How does an electromagnetic field arise? Electromagnetic field
An electromagnetic field is an alternating electrical and magnetic fields.
The electromagnetic field theory was created by James Maxwell in 1865.
He theoretically proved that:
any change in the magnetic field with time leads to the appearance of a changing electric field, and any change in the electric field with time generates a changing magnetic field.
If electric charges move with acceleration, then the electric field created by them periodically changes and itself creates an alternating magnetic field in space, etc.
The sources of the electromagnetic field can be:
- moving magnet;
- an electric charge moving with acceleration or oscillating (as opposed to a charge moving at a constant speed, for example, in the case direct current in the conductor, a constant magnetic field is created here).
Electric field always exists around an electric charge, in any frame of reference, magnetic - in the one relative to which electric charges move.
The electromagnetic field exists in the frame of reference, relative to which electric charges move with acceleration.
TRY SOLUTION
A piece of amber was rubbed against a cloth and charged with static electricity. What field can be found around immobile amber? Around moving?
A charged body is at rest relative to the earth's surface. The car moves uniformly and rectilinearly relative to the surface of the earth. Is it possible to detect a constant magnetic field in the reference frame associated with the car?
What field arises around an electron if it: is at rest; moving at a constant speed; moving with acceleration?
A kinescope creates a stream of uniformly moving electrons. Is it possible to detect a magnetic field in a frame of reference associated with one of the moving electrons?
ELECTROMAGNETIC WAVES
Electromagnetic waves are an electromagnetic field propagating in space at a finite speed, depending on the properties of the medium
Properties of electromagnetic waves:
- propagate not only in matter, but also in vacuum;
- propagate in vacuum at the speed of light (С = 300,000 km/s);
are transverse waves
- these are traveling waves (transfer energy).
The source of electromagnetic waves are rapidly moving electric charges.
Oscillations of electric charges are accompanied by electromagnetic radiation having a frequency equal to the frequency of charge oscillations.
SCALE OF ELECTROMAGNETIC WAVES
All the space around us is permeated with electromagnetic radiation. The sun, the bodies around us, transmitter antennas emit electromagnetic waves, which, depending on their frequency of oscillation, have different names.
Radio waves are electromagnetic waves (with a wavelength from more than 10,000m to 0.005m) that are 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.
Radio waves of different lengths propagate differently.
Electromagnetic radiation with a wavelength less than 0.005 m but greater than 770 nm, i.e., lying between the radio wave range and the visible light range, is called infrared radiation (IR).
Infrared radiation is emitted by any heated body. Sources of infrared radiation are stoves, water heaters, electric incandescent lamps. With the help of special devices, infrared radiation can be converted into visible light and images of heated objects can be obtained in complete darkness. Infrared radiation is used for drying painted products, building walls, wood.
Visible light includes radiation with a wavelength of approximately 770nm to 380nm, from red to violet light. The values of this section of the spectrum of electromagnetic radiation in human life are exceptionally large, since almost all information about the world around a person receives through vision. Light is a prerequisite for the development of green plants and therefore necessary condition for the existence of life on earth.
Invisible to the eye, electromagnetic radiation with a wavelength shorter than that of violet light is called ultraviolet radiation (UV). Ultraviolet radiation can kill pathogenic bacteria, so it is widely used in medicine. Ultraviolet radiation in the composition of sunlight causes biological processes that lead to darkening of human skin - sunburn. Discharge lamps are used as sources of ultraviolet radiation in medicine. The tubes of such lamps are made of quartz, which is transparent to ultraviolet rays; therefore these lamps are called quartz lamps.
X-rays (Ri) are invisible to the atom. They pass without significant absorption through significant layers of material that is opaque to visible light. X-rays are detected by their ability to cause a certain glow of certain crystals and act on photographic film. The ability of X-rays to penetrate through thick layers of substances is used to diagnose diseases of human internal organs.
Details Category: Electricity and magnetism Posted on 06/05/2015 20:46 Views: 11962Variable electric and magnetic fields under certain conditions can give rise to 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 is capable of generating an electric current. This was proved by the English physicist Michael Faraday, who discovered the phenomenon of electromagnetic induction in 1831. He is also the author of the term "electromagnetic field".
In those days, 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, on the other hand, believed that emptiness does not exist in nature, and the interaction occurs at a finite speed through a certain material medium. This medium for electric charges is electromagnetic field. And it propagates at a speed equal to the speed of light.
Maxwell's theory
Combining the results of previous studies, English physicist James Clerk Maxwell in 1864 created electromagnetic field theory. According to it, a changing magnetic field generates a changing electric field, and an alternating electric field generates an alternating magnetic field. Of course, at 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 the appearance of each other. 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 vortex. His lines of force are closed.
This theory is phenomenological. This means that it is based on assumptions and observations, and does not consider the cause that causes the occurrence of electric and magnetic fields.
Properties of the electromagnetic field
The electromagnetic field is a combination of electric and magnetic fields, therefore, at each point in its space, it is described by two main quantities: the strength of the electric field E and magnetic field induction AT .
Since the electromagnetic field is a process of transforming an electric field into a magnetic field, and then a magnetic field into an electric one, its state is constantly changing. Spreading 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 intensity and induction vectors of the electromagnetic field are mutually perpendicular, and the plane in which they lie is perpendicular to the direction of wave propagation.
In the theory of long-range action, the propagation velocity of electromagnetic waves was considered to be 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 short-range theory.
Maxwell's theory was experimentally confirmed in 1888 by the German physicist Heinrich Rudolf Hertz. He proved that electromagnetic waves exist. Moreover, he measured the speed of propagation of electromagnetic waves in vacuum, which turned out to be equal to the speed of light.
In integral form, this law looks like this:
Gauss' law for a magnetic field
The flux of magnetic induction through a closed surface is zero.
The physical meaning of this law is that there are no magnetic charges in nature. The poles of a magnet cannot be separated. The lines of force of the magnetic field are closed.
Faraday's law of induction
A change in magnetic induction causes the appearance of a vortex electric field.
,
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 change in electric induction generate a vortex magnetic field.
,
,
E is the electric field strength;
H is the magnetic field strength;
AT- magnetic induction. This is a vector quantity showing how strong the magnetic field acts on a charge of q moving at a speed v;
D- electrical induction, or electrical displacement. It is a vector quantity equal to the sum of the intensity vector and the polarization vector. Polarization is caused by the displacement of electric charges under the action of an external electric field relative to their position when such a field is absent.
Δ is the Nabla operator. The action of this operator on a specific field is 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 is the speed of light in vacuum.
The science that studies the electromagnetic field is called electrodynamics. She considers its interaction with bodies that have an electric charge. Such an interaction is called electromagnetic. Classical electrodynamics describes only the continuous properties of an electromagnetic field using Maxwell's equations. Modern quantum electrodynamics considers that the electromagnetic field also has discrete (discontinuous) properties. And such an electromagnetic interaction occurs with the help of indivisible particles-quanta that do not have mass and charge. The quantum of the electromagnetic field is called photon .
The electromagnetic field around us
An electromagnetic field is formed around any conductor with alternating current. The 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, cordless phones, mobile phones, electric shavers - in a word, everything that is associated with consumption or transmission of electricity. Powerful sources of electromagnetic fields are television transmitters, antennas of cellular telephone stations, radar stations, microwave ovens, etc. And since there are quite a lot of such devices around us, then electromagnetic fields surround us everywhere. These fields affect environment and a person. It cannot be said that this influence is always negative. Electric and magnetic fields have existed around a person for a long time, but the power of their radiation a few decades ago was hundreds of times lower than today.
To a certain level, electromagnetic radiation can be safe for humans. So, in medicine, with the help of low-intensity electromagnetic radiation, tissues heal, eliminate inflammatory processes have an analgesic effect. UHF devices relieve spasms of the smooth muscles of the intestines and stomach, improve metabolic processes in the cells of the body, reducing the tone of capillaries, and lower blood pressure.
But strong electromagnetic fields cause malfunctions in the work of the cardiovascular, immune, endocrine and nervous systems of a person, can cause insomnia, headaches, and stress. The danger is that their impact is almost imperceptible to humans, and violations occur gradually.
How can we protect ourselves from the electromagnetic radiation around us? It is impossible to do this completely, so you need to try to minimize its impact. First of all, you need to place Appliances so that they are away from the places where we are most often. For example, do not sit too close to the TV. After all, the farther 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 mains.
Human health is also affected by natural electromagnetic fields - cosmic radiation, the Earth's magnetic field.
What is an electromagnetic field, how it affects human health and why measure it - you will learn from this article. Continuing to acquaint you with the assortment of our store, we will tell you about useful devices - indicators of electromagnetic field strength (EMF). They can be used both in businesses and at home.
What is an electromagnetic field?
The modern world is inconceivable without household appliances, mobile phones, electricity, trams and trolleybuses, televisions and computers. We are used to them and do not think at all that any electrical device creates an electromagnetic field around itself. It is invisible, but affects any living organisms, including humans.
An electromagnetic field is a special form of matter that occurs when moving particles interact with electric charges. The electric and magnetic fields are interconnected with each other and can give rise to one another - which is why, as a rule, they are spoken of together as a single, electromagnetic field.
The main sources of electromagnetic fields include:
- power lines;
— transformer substations;
– electrical wiring, telecommunications, TV and Internet cables;
– cell towers, radio and TV towers, amplifiers, cell and satellite phone antennas, WiFi routers;
— computers, TVs, displays;
- household electrical appliances;
– induction and microwave (MW) ovens;
— electric transport;
- radars.
Effect of electromagnetic fields on human health
Electromagnetic fields affect any biological organisms - plants, insects, animals, people. Scientists studying the effect of electromagnetic fields on humans have come to the conclusion that prolonged and regular exposure to electromagnetic fields can lead to:
- increased fatigue, sleep disturbances, headaches, decreased pressure, decreased heart rate;
- disorders in the immune, nervous, endocrine, sexual, hormonal, cardiovascular systems;
- the development of oncological diseases;
- the development of diseases of the central nervous system;
- allergic reactions.
EMI protection
Exist sanitary norms, which establish the maximum allowable levels of electromagnetic field strength depending on the time spent in the hazardous area - for residential premises, workplaces, places near sources of a strong field. If it is not possible to reduce the radiation structurally, for example, from an electromagnetic transmission line (EMF) or a cell tower, then service instructions, protective equipment for working personnel, and sanitary-quarantine restricted access zones are developed.
Various instructions regulate the time a person stays in the danger zone. Shielding nets, 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 equipped with warning signs “Do not enter, it is dangerous!” and sign of danger electromagnetic field.
Special services with the help of devices constantly monitor the level of EMF intensity at workplaces and in residential premises. You can take care of your own health by buying portable device "Impulse" or kit "Impulse" + nitrate tester "SOEKS".
Why do we need household devices for measuring the strength of the electromagnetic field?
The electromagnetic field negatively affects human health, so it is useful to know which places you visit (at home, in the office, on personal plot, in a garage) can be dangerous. You must understand that an increased electromagnetic background can be created not only by your electrical devices, telephones, televisions and computers, but also faulty wiring, electrical appliances of neighbors, industrial facilities located nearby.
Experts have found that a short-term exposure to EMF on a person is practically harmless, but a long stay in an area with an increased electromagnetic background is dangerous. These are the zones that can be detected using devices of the “Impulse” type. So, you can check the places where you spend the most time; nursery and your bedroom; study. The instrument contains the values set normative documents so that you can immediately assess the degree of danger to you and your loved ones. It is possible that after the examination you decide to move the computer away from the bed, get rid of cell phone with a reinforced antenna, change the old microwave oven to a new one, replace the refrigerator door insulation with the No Frost mode.
electricity around us
Electromagnetic field (definition from TSB)- this is a special form of matter, through which the interaction between electrically charged particles is carried out. Based on this definition, it is not clear what is primary - the existence of charged particles or the presence of a field. Perhaps only due to the presence of an electromagnetic field, particles can receive a charge. Just like the chicken and egg story. The bottom line is that charged particles and the electromagnetic field are inseparable from each other and cannot exist without each other. Therefore, the definition does not give us the opportunity to understand the essence of the phenomenon of the electromagnetic field, and the only thing to remember is that this special form of matter! The electromagnetic field theory was developed by James Maxwell in 1865.
What is an electromagnetic field? One can imagine that we live in the electromagnetic Universe, which is entirely permeated by the electromagnetic field, and various particles and substances, depending on their structure and properties, acquire a positive or negative charge under the influence of the electromagnetic field, accumulate it, or remain electrically neutral. Accordingly, electromagnetic fields can be divided into two types: static, that is, emitted by charged bodies (particles) and integral to them, and dynamic, propagating in space, being torn off from the source that radiated it. A dynamic electromagnetic field in physics is represented as two mutually perpendicular waves: electric (E) and magnetic (H).
The fact that an electric field is generated by an alternating magnetic field, and magnetic field - alternating electric, leads to the fact that electric and magnetic alternating fields do not exist separately from each other. The electromagnetic field of stationary or uniformly moving charged particles is directly related to the particles themselves. With the accelerated movement of these charged particles, the electromagnetic field "breaks away" from them and exists independently in the form of electromagnetic waves, not disappearing with the elimination of the source.
Sources of electromagnetic fields
Natural (natural) sources of electromagnetic fields
Natural (natural) sources of EMF are divided into the following groups:
Earth's magnetic field. The magnitude of the Earth's geomagnetic field varies over the earth's surface from 35 µT at the equator to 65 µT near the poles.
Earth's electric field is directed normally to the earth's surface, which is negatively charged with respect to upper layers atmosphere. The electric field strength near the Earth's surface is 120…130 V/m and decreases approximately exponentially with height. Annual changes in EP are similar in nature throughout the Earth: the maximum intensity is 150...250 V/m in January-February and the minimum is 100...120 V/m in June-July.
atmospheric electricity are electrical phenomena in the earth's atmosphere. In the air (link) there are always positive and negative electric charges - ions that arise under the influence of radioactive substances, cosmic rays and ultraviolet radiation from the Sun. Earth negatively charged; there is a large potential difference between it and the atmosphere. The strength of the electrostatic field increases sharply during thunderstorms. The frequency range of atmospheric discharges lies between 100 Hz and 30 MHz.
extraterrestrial sources include radiation outside the Earth's atmosphere.
Biological electromagnetic background. Biological objects, like others physical bodies, at temperatures above absolute zero emit EMF in the range of 10 kHz - 100 GHz. This is due to the chaotic movement of charges - ions, in the human body. The power density of such radiation in humans is 10 mW / cm2, which for an adult gives a total power of 100 watts. The human body also emits EMF at 300 GHz with a power density of about 0.003 W/m2.
Anthropogenic sources of electromagnetic fields
Anthropogenic sources are divided into 2 groups:
Sources of low-frequency radiation (0 - 3 kHz)
This group includes all systems for the production, transmission and distribution of electricity (power lines, transformer substations, power plants, various cable systems), home and office electrical and electronic equipment, including PC monitors, electric vehicles, railway transport and its infrastructure, as well as metro, trolleybus and tram transport.
Already today, the electromagnetic field on 18-32% of the territory of cities is formed as a result of car traffic. Electromagnetic waves, arising from the movement of vehicles, interfere with television and radio reception, and may also have harmful effect on the human body.
RF sources (3 kHz to 300 GHz)
This group includes functional transmitters - sources of an electromagnetic field for the purpose of transmitting or receiving information. These are commercial transmitters (radio, television), radio telephones (car, radio telephones, radio CB, amateur radio transmitters, industrial radio telephones), directional radio communications (satellite radio communications, ground relay stations), navigation (air traffic, shipping, radio point), locators (air communication, shipping, traffic locators, air traffic control). This also includes various technological equipment, using microwave radiation, variable (50 Hz - 1 MHz) and pulsed fields, household equipment(microwave ovens), means of visual display of information on cathode ray tubes (PC monitors, televisions, etc.). For scientific research In medicine, ultra-high frequency currents are used. The electromagnetic fields arising from the use of such currents represent a certain occupational hazard, therefore, it is necessary to take measures to protect against their effects on the body.
The main technogenic sources are:
A feature of exposure in urban conditions is the impact on the population of both the total electromagnetic background (integral parameter) and strong EMF from individual sources (differential parameter).
