Electromagnetic waves and their propagation. Electromagnetic waves - properties and characteristics

In 1864, James Clerk Maxwell predicted the possibility of electromagnetic waves existing in space. He put forward this statement based on conclusions arising from the analysis of all experimental data known at that time regarding electricity and magnetism.

Maxwell mathematically unified the laws of electrodynamics, linking electrical and magnetic phenomena, and thus came to the conclusion that electric and magnetic fields changing over time generate each other.

Initially, he focused on the fact that the relationship between magnetic and electrical phenomena is not symmetrical, and introduced the term “vortex electric field”, offering his truly new explanation of the phenomenon of electromagnetic induction discovered by Faraday: “every change in the magnetic field leads to the appearance of the surrounding space of a vortex electric field having closed lines of force.”

According to Maxwell, the opposite statement was also true: “a changing electric field gives rise to a magnetic field in the surrounding space,” but this statement initially remained only a hypothesis.

Maxwell wrote down a system of mathematical equations that consistently described the laws of mutual transformations of magnetic and electric fields; these equations later became the basic equations of electrodynamics, and began to be called “Maxwell’s equations” in honor of the great scientist who wrote them down. Maxwell's hypothesis, based on the written equations, had several extremely important conclusions for science and technology, which are given below.

Electromagnetic waves really exist

Transverse electromagnetic waves can exist in space, which are propagating over time. The fact that the waves are transverse is indicated by the fact that the vectors of magnetic induction B and electric field strength E are mutually perpendicular and both lie in a plane perpendicular to the direction of propagation of the electromagnetic wave.

The speed of propagation of electromagnetic waves in a substance is finite, and it is determined by the electrical and magnetic properties of the substance through which the wave propagates. The length of the sinusoidal wave λ is related to the speed υ by a certain exact ratio λ = υ / f, and depends on the frequency f of the field oscillations. The speed c of an electromagnetic wave in a vacuum is one of the fundamental physical constants - the speed of light in a vacuum.

Since Maxwell declared the finite speed of propagation of an electromagnetic wave, this created a contradiction between his hypothesis and the theory of long-range action accepted at that time, according to which the speed of propagation of waves should be infinite. Maxwell's theory was therefore called the theory of short-range action.

In an electromagnetic wave, the transformation of electric and magnetic fields into each other simultaneously occurs, therefore the volumetric densities of magnetic energy and electrical energy are equal to each other. Therefore, it is true that the moduli of electric field strength and magnetic field induction are related to each other at each point in space by the following relationship:

An electromagnetic wave, in the process of its propagation, creates a flow of electromagnetic energy, and if we consider an area in a plane perpendicular to the direction of propagation of the wave, then in a short time a certain amount of electromagnetic energy will move through it. Electromagnetic energy flux density is the amount of energy transferred by an electromagnetic wave through the surface of a unit area per unit time. By substituting the values ​​of speed, as well as magnetic and electrical energy, we can obtain an expression for the flux density in terms of the values ​​of E and B.

Since the direction of propagation of wave energy coincides with the direction of the speed of wave propagation, the flow of energy propagating in an electromagnetic wave can be specified using a vector directed in the same way as the speed of wave propagation. This vector was called the “Poynting vector” - in honor of the British physicist Henry Poynting, who developed the theory of propagation of electromagnetic field energy flow in 1884. Wave energy flux density is measured in W/sq.m.

When an electric field acts on a substance, small currents appear in it, representing the ordered movement of electrically charged particles. These currents in the magnetic field of an electromagnetic wave are subject to the action of the Ampere force, which is directed deep into the substance. The Ampere force ultimately generates pressure.

This phenomenon was later, in 1900, studied and experimentally confirmed by the Russian physicist Pyotr Nikolaevich Lebedev, whose experimental work was very important for confirming Maxwell's theory of electromagnetism and its acceptance and approval in the future.

The fact that an electromagnetic wave exerts pressure allows one to judge that the electromagnetic field has a mechanical impulse, which can be expressed for a unit volume through the volumetric density of electromagnetic energy and the speed of wave propagation in vacuum:

Since momentum is associated with the movement of mass, it is possible to introduce such a concept as electromagnetic mass, and then for a unit volume this relationship (in accordance with STR) will take on the character of a universal law of nature, and will be valid for any material bodies, regardless of the form of matter. And the electromagnetic field is then akin to a material body - it has energy W, mass m, momentum p and a final speed of propagation v. That is, the electromagnetic field is one of the forms of matter that actually exists in nature.

For the first time in 1888, Heinrich Hertz experimentally confirmed Maxwell's electromagnetic theory. He experimentally proved the reality of electromagnetic waves and studied their properties such as refraction and absorption in various media, as well as reflection of waves from metal surfaces.

Hertz measured the wavelength and showed that the speed of propagation of an electromagnetic wave is equal to the speed of light. Hertz's experimental work was the last step towards the recognition of Maxwell's electromagnetic theory. Seven years later, in 1895, Russian physicist Alexander Stepanovich Popov used electromagnetic waves to create wireless communications.

In direct current circuits, charges move at a constant speed, and in this case electromagnetic waves are not emitted into space. For radiation to take place, it is necessary to use an antenna in which alternating currents are excited, that is, currents that quickly change their direction.

In its simplest form, an electric dipole of small size, whose dipole moment would quickly change with time, is suitable for emitting electromagnetic waves. It is precisely this kind of dipole that is called today the “Hertz dipole,” the size of which is several times smaller than the wavelength it emits.

When radiated by a Hertzian dipole, the maximum flow of electromagnetic energy falls on a plane perpendicular to the dipole axis. There is no radiation of electromagnetic energy along the dipole axis. In Hertz's most important experiments, elementary dipoles were used to both emit and receive electromagnetic waves, and the existence of electromagnetic waves was proven.

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 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 an induced current in it. From Maxwell's theory it follows that the reverse process is also possible. 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 the wavelength, its speed, period and frequency of oscillations obtained for mechanical waves is also true for electromagnetic waves:

Voltage vector fluctuations E and magnetic induction vector B occur in mutually perpendicular planes and perpendicular to the direction of wave propagation (velocity vector).

An electromagnetic wave transfers energy.

Electromagnetic wave range

Around us is a complex world of 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 visible light range are called infrared radiation. Infrared radiation is emitted by any heated body. The sources of infrared radiation are stoves, batteries, and incandescent electric lamps. 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 violet. 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.

Electromagnetic radiation with a wavelength shorter than violet, invisible to the eye, 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 an open oscillating 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.

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 the magnetic field changes, a vortex electric field appears.

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 electrical 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 speed of propagation. 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 of the alternating current passing through the 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 radio waves moving away 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 a certain point in time at the Moscow Broadcasting Station, 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 distinguishes them from sound waves, which absolutely require air or some other dense medium, such as water.

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 move them apart to the 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.

Associated with the antenna wire are both an electric field, the lines of force of which begin on positive charges 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.

Every time an electric current changes its frequency or direction, it generates electromagnetic waves - oscillations of electric and magnetic force fields in space. One example is the changing current in the antenna of a radio transmitter, which creates rings of radio waves propagating in space.

The energy of an electromagnetic wave depends on its length - the distance between two adjacent “peaks”. The shorter the wavelength, the higher its energy. In descending order of their length, electromagnetic waves are divided into radio waves, infrared radiation, visible light, ultraviolet, x-rays and gamma radiation. The wavelength of gamma radiation does not reach even one hundred billionth of a meter, while radio waves can have a length measured in kilometers.

Electromagnetic waves propagate in space at the speed of light, and the lines of force of their electric and magnetic fields are located at right angles to each other and to the direction of motion of the wave.

Electromagnetic waves radiate out in gradually widening circles from the transmitting antenna of a two-way radio station, similar to the way waves do when a pebble falls into a pond. The alternating electric current in the antenna creates waves consisting of electric and magnetic fields.

Electromagnetic wave circuit

An electromagnetic wave travels in a straight line, and its electric and magnetic fields are perpendicular to the flow of energy.

Refraction of electromagnetic waves

Just like light, all electromagnetic waves are refracted when they enter matter at any angle other than right angles.

Reflection of electromagnetic waves

If electromagnetic waves fall on a metal parabolic surface, they are focused at a point.

The rise of electromagnetic waves

the false pattern of electromagnetic waves emanating from a transmitting antenna arises from a single oscillation of electrical current. When current flows up the antenna, the electric field (red lines) is directed from top to bottom, and the magnetic field (green lines) is directed counterclockwise. If the current changes its direction, the same happens to the electric and magnetic fields.

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 influences (physiological, photochemical, photoelectric, etc.) are caused by oscillations of the 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.