phone towers, radio and television towers, amplifiers, antennas for cell and satellite phones, Wi-Fi routers;
— computers, televisions, displays;
— household electrical appliances;
— induction and microwave ovens;
— electric transport;
— radars.
The influence of electromagnetic fields on human health
Electromagnetic fields affect any biological organisms- on plants, insects, animals, people. Scientists studying the effects of EMF on humans have concluded that prolonged and regular exposure to electromagnetic fields can lead to:
- increased fatigue, sleep disturbances, headaches, decreased blood pressure, decreased heart rate;
- disorders in the immune, nervous, endocrine, reproductive, hormonal, cardiovascular systems;
— development of oncological diseases;
- development of diseases of the central nervous system;
- allergic reactions.
EMF protection
There are sanitary standards that establish the maximum permissible levels electromagnetic field strength depending on the time spent in danger zone- for residential premises, workplaces, places near sources strong field. If it is not possible to reduce radiation structurally, for example, from a line electromagnetic transmissions(EMF) or cell tower, then service instructions, protective equipment for working personnel, and sanitary quarantine zones of limited access are developed.
Various instructions regulate the time a person stays in the danger zone. Shielding meshes, films, glazing, suits made of metallized fabric based on polymer fibers can reduce the intensity of electromagnetic radiation by thousands of times. At the request of GOST, EMF radiation zones are fenced off and provided with warning signs “Do not enter, dangerous!” and an electromagnetic field hazard sign.
Special services use instruments to constantly monitor the level of EMF intensity in workplaces and residential premises. You can take care of your health yourself by purchasing a portable device “Impulse” or a set “Impulse” + nitrate tester “SOEKS”.
Why do we need household electromagnetic field strength measuring devices?
The electromagnetic field negatively affects human health, so it is useful to know which places you visit (at home, in the office, in the garden, in the garage) may pose a danger. You must understand that increased electromagnetic background can be created not only by your electrical appliances, telephones, televisions and computers, but also by faulty wiring, electrical appliances of your neighbors, industrial facilities located nearby.
Experts have found that short-term exposure to EMF on a person is practically harmless, but long-term stay in an area with a high electromagnetic background is dangerous. These are the zones that can be detected using “Impulse” type devices. This way, you can check the places where you spend the most time; a nursery and your own bedroom; study. The device contains the values set regulatory documents, so you can immediately assess the degree of danger for you and your loved ones. It is possible that after the examination you will decide to move the computer away from the bed, get rid of the cell phone with an amplified antenna, replace the old microwave oven with a new one, replace the insulation of the refrigerator door with the No Frost mode.
In 1860-1865 one of greatest physicists 19th century James Clerk Maxwell created a theory electromagnetic field. According to Maxwell, the phenomenon of electromagnetic induction is explained by in the following way. If at a certain point in space the magnetic field changes in time, then an electric field is also formed there. If there is a closed conductor in the field, then the electric field causes in it induced current. From Maxwell's theory it follows that it is also possible reverse process. If in a certain region of space the electric field changes with time, then a magnetic field is also formed there.
Thus, any change in the magnetic field over time gives rise to a changing electric field, and any change in the electric field over time gives rise to a changing magnetic field. These alternating electric and magnetic fields generating each other form a single electromagnetic field.
Properties of electromagnetic waves
The most important result that follows from the theory of the electromagnetic field formulated by Maxwell was the prediction of the possibility of the existence of electromagnetic waves. Electromagnetic wave- propagation of electromagnetic fields in space and time.
Electromagnetic waves, unlike elastic (sound) waves, can propagate in a vacuum or any other substance.
Electromagnetic waves in a vacuum propagate at speed c=299 792 km/s, that is, at the speed of light.
In matter, the speed of an electromagnetic wave is less than in a vacuum. The relationship between wavelength, its speed, period and frequency of oscillations obtained for mechanical waves are also fulfilled for electromagnetic waves:
Voltage vector fluctuations E and magnetic induction vector B occur mutually perpendicular planes and perpendicular to the direction of wave propagation (velocity vector).
An electromagnetic wave transfers energy.
Electromagnetic wave range
Around us complex world electromagnetic waves of various frequencies: radiation from computer monitors, cell phones, microwave ovens, televisions, etc. Currently, all electromagnetic waves are divided by wavelength into six main ranges.
Radio waves- these are electromagnetic waves (with a wavelength from 10000 m to 0.005 m), used to transmit signals (information) over a distance without wires. In radio communications, radio waves are created by high-frequency currents flowing in an antenna.
Electromagnetic radiation with a wavelength from 0.005 m to 1 micron, i.e. lying between the radio wave range and the range visible light, are called infrared radiation. Infrared radiation is emitted by any heated body. The sources of infrared radiation are stoves, batteries, electric lamps incandescent Using special devices, infrared radiation can be converted into visible light and images of heated objects can be obtained in complete darkness.
TO visible light include radiation with a wavelength of approximately 770 nm to 380 nm, from red to purple. The significance of this part of the spectrum of electromagnetic radiation in human life is extremely great, since a person receives almost all information about the world around him through vision.
Invisible to the eye electromagnetic radiation with a wavelength shorter than violet is called ultraviolet radiation. It can kill pathogenic bacteria.
X-ray radiation invisible to the eye. It passes without significant absorption through significant layers of a substance that is opaque to visible light, which is used to diagnose diseases of internal organs.
Gamma radiation called electromagnetic radiation emitted by excited nuclei and arising from the interaction of elementary particles.
Principle of radio communication
An oscillatory circuit is used as a source of electromagnetic waves. For effective radiation, the circuit is “opened”, i.e. create conditions for the field to “go” into space. This device is called open oscillatory circuit - antenna.
Radio communication is the transmission of information using electromagnetic waves, the frequencies of which are in the range from to Hz.
Radar (radar)
A device that transmits ultrashort waves and immediately receives them. Radiation is carried out in short pulses. The pulses are reflected from objects, allowing, after receiving and processing the signal, to establish the distance to the object.
Speed radar works on a similar principle. Think about how radar detects the speed of a moving car.