The electromagnetic field is short and simple. Electromagnetic field
In 1860-1865. one of the greatest physicists of the 19th century James Clerk Maxwell created a theory electromagnetic field. According to Maxwell, the phenomenon of electromagnetic induction is explained as follows. If at some point in space the magnetic field changes in time, then there is formed and electric field... If there is a closed conductor in the field, then the electric field causes an induction current in it. It follows from Maxwell's theory that the reverse process is also possible. If in a certain region of space the electric field changes in time, then a magnetic field is also formed here.
Thus, 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. These mutually generating variables are electrical and magnetic fields 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 is the prediction of the possibility of the existence 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 a 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 the wavelength, its speed, period and oscillation frequency obtained for mechanical waves is also true for electromagnetic waves:
Fluctuations of the tension vector E and the vector of magnetic induction B occur in mutually perpendicular planes and perpendicular to the direction of wave propagation (velocity vector).
An electromagnetic wave carries energy.
Electromagnetic wave range
A complex world of electromagnetic waves around us different frequencies: radiation from computer monitors, cell phones, microwave ovens, televisions, etc. Currently, all electromagnetic waves are divided into six main wavelength ranges.
Radio waves- these are electromagnetic waves (with a wavelength of 10,000 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 through the antenna.
Electromagnetic radiation with a wavelength of 0.005 m to 1 μm, i.e. lying between the range of radio waves and the range of visible light are called infrared radiation... Infrared radiation is emitted by any heated body. Ovens, batteries, electric incandescent lamps serve as sources of infrared radiation. 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.
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 with the help of sight.
Electromagnetic radiation invisible to the eye with a wavelength less than that of violet is called ultraviolet radiation. It is capable of killing 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 is called the electromagnetic radiation emitted by excited nuclei and arising from the interaction of elementary particles.
The principle of radio communication
The oscillating 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 an open oscillating circuit - antenna.
Radio communication is called the transmission of information using electromagnetic waves, the frequencies of which are in the range from to Hz.
Radar (radar)
A device that transmits ultra-short waves and receives them immediately. 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 in a similar way. Think about how radar detects the speed of a moving car.
Electromagnetic field, a special form of matter. Interaction between charged particles is carried out by means of an electromagnetic field.
The behavior of the electromagnetic field is studied by classical electrodynamics. The electromagnetic field is described by Maxwell's equations, which relate the quantities characterizing the field with its sources, that is, with charges and currents distributed in space. The electromagnetic field of stationary or uniformly moving charged particles is inextricably linked with these particles; with accelerated motion of particles, the electromagnetic field "breaks away" from them and exists independently in the form of electromagnetic waves.
It follows from Maxwell's equations that an alternating electric field generates a magnetic, and an alternating magnetic field generates an electric, therefore, an electromagnetic field can exist in the absence of charges. Generation of an electromagnetic field by an alternating magnetic field and a magnetic field by an alternating electric leads to the fact that electric and magnetic fields do not exist separately, independently of each other. Therefore, the electromagnetic field is a type of matter, determined at all points by two vector quantities that characterize its two components - "electric field" and "magnetic field", and exerting a force effect on charged particles, depending on their speed and the magnitude of their charge.
An electromagnetic field in a vacuum, that is, in a free state, not associated with particles of matter, exists in the form of electromagnetic waves, and propagates in a vacuum in the absence of very strong gravitational fields at a speed equal to the speed of light c= 2.998. 10 8 m / s. Such a field is characterized by an electric field strength E and magnetic induction V... To describe the electromagnetic field in a medium, the values of electrical induction are also used D and magnetic field strength H... In matter, as well as in the presence of very strong gravitational fields, that is, near very large masses of matter, the propagation velocity of the electromagnetic field is less than c.
The components of the vectors characterizing the electromagnetic field form, according to the theory of relativity, a single physical quantity- the tensor of the electromagnetic field, the components of which are transformed when passing from one inertial reference frame to another in accordance with the Lorentz transformations.
An electromagnetic field has energy and momentum. The existence of a pulse of an electromagnetic field was first discovered experimentally in the experiments of PN Lebedev on measuring the pressure of light in 1899. An electromagnetic field always has energy. Energy density of the electromagnetic field = 1/2 (ED + VN).
The electromagnetic field spreads through space. The energy flux density of the electromagnetic field is determined by the Poyting vector S =, unit of measurement W / m 2. The direction of the Poiting vector is perpendicular E and H and coincides with the direction of propagation of electromagnetic energy. Its value is equal to the energy transferred through a unit area perpendicular to S per unit of time. Field pulse density in vacuum K = S / s 2 = / s 2.
At high frequencies of the electromagnetic field, its quantum properties become essential and the electromagnetic field can be considered as a flux of field quanta - photons. In this case, the electromagnetic field is described
Details Category: Electricity and Magnetism Published 05.06.2015 20:46 Hits: 11962Alternating electric and magnetic fields under certain conditions can generate each other. They form an electromagnetic field, which is not at all their combination. It is a single whole in which these two fields cannot exist without each other.
From the history
The experience of the Danish scientist Hans Christian Oersted, carried out in 1821, showed that electricity generates a magnetic field. In turn, the changing magnetic field is capable of generating an electric current. This was proved by the English physicist Michael Faraday, who discovered in 1831 the phenomenon of electromagnetic induction. He is also the author of the term "electromagnetic field".
At that time, the concept of Newton's long-range action was adopted in physics. It was believed that all bodies act on each other through the void with 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 that interaction occurs at a finite speed through a certain material environment. This medium for electric charges is electromagnetic field... And it spreads at a speed equal to the speed of light.
Maxwell's theory
By combining the results of previous studies, English physicist James Clerk Maxwell in 1864 created electromagnetic field theory... According to her, a changing magnetic field generates a changing electric field, and an alternating electric field generates an alternating magnetic field. Of course, in the beginning one of the fields is created by a source of charges or currents. But in the future, these fields may 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 any 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. Its lines of force are closed.
This theory is phenomenological. This means that it was created on the basis of assumptions and observations, and does not consider the cause of the occurrence of electric and magnetic fields.
Electromagnetic field properties
The electromagnetic field is a combination of electric and magnetic fields, therefore, at each point of its space, it is described by two main quantities: the strength of the electric field E and magnetic induction V .
Since the electromagnetic field is the process of converting an electric field into a magnetic, 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-ray and gamma radiation.
The vectors of the intensity and induction 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 speed of propagation of electromagnetic waves was considered to be infinitely large. However, Maxwell proved that this is 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.
Experimentally, Maxwell's theory was 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 a vacuum, which turned out to be equal to the speed of light.
In integral form, this law looks like this:
Gauss's 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 magnetic field lines 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 electrical induction generate a vortex magnetic field.
,
,
E- electric field strength;
H- magnetic field strength;
V- magnetic induction. This is a vector quantity that shows with what force a magnetic field acts on a charge of q value moving with 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 - Nabla operator. The action of this operator on a specific field is called the rotor of this field.
Δ x E = rot E
ρ - the density of the external electric charge;
j- current density - a value showing the strength of the current flowing through a unit area;
with- the speed of light in a vacuum.
The study of the electromagnetic field is engaged in a science called electrodynamics... She considers its interaction with bodies that have an electric charge. This interaction is called electromagnetic... Classical electrodynamics describes only the continuous properties of the electromagnetic field using Maxwell's equations. Modern quantum electrodynamics believes that the electromagnetic field also has discrete (discontinuous) properties. And such electromagnetic interaction occurs with the help of indivisible quantum particles that do not have mass and charge. The quantum of the electromagnetic field is called photon .
Electromagnetic field around us
An electromagnetic field is generated around any conductor with alternating current... Sources of electromagnetic fields are power lines, electric motors, transformers, urban electric transport, rail transport, electrical and electronic household appliances - televisions, computers, refrigerators, irons, vacuum cleaners, radio telephones, mobile phones, electric shavers - in a word, everything related to the 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 few such devices around us, electromagnetic fields surround us everywhere. These fields affect environment and a person. 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 the current one.
Up to a certain level, electromagnetic radiation can be harmless to 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 smooth muscles of the intestine 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 cardiovascular, immune, endocrine and nervous systems human, can cause insomnia, headaches, stress. The danger is that their effect is practically invisible to a person, and violations arise gradually.
How can we protect ourselves from the electromagnetic radiation that surrounds us? This cannot be done completely, so you need to try to minimize its impact. First of all, you need to arrange Appliances so that they are away from where we are most often. For example, you don't have to 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 appliance plugged in. But the electromagnetic field disappears only when the device is disconnected from the electrical network.
Human health is also affected by natural electromagnetic fields - cosmic radiation, the Earth's magnetic field.
Electromagnetic field
An electromagnetic field refers to the type of matter that occurs around moving charges. It consists of electric as well as magnetic fields. Their existence is interconnected, since they cannot exist separately and independently of each other, because one field generates another.
Now let's try to approach the topic of the electromagnetic field in more detail. From the definition, it can be concluded that in the event of a change in the electric field, prerequisites for the occurrence of a magnetic field appear. And since the electric field tends to change over time and cannot be called unchanged, the magnetic field is also variable.
When one field changes, another is spawned. And regardless of what the subsequent field will be, the source will be the previous field, that is, a conductor with current, and not its original source.
And even if the current in the conductor is turned off, the electromagnetic field will not disappear anywhere, but will continue to exist and will spread in space.
Properties of electromagnetic waves
Maxwell's theory. Vortex electric field
James Clerk Maxwell, a famous British physicist, wrote a work in 1857 in which he cited evidence that fields such as electric and magnetic are closely related.
According to his theory, it followed that the alternating magnetic field has the ability to create such a new electric field, which differs from the previous electric field created with the help of a current source, since this new electric field is vortex.
And here you and I see that a vortex electric field is a field in which the lines of force are closed. That is, it should be noted that the electric field has the same closed lines as the magnetic field.
From this it follows that an alternating magnetic field is capable of creating a vortex electric field, and a vortex electric field has the ability to make charges move. And as a result, we get an induction electric current. It follows from Maxwell's work that such fields as electric and magnetic coexist closely with each other.
That is, for the existence of a magnetic field, a moving electric charge is required. Well, the electric field is created due to the resting electric charge. This is the kind of transparent relationship that exists between the fields. From this we can draw another conclusion that in different systems countdown can be observed different kinds fields.
If we follow Maxwell's theory, then we can summarize that alternating electric and magnetic fields are not able to exist separately, because when the magnetic field changes, it generates an electric field, and a changing electric field generates a magnetic one.
Natural sources of electromagnetic fields
For a modern person, it is not a secret that the electromagnetic fields, although they remain invisible to our eyes, surround us everywhere.
Natural sources of EMF include:
First, it is the permanent electric and magnetic field of the Earth.
Secondly, such sources include radio waves that convert such space sources like the sun, stars, etc.
Thirdly, these sources are also atmospheric processes such as lightning discharges, etc.
Anthropogenic (artificial) sources of electromagnetic fields
except natural sources emergence of EMF, they also arise due to anthropogenic sources. These sources include X-rays, which are used in medical institutions. They are also used to transmit information using various radio stations, mobile stations and also TV antennas. And the electricity that is in each outlet also generates an EMF, but it is true, of a lower frequency.
Impact of EMF on human health
Modern society currently does not think of its own life, without such benefits of civilization as the presence of various household appliances, computers, mobile communications. They, of course, make our life easier, but they create electromagnetic fields around us. Naturally, you and I cannot see EMF, but they surround us everywhere. They are present in our homes, at work, and even in transport.
We can safely say that modern man lives in a continuous electromagnetic field, which, unfortunately, has a huge impact on human health. With prolonged exposure to the electromagnetic field on the human body, such unpleasant symptoms as chronic fatigue, irritability, disturbed sleep, attention and memory appear. Such prolonged exposure to EMF can cause headaches, infertility, disturbances in the functioning of the nervous and cardiac systems, as well as the appearance of oncological diseases in a person.
