What is an electromagnetic wave and how is it formed? How did electromagnetic waves “appear”? Electromagnetic radiation and its types

M. Faraday introduced the concept of field:

an electrostatic field arises around a stationary charge,

A magnetic field arises around moving charges (current).

In 1830, M. Faraday discovered the phenomenon of electromagnetic induction: when changing magnetic field a vortex occurs electric field.

Figure 2.7 - Vortex electric field

Where,
- electric field strength vector,
- vector of magnetic induction.

An alternating magnetic field creates a vortex electric field.

In 1862 D.K. Maxwell put forward a hypothesis: when the electric field changes, a vortex magnetic field appears.

The idea of ​​a single electromagnetic field arose.

Figure 2.8 - Unified electromagnetic field.

An alternating electric field creates a vortex magnetic field.

Electromagnetic field- this is a special form of matter - a combination of electric and magnetic fields. Alternating electric and magnetic fields exist simultaneously and form a single electromagnetic field. It is material:

Manifests itself in action on both stationary and moving charges;

Spreads at a high but finite speed;

It exists regardless of our will and desires.

When the charge speed is zero, there is only an electric field. At a constant charge speed, an electromagnetic field arises.

With the accelerated movement of a charge, an electromagnetic wave is emitted, which propagates in space at a finite speed .

The development of the idea of ​​electromagnetic waves belongs to Maxwell, but Faraday already guessed about their existence, although he was afraid to publish the work (it was read more than 100 years after his death).

The main condition for the occurrence of an electromagnetic wave is the accelerated movement of electric charges.

What an electromagnetic wave is can be easily illustrated using the following example. If you throw a pebble onto the surface of the water, waves will form on the surface, spreading out in circles. They move from the source of their origin (disturbance) with a certain propagation speed. For electromagnetic waves, disturbances are electric and magnetic fields moving in space. An electromagnetic field that changes over time necessarily causes the appearance of an alternating magnetic field, and vice versa. These fields are mutually related.

The main source of the spectrum of electromagnetic waves is the Sun star. Part of the spectrum of electromagnetic waves is visible to the human eye. This spectrum lies within the range of 380...780 nm (Fig. 2.1). In the visible spectrum, the eye senses light differently. Electromagnetic vibrations with different wavelengths cause the sensation of light with different colors.

Figure 2.9 - Spectrum of electromagnetic waves

Part of the electromagnetic wave spectrum is used for radiotelevision and communications purposes. The source of electromagnetic waves is a wire (antenna) in which electric charges oscillate. The process of field formation, which began near the wire, gradually, point by point, covers the entire space. The higher the frequency alternating current, passing through a wire and generating an electric or magnetic field, the more intense the radio waves of a given length created by the wire.

Radio(lat. radio - radiate, emit rays ← radius - ray) - a type of wireless communication in which radio waves, freely propagating in space, are used as a signal carrier.

Radio waves(from radio...), electromagnetic waves with wavelength > 500 µm (frequency< 6×10 12 Гц).

Radio waves are electric and magnetic fields that vary over time. The speed of propagation of radio waves in free space is 300,000 km/s. From this, the radio wavelength (m) can be determined.

λ=300/f, wheref - frequency (MHz)

Sound vibrations in the air created during a telephone conversation are converted by a microphone into electrical vibrations of sound frequency, which are transmitted through wires to the subscriber’s equipment. There, at the other end of the line, they are converted, using the telephone emitter, into air vibrations, perceived by the subscriber as sounds. In telephony, the means of communication of the circuit are wires, in radio broadcasting - radio waves.

The “heart” of the transmitter of any radio station is a generator - a device that produces oscillations of a high, but strictly constant frequency for a given radio station. These radio frequency oscillations, amplified to the required power, enter the antenna and excite electromagnetic oscillations of exactly the same frequency - radio waves - in the space surrounding it. The speed of removal of radio waves from a radio station antenna is equal to the speed of light: 300,000 km/s, which is almost a million times faster than the propagation of sound in air. This means that if the transmitter was turned on at the Moscow Broadcasting Station at a certain point in time, then its radio waves will reach Vladivostok in less than 1/30 s, and the sound during this time will have time to spread only 10-11 m.

Radio waves propagate not only in the air, but also where there is no air, for example, in outer space. This is how they differ from sound waves, for which air or some other dense medium, such as water, is absolutely necessary.

Electromagnetic wave – electromagnetic field propagating in space (oscillations of vectors
). Near the charge, the electric and magnetic fields change with a phase shift p/2.

Figure 2.10 - Unified electromagnetic field.

At a large distance from the charge, the electric and magnetic fields change in phase.

Figure 2.11 - In-phase change in electric and magnetic fields.

Electromagnetic wave is transverse. The direction of the speed of the electromagnetic wave coincides with the direction of movement of the right screw when turning the handle of the vector gimlet to vector .

Figure 2.12 - Electromagnetic wave.

Moreover, in an electromagnetic wave the relation is satisfied
, where c is the speed of light in vacuum.

Maxwell theoretically calculated the energy and speed of electromagnetic waves.

Thus, wave energy is directly proportional to the fourth power of frequency. This means that in order to detect a wave more easily, it must be of high frequency.

Electromagnetic waves were discovered by G. Hertz (1887).

A closed oscillatory circuit does not emit electromagnetic waves: all the energy of the electric field of the capacitor is converted into the energy of the magnetic field of the coil. The oscillation frequency is determined by the parameters of the oscillatory circuit:
.

Figure 2.13 - Oscillatory circuit.

To increase the frequency, it is necessary to reduce L and C, i.e. unfold the coil to a straight wire and, because
, reduce the area of ​​the plates and spread them apart maximum distance. From this we can see that we will essentially have a straight conductor.

Such a device is called a Hertz vibrator. The middle is cut and connected to a high-frequency transformer. Between the ends of the wires on which small ball conductors are fixed, an electric spark jumps, which is the source of the electromagnetic wave. The wave propagates so that the electric field strength vector oscillates in the plane in which the conductor is located.

Figure 2.14 - Hertz vibrator.

If you place the same conductor (antenna) parallel to the emitter, then the charges in it will begin to oscillate and weak sparks will jump between the conductors.

Hertz discovered electromagnetic waves experimentally and measured their speed, which coincided with that calculated by Maxwell and equal to c = 3. 10 8 m/s.

An alternating electric field generates an alternating magnetic field, which, in turn, generates an alternating electric field, that is, an antenna that excites one of the fields causes the appearance of a single electromagnetic field. The most important property of this field is that it propagates in the form of electromagnetic waves.

The speed of propagation of electromagnetic waves in a lossless medium depends on the relative dielectric and magnetic permeability of the medium. For air, the magnetic permeability of the medium is equal to unity, therefore, the speed of propagation of electromagnetic waves in this case is equal to the speed of light.

The antenna can be a vertical wire powered by a high-frequency generator. The generator expends energy to accelerate the movement of free electrons in the conductor, and this energy is converted into an alternating electromagnetic field, that is, electromagnetic waves. The higher the frequency of the generator current, the faster the electromagnetic field changes and the more intense the healing of waves.

The antenna wire is connected like an electric field, power lines which begin on positive and end on negative charges, and a magnetic field, the lines of which close around the current of the wire. The shorter the oscillation period, the less time remains for the energy of bound fields to return to the wire (that is, to the generator) and the more it turns into free fields, which further propagate in the form of electromagnetic waves. Effective radiation of electromagnetic waves occurs under the condition that the wavelength and the length of the emitting wire are commensurate.

Thus, it can be determined that radio wave- this is an electromagnetic field not associated with the emitter and channel-forming devices, freely propagating in space in the form of a wave with an oscillation frequency from 10 -3 to 10 12 Hz.

Oscillations of electrons in the antenna are created by a source of periodically varying emf with a period T. If at some moment the field at the antenna had a maximum value, then it will have the same value after a while T. During this time, the electromagnetic field that initially existed at the antenna will move a distance

λ = υТ (1)

The minimum distance between two points in space at which the field has the same value is called wavelength. As follows from (1), the wavelength λ depends on the speed of its propagation and the period of oscillation of electrons in the antenna. Because frequency current f = 1/T, then the wavelength λ = υ / f .

The radio link includes the following main parts:

Transmitter

Receiver

The environment in which radio waves propagate.

The transmitter and receiver are controllable elements of a radio link, since you can increase the transmitter power, connect a more efficient antenna and increase the sensitivity of the receiver. The medium is an uncontrolled element of the radio link.

The difference between a radio communication line and wired lines is that in wired lines, wires or cables, which are controllable elements (you can change their electrical parameters), are used as a connecting link.

Electromagnetic waves is the process of propagation of an alternating electromagnetic field in space. Theoretically, the existence of electromagnetic waves was predicted by the English scientist Maxwell in 1865, and they were first experimentally obtained by the German scientist Hertz in 1888.

From Maxwell's theory follow formulas that describe the oscillations of vectors and. Plane monochromatic electromagnetic wave propagating along the axis x, is described by the equations

Here E And H- instantaneous values, and E m and H m - amplitude values ​​of the electric and magnetic field strength, ω - circular frequency, k- wave number. Vectors and oscillate with the same frequency and phase, are mutually perpendicular and, in addition, perpendicular to the vector - the speed of wave propagation (Fig. 3.7). That is, electromagnetic waves are transverse.

In a vacuum, electromagnetic waves travel at speed. In a medium with dielectric constant ε and magnetic permeability µ the speed of propagation of an electromagnetic wave is equal to:

The frequency of electromagnetic oscillations, as well as the wavelength, can, in principle, be anything. The classification of waves by frequency (or wavelength) is called the electromagnetic wave scale. Electromagnetic waves are divided into several types.

Radio waves have a wavelength from 10 3 to 10 -4 m.

Light waves include:

X-ray radiation - .

Light waves are electromagnetic waves that include the infrared, visible and ultraviolet parts of the spectrum. The wavelengths of light in a vacuum corresponding to the primary colors of the visible spectrum are shown in the table below. The wavelength is given in nanometers.

Table

Light waves have the same properties as electromagnetic waves.

1. Light waves are transverse.

2. The vectors and oscillate in a light wave.

Experience shows that all types of effects (physiological, photochemical, photoelectric, etc.) are caused by vibrations electric vector. He is called light vector .

Amplitude of the light vector E m is often denoted by the letter A and instead of equation (3.30), equation (3.24) is used.

3. Speed ​​of light in vacuum.

The speed of a light wave in a medium is determined by formula (3.29). But for transparent media (glass, water) it is usual.

For light waves, the concept of absolute refractive index is introduced.

Absolute refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium

From (3.29), taking into account the fact that for transparent media, we can write the equality.

For vacuum ε = 1 and n= 1. For any physical environment n> 1. For example, for water n= 1.33, for glass. A medium with a higher refractive index is called optically denser. The ratio of absolute refractive indices is called relative refractive index:

4. The frequency of light waves is very high. For example, for red light with wavelength.

When light passes from one medium to another, the frequency of the light does not change, but the speed and wavelength change.

For vacuum - ; for environment - , then

.

Hence the wavelength of light in the medium is equal to the ratio of the wavelength of light in vacuum to the refractive index

5. Because the frequency of light waves is very high , then the observer’s eye does not distinguish individual vibrations, but perceives average energy flows. This introduces the concept of intensity.

Intensity is the ratio of the average energy transferred by the wave to the period of time and to the area of ​​the site perpendicular to the direction of propagation of the wave:

Since the wave energy is proportional to the square of the amplitude (see formula (3.25)), the intensity is proportional to the average value of the square of the amplitude

The characteristic of light intensity, taking into account its ability to cause visual sensations, is luminous flux - F .

6. The wave nature of light manifests itself, for example, in phenomena such as interference and diffraction.

An electromagnetic wave is a disturbance of the electromagnetic field that is transmitted in space. Its speed matches the speed of light

2. Describe Hertz’s experiment in detecting electromagnetic waves

In Hertz's experiment, the source of electromagnetic disturbance was electromagnetic oscillations that arose in a vibrator (a conductor with an air gap in the middle). To this interval it was submitted high voltage, it caused spark discharge. After a moment, a spark discharge appeared in the resonator (a similar vibrator). The most intense spark occurred in the resonator, which was located parallel to the vibrator.

3. Explain the results of Hertz’s experiment using Maxwell’s theory. Why is an electromagnetic wave transverse?

The current through the discharge gap creates induction around itself, magnetic flux increases, arises induced current offsets. The voltage at point 1 (Fig. 155, b of the textbook) is directed counterclockwise in the plane of the drawing, at point 2 the current is directed upward and causes induction at point 3, the tension is directed upward. If the voltage is sufficient for electrical breakdown of the air in the gap, then a spark occurs and current flows in the resonator.

Because the directions of the magnetic field induction vectors and the electric field strength are perpendicular to each other and to the direction of the wave.

4. Why does the radiation of electromagnetic waves occur with the accelerated movement of electric charges? How does the electric field strength in an emitted electromagnetic wave depend on the acceleration of the emitting charged particle?

The strength of the current is proportional to the speed of movement of charged particles, so an electromagnetic wave occurs only if the speed of movement of these particles depends on time. The intensity in the emitted electromagnetic wave is directly proportional to the acceleration of the radiating charged particle.

5. How does the energy density of the electromagnetic field depend on the electric field strength?

The energy density of the electromagnetic field is directly proportional to the square of the electric field strength.

Electromagnetic waves (the table of which will be given below) are disturbances of magnetic and electric fields distributed in space. There are several types of them. Physics studies these disturbances. Electromagnetic waves are formed due to the fact that an alternating electric field generates a magnetic field, which, in turn, generates an electric one.

History of research

The first theories, which can be considered the oldest versions of hypotheses about electromagnetic waves, date back at least to the time of Huygens. During that period, the assumptions reached pronounced quantitative development. Huygens in 1678 released a kind of “sketch” of the theory - “Treatise on Light”. In 1690, he published another remarkable work. It outlined the qualitative theory of reflection and refraction in the form in which it is still presented today in school textbooks (“Electromagnetic Waves,” 9th grade).

At the same time, Huygens' principle was formulated. With its help, it became possible to study the movement of the wave front. This principle subsequently found its development in the works of Fresnel. The Huygens-Fresnel principle was of particular importance in the theory of diffraction and the wave theory of light.

In the 1660-1670s, Hooke and Newton made major experimental and theoretical contributions to research. Who discovered electromagnetic waves? Who conducted the experiments to prove their existence? What types of electromagnetic waves are there? More on this later.

Maxwell's rationale

Before talking about who discovered electromagnetic waves, it should be said that the first scientist who generally predicted their existence was Faraday. He put forward his hypothesis in 1832. Maxwell subsequently worked on the construction of the theory. By 1865 he completed this work. As a result, Maxwell strictly formulated the theory mathematically, justifying the existence of the phenomena under consideration. He also determined the speed of propagation of electromagnetic waves, which coincided with the then used value of the speed of light. This, in turn, allowed him to substantiate the hypothesis that light is one of the types of radiation under consideration.

Experimental detection

Maxwell's theory was confirmed by Hertz's experiments in 1888. Here it should be said that the German physicist conducted his experiments to refute the theory, despite its mathematical justification. However, thanks to his experiments, Hertz became the first to practically discover electromagnetic waves. In addition, during his experiments, the scientist identified the properties and characteristics of the radiation.

Hertz obtained electromagnetic oscillations and waves by exciting a series of pulses of a rapidly varying flow in a vibrator using a high voltage source. High frequency currents can be detected using a circuit. The higher the capacitance and inductance, the higher the oscillation frequency will be. But at the same time, a high frequency does not guarantee an intense flow. To carry out his experiments, Hertz used a fairly simple device, which today is called the “Hertz vibrator.” The device is an open type oscillatory circuit.

Schematic of Hertz's experiment

Registration of radiation was carried out using a receiving vibrator. This device had the same design as the emitting device. Under the influence of an electromagnetic wave of an electric alternating field, a current oscillation was excited in the receiving device. If in this device its natural frequency and the frequency of the flow coincided, then resonance appeared. As a result, disturbances in the receiving device occurred with greater amplitude. The researcher discovered them by observing sparks between the conductors in a small gap.

Thus, Hertz became the first to discover electromagnetic waves and prove their ability to be reflected well from conductors. He practically substantiated the formation of standing radiation. In addition, Hertz determined the speed of propagation of electromagnetic waves in air.

Characteristics Study

Electromagnetic waves propagate in almost all media. In a space filled with matter, radiation can in some cases be distributed quite well. But at the same time they change their behavior somewhat.

Electromagnetic waves in a vacuum are detected without attenuation. They are distributed to any, arbitrarily long distance. The main characteristics of waves include polarization, frequency and length. The properties are described within the framework of electrodynamics. However, more specific branches of physics deal with the characteristics of radiation from certain regions of the spectrum. These include, for example, optics.

The study of hard electromagnetic radiation at the short-wave spectral end is carried out by the high-energy section. Taking into account modern ideas, dynamics ceases to be an independent discipline and is combined with one theory.

Theories used in the study of properties

Today there are various methods, facilitating the modeling and study of the manifestations and properties of vibrations. Quantum electrodynamics is considered the most fundamental of the tested and completed theories. From it, through certain simplifications, it becomes possible to obtain the methods listed below, which are widely used in various fields.

The description of relatively low-frequency radiation in a macroscopic environment is carried out using classical electrodynamics. It is based on Maxwell's equations. However, there are simplifications in applications. Optical study uses optics. Wave theory is used in cases where some parts optical system are close in size to wavelengths. Quantum optics is used when the processes of scattering and absorption of photons are significant.

Geometric optical theory is a limiting case in which the wavelength can be ignored. There are also several applied and fundamental sections. These include, for example, astrophysics, the biology of visual perception and photosynthesis, and photochemistry. How are electromagnetic waves classified? A table clearly depicting the distribution into groups is presented below.

Classification

There are frequency ranges of electromagnetic waves. There are no sharp transitions between them; sometimes they overlap each other. The boundaries between them are quite arbitrary. Due to the fact that the flow is distributed continuously, the frequency is strictly related to the length. Below are the ranges of electromagnetic waves.

Ultrashort radiation is usually divided into micrometer (submillimeter), millimeter, centimeter, decimeter, meter. If electromagnetic radiation less than a meter, then it is usually called ultra-high frequency oscillation (microwave).

Types of electromagnetic waves

Above are the ranges of electromagnetic waves. What types of streams are there? The group includes gamma and x-rays. It should be said that both ultraviolet and even visible light are capable of ionizing atoms. The boundaries within which gamma and X-ray fluxes are located are determined very conditionally. As a general guideline, the limits of 20 eV - 0.1 MeV are accepted. Gamma fluxes in the narrow sense are emitted by the nucleus, X-ray fluxes are emitted by the electron atomic shell in the process of knocking out electrons from low-lying orbits. However, this classification is not applicable to hard radiation generated without the participation of nuclei and atoms.

X-ray fluxes are formed when charged fast particles (protons, electrons and others) slow down and as a result of processes that occur inside atomic electron shells. Gamma oscillations arise as a result of processes inside the nuclei of atoms and during the transformation of elementary particles.

Radio streams

Due to of great importance lengths, consideration of these waves can be carried out without taking into account the atomistic structure of the medium. As an exception, only the shortest flows, which are adjacent to the infrared region of the spectrum, act. In the radio range quantum properties vibrations appear rather weakly. Nevertheless, they must be taken into account, for example, when analyzing molecular time and frequency standards during cooling of equipment to a temperature of several kelvins.

Quantum properties are also taken into account when describing generators and amplifiers in the millimeter and centimeter ranges. The radio stream is formed during the movement of alternating current through conductors of the corresponding frequency. And a passing electromagnetic wave in space excites the corresponding one. This property used in the design of antennas in radio engineering.

Visible threads

Ultraviolet and infrared visible radiation amounts to in a broad sense words, the so-called optical part of the spectrum. The selection of this area is determined not only by the proximity of the corresponding zones, but also by the similarity of the instruments used in the research and developed primarily during the study of visible light. These, in particular, include mirrors and lenses for focusing radiation, diffraction gratings, prisms and others.

The frequencies of optical waves are comparable to those of molecules and atoms, and their lengths are comparable to intermolecular distances and molecular sizes. Therefore, phenomena that are caused by the atomic structure of matter become significant in this area. For the same reason, light, along with wave properties, also has quantum properties.

The emergence of optical flows

The most famous source is the Sun. The star's surface (photosphere) has a temperature of 6000° Kelvin and emits bright white light. The highest value of the continuous spectrum is located in the “green” zone - 550 nm. This is also where the maximum visual sensitivity is located. Oscillations in the optical range occur when bodies are heated. Infrared flows are therefore also called thermal flows.

The more the body heats up, the higher the frequency where the maximum of the spectrum is located. With a certain increase in temperature, incandescence (glow in the visible range) is observed. In this case, red appears first, then yellow, and so on. The creation and recording of optical flows can occur in biological and chemical reactions, one of which is used in photography. For most creatures living on Earth, photosynthesis serves as a source of energy. This biological reaction occurs in plants under the influence of optical solar radiation.

Features of electromagnetic waves

The properties of the medium and the source influence the characteristics of the flows. This establishes, in particular, the time dependence of the fields, which determines the type of flow. For example, when the distance from the vibrator changes (as it increases), the radius of curvature becomes larger. As a result, a plane electromagnetic wave is formed. Interaction with the substance also occurs in different ways.

The processes of absorption and emission of fluxes, as a rule, can be described using classical electrodynamic relations. For waves in the optical region and for hard rays, their quantum nature should be taken into account even more.

Stream sources

Despite the physical difference, everywhere - in a radioactive substance, a television transmitter, an incandescent lamp - electromagnetic waves are excited by electric charges that move with acceleration. There are two main types of sources: microscopic and macroscopic. In the first, there is an abrupt transition of charged particles from one to another level inside molecules or atoms.

Microscopic sources emit x-ray, gamma, ultraviolet, infrared, visible, and in some cases long-wave radiation. An example of the latter is the line in the spectrum of hydrogen, which corresponds to a wavelength of 21 cm. This phenomenon is of particular importance in radio astronomy.

Macroscopic type sources are emitters in which free electrons conductors undergo periodic synchronous oscillations. In systems of this category, flows from millimeter-scale to the longest (in power lines) are generated.

Structure and strength of flows

Accelerated and periodically changing currents influence each other with certain forces. The direction and their magnitude depend on such factors as the size and configuration of the region in which the currents and charges are contained, their relative direction and magnitude. They also have a significant impact electrical characteristics specific environment, as well as changes in charge concentration and source current distribution.

Due to the general complexity of the problem statement, it is impossible to present the law of forces in the form of a single formula. The structure, called the electromagnetic field and considered, if necessary, as a mathematical object, is determined by the distribution of charges and currents. It, in turn, is created by a given source taking into account boundary conditions. The conditions are determined by the shape of the interaction zone and the characteristics of the material. If we are talking about unlimited space, these circumstances are supplemented. The radiation condition acts as a special additional condition in such cases. Due to it, the “correctness” of the field behavior at infinity is guaranteed.

Chronology of study

Lomonosov in some of his provisions anticipates individual postulates of the theory of the electromagnetic field: the “rotary” (rotational) movement of particles, the “oscillating” (wave) theory of light, its commonality with the nature of electricity, etc. Infrared flows were discovered in 1800 by Herschel (English scientist), and the following year, 1801, Ritter described ultraviolet. Radiation of a shorter range than ultraviolet was discovered by Roentgen in 1895, on November 8. Subsequently it received the name X-ray.

The influence of electromagnetic waves has been studied by many scientists. However, the first to explore the possibilities of flows and the scope of their application was Narkevich-Iodko (Belarusian scientist). He studied the properties of flows in relation to practical medicine. Gamma radiation was discovered by Paul Willard in 1900. During the same period, Planck conducted theoretical research properties of a black body. In the process of studying, he discovered the quantum nature of the process. His work marked the beginning of the development. Subsequently, several works by Planck and Einstein were published. Their research led to the formation of such a concept as the photon. This, in turn, marked the beginning of the creation quantum theory electromagnetic fluxes. Its development continued in the works of leading scientific figures of the twentieth century.

Further research and work on the quantum theory of electromagnetic radiation and its interaction with matter ultimately led to the formation of quantum electrodynamics in the form in which it exists today. Among the outstanding scientists who studied this issue, should be named, in addition to Einstein and Planck, Bohr, Bose, Dirac, de Broglie, Heisenberg, Tomonagu, Schwinger, Feynman.

Conclusion

The importance of physics in modern world big enough. Almost everything that is used in human life today appeared thanks to practical use research of great scientists. The discovery of electromagnetic waves and their study, in particular, led to the creation of conventional, and subsequently mobile phones, radio transmitters. Special meaning practical use has such theoretical knowledge in the field of medicine, industry, and technology.

This widespread use is due to the quantitative nature of science. All physical experiments rely on measurements, comparison of the properties of the phenomena being studied with existing standards. It is for this purpose that a complex has been developed within the discipline measuring instruments and units. A number of patterns are common to all existing material systems. For example, the laws of conservation of energy are considered general physical laws.

Science as a whole is called fundamental in many cases. This is due, first of all, to the fact that other disciplines provide descriptions, which, in turn, obey the laws of physics. Thus, in chemistry, atoms, substances formed from them, and transformations are studied. But Chemical properties bodies are determined physical characteristics molecules and atoms. These properties describe such branches of physics as electromagnetism, thermodynamics and others.

An electromagnetic wave is a process of sequential, interconnected changes in the strength vectors of the electric and magnetic fields, directed perpendicular to the wave propagation beam, in which a change in the electric field causes changes in the magnetic field, which, in turn, cause changes in the electric field.

Wave (wave process) - the process of propagation of oscillations in continuum. When a wave propagates, the particles of the medium do not move with the wave, but oscillate around their equilibrium positions. Together with the wave, only states are transferred from particle to particle of the medium oscillatory motion and his energy. Therefore, the main property of all waves, regardless of their nature, is the transfer of energy without transfer of matter

Electromagnetic waves always occur when there is a changing electric field in space. This changing electric field is most often caused by the movement of charged particles, and how special case such movement by alternating electric current.

The electromagnetic field is an interconnected oscillation of the electric (E) and magnetic (B) fields. The propagation of a single electromagnetic field in space is carried out through electromagnetic waves.

Electromagnetic wave - electromagnetic vibrations propagating in space and transferring energy

Features of electromagnetic waves, the laws of their excitation and propagation are described by Maxwell's equations (which are not discussed in this course). If in some region of space there are electric charges and currents, their change over time leads to the emission of electromagnetic waves. The description of their propagation is similar to the description of mechanical waves.

If the medium is homogeneous and the wave propagates along the X axis with speed v, then electric (E) and magnetic (B) the field components at each point of the medium vary according to a harmonic law with the same circular frequency (ω) and in the same phase (plane wave equation):

where x is the coordinate of the point, and t is the time.

Vectors B and E are mutually perpendicular, and each of them is perpendicular to the direction of wave propagation (X axis). Therefore electromagnetic waves are transverse

Sinusoidal (harmonic) electromagnetic wave. Vectors , and are mutually perpendicular

1) Electromagnetic waves propagate in matter with terminal speed

Speed c propagation of electromagnetic waves in a vacuum is one of the fundamental physical constants.

Maxwell's conclusion about the finite speed of propagation of electromagnetic waves was in conflict with the accepted view at that time long-range theory , in which the speed of propagation of electric and magnetic fields was assumed to be infinitely large. Therefore, Maxwell's theory is called the theory short range.

In an electromagnetic wave, mutual transformations of electric and magnetic fields occur. These processes occur simultaneously, and the electric and magnetic fields act as equal “partners”. Therefore, the volumetric densities of electric and magnetic energy are equal to each other: w e = w m.

4. Electromagnetic waves carry energy. When waves propagate, a flow occurs electromagnetic energy. If you select a site S(Fig. 2.6.3), oriented perpendicular to the direction of wave propagation, then in a short time Δ t energy Δ will flow through the platform W um, equal

Substituting here the expressions for w uh, w m and υ, we can get:

Where E 0 – amplitude of electric field strength oscillations.

Energy flux density in SI is measured in watts per square meter (W/m2).

5. From Maxwell's theory it follows that electromagnetic waves must exert pressure on an absorbing or reflecting body. The pressure of electromagnetic radiation is explained by the fact that under the influence of the electric field of the wave, weak currents arise in the substance, that is, the ordered movement of charged particles. These currents are affected by the Ampere force from the magnetic field of the wave, directed into the thickness of the substance. This force creates the resulting pressure. Usually the pressure of electromagnetic radiation is negligible. For example, the pressure of solar radiation arriving on Earth on an absolutely absorbing surface is approximately 5 μPa. The first experiments to determine the radiation pressure on reflecting and absorbing bodies, which confirmed the conclusion of Maxwell's theory, were carried out by P. N. Lebedev in 1900. Lebedev's experiments were of great importance for the approval of Maxwell's electromagnetic theory.

The existence of pressure of electromagnetic waves allows us to conclude that the electromagnetic field is inherent mechanical impulse. The pulse of the electromagnetic field in a unit volume is expressed by the relation

This implies:

This relationship between the mass and energy of the electromagnetic field in a unit volume is a universal law of nature. According to the special theory of relativity, it is true for any bodies, regardless of their nature and internal structure.

Thus, the electromagnetic field has all the characteristics material bodies– energy, final velocity of propagation, momentum, mass. This suggests that the electromagnetic field is one of the forms of existence of matter.

6. First experimental confirmation Maxwell's electromagnetic theory was given approximately 15 years after the creation of the theory in the experiments of G. Hertz (1888). Hertz not only experimentally proved the existence of electromagnetic waves, but for the first time began to study their properties - absorption and refraction in different media, reflection from metal surfaces, etc. He was able to experimentally measure the wavelength and speed of propagation of electromagnetic waves, which turned out to be equal to the speed of light .

Hertz's experiments played a decisive role in the proof and recognition of Maxwell's electromagnetic theory. Seven years after these experiments, electromagnetic waves found application in wireless communication(A.S. Popov, 1895).

7. Electromagnetic waves can only be excited accelerated moving charges. Chains direct current, in which charge carriers move at a constant speed, are not a source of electromagnetic waves. In modern radio engineering, electromagnetic waves are emitted using antennas various designs, in which rapidly alternating currents are excited.

The simplest system emitting electromagnetic waves is a small-sized electric dipole, dipole moment p (t) which changes rapidly over time.

Such an elementary dipole is called Hertz dipole . In radio engineering, a Hertz dipole is equivalent to a small antenna, the size of which is much smaller than the wavelength λ (Fig. 2.6.4).

Rice. 2.6.5 gives an idea of ​​the structure of the electromagnetic wave emitted by such a dipole.

It should be noted that the maximum flow of electromagnetic energy is emitted in a plane perpendicular to the dipole axis. The dipole does not radiate energy along its axis. Hertz used an elementary dipole as a transmitting and receiving antenna to experimentally prove the existence of electromagnetic waves.