Instructions
Take two batteries and connect them with electrical tape. Connect the batteries so that their ends are different, that is, the plus is opposite the minus and vice versa. Use paper clips to attach a wire to the end of each battery. Next, place one of the paper clips on top of the batteries. If the paperclip does not reach the center of each paperclip, it may need to be bent to the correct length. Secure the structure with tape. Make sure the ends of the wires are clear and the edge of the paperclip reaches the center of each battery. Connect the batteries from the top, do the same on the other side.
Take copper wire. Leave about 15 centimeters of the wire straight, and then start wrapping it around the glass cup. Make about 10 turns. Leave another 15 centimeters straight. Connect one of the wires from the power supply to one of the free ends of the resulting copper coil. Make sure the wires are well connected to each other. When connected, the circuit produces a magnetic field. Connect the other wire of the power supply to the copper wire.
When current flows through the coil, the coil placed inside will be magnetized. Paper clips will stick together, and parts of a spoon or fork or screwdriver will become magnetized and attract other metal objects while current is applied to the coil.
note
The coil may be hot. Make sure there are no flammable substances nearby and be careful not to burn your skin.
Helpful advice
The most easily magnetized metal is iron. When checking the field, do not select aluminum or copper.
In order to make an electromagnetic field, you need to make its source radiate. At the same time, it must produce a combination of two fields, electric and magnetic, which can propagate in space, generating each other. An electromagnetic field can propagate in space in the form of an electromagnetic wave.
You will need
- - insulated wire;
- - nail;
- - two conductors;
- - Ruhmkorff coil.
Instructions
Take an insulated wire with low resistance, copper is best. Wind it around a steel core; a regular nail 100 mm long (one hundred square meters) will do. Connect the wire to a power source; a regular battery will do. Electricity will arise field, which will generate an electric current in it.
Directed movement of charged (electric current) will in turn give rise to magnetic field, which will be concentrated in a steel core, with a wire wound around it. The core transforms and attracts ferromagnets (nickel, cobalt, etc.). The resulting field can be called electromagnetic, since electric field magnetic.
To obtain a classical electromagnetic field, it is necessary that both electric and magnetic field changed over time, then electrical field will generate magnetic and vice versa. To do this, moving charges need to be accelerated. The easiest way to do this is to make them hesitate. Therefore, to obtain an electromagnetic field, it is enough to take a conductor and plug it into a regular household network. But it will be so small that it will not be possible to measure it with instruments.
To obtain a sufficiently powerful magnetic field, make a Hertz vibrator. To do this, take two straight identical conductors and fasten them so that the gap between them is 7 mm. This will be an open oscillatory circuit, with low electrical capacity. Connect each of the conductors to Ruhmkorff clamps (it allows you to receive high voltage pulses). Connect the circuit to the battery. Discharges will begin in the spark gap between the conductors, and the vibrator itself will become a source of an electromagnetic field.
Video on the topic
The introduction of new technologies and the widespread use of electricity has led to the emergence of artificial electromagnetic fields, which most often have a harmful effect on humans and the environment. These physical fields arise where there are moving charges.
The nature of the electromagnetic field
The electromagnetic field is a special type of matter. It occurs around conductors along which electric charges move. The force field consists of two independent fields - magnetic and electric, which cannot exist in isolation from one another. When an electric field arises and changes, it invariably generates a magnetic field.
One of the first to study the nature of alternating fields in the middle of the 19th century was James Maxwell, who is credited with creating the theory of the electromagnetic field. The scientist showed that electric charges moving with acceleration create an electric field. Changing it generates a field of magnetic forces.
The source of an alternating magnetic field can be a magnet if it is set in motion, as well as an electric charge that oscillates or moves with acceleration. If a charge moves at a constant speed, then a constant current flows through the conductor, which is characterized by a constant magnetic field. Propagating in space, the electromagnetic field transfers energy, which depends on the magnitude of the current in the conductor and the frequency of the emitted waves.
Impact of electromagnetic field on humans
The level of all electromagnetic radiation created by man-made technical systems is many times higher than the natural radiation of the planet. This is a thermal effect that can lead to overheating of body tissues and irreversible consequences. For example, prolonged use of a mobile phone, which is a source of radiation, can lead to an increase in the temperature of the brain and the lens of the eye.
Electromagnetic fields generated when using household appliances can cause the appearance of malignant tumors. This especially applies to children's bodies. A person's prolonged presence near a source of electromagnetic waves reduces the efficiency of the immune system and leads to heart and vascular diseases.
Of course, it is impossible to completely abandon the use of technical means that are a source of electromagnetic fields. But you can use the simplest preventive measures, for example, use your phone only with a headset, and do not leave appliance cords in electrical outlets after using equipment. In everyday life, it is recommended to use extension cords and cables that have protective shielding.
Shmelev V.E., Sbitnev S.A.
"THEORETICAL FUNDAMENTALS OF ELECTRICAL ENGINEERING"
"ELECTROMAGNETIC FIELD THEORY"
Chapter 1. Basic concepts of electromagnetic field theory
§ 1.1. Definition of the electromagnetic field and its physical quantities.
Mathematical apparatus of the theory of electromagnetic field
Electromagnetic field(EMF) is a type of matter that exerts a force on charged particles and is determined at all points by two pairs of vector quantities that characterize its two sides - electric and magnetic fields.
Electric field- this is a component of EMF, which is characterized by the effect on an electrically charged particle with a force proportional to the charge of the particle and independent of its speed.
A magnetic field is a component of EMF, which is characterized by the effect on a moving particle with a force proportional to the charge of the particle and its speed.
The basic properties and methods of calculating EMFs studied in the course of theoretical foundations of electrical engineering involve a qualitative and quantitative study of EMFs found in electrical, electronic and biomedical devices. For this purpose, the equations of electrodynamics in integral and differential forms are most suitable.
The mathematical apparatus of electromagnetic field theory (TEMF) is based on scalar field theory, vector and tensor analysis, as well as differential and integral calculus.
Control questions
1. What is an electromagnetic field?
2. What are called electric and magnetic fields?
3. What is the mathematical apparatus of the electromagnetic field theory based on?
§ 1.2. Physical quantities characterizing EMF
Electric field strength vector at the point Q is the vector of force acting on an electrically charged stationary particle placed at a point Q, if this particle has a unit positive charge.
According to this definition, the electric force acting on a point charge q is equal to:
Where E measured in V/m.
The magnetic field is characterized vector of magnetic induction. Magnetic induction at some observation point Q is a vector quantity whose modulus is equal to the magnetic force acting on a charged particle located at a point Q, having a unit charge and moving with a unit speed, and the vectors of force, speed, magnetic induction, as well as the charge of the particle satisfy the condition
.
The magnetic force acting on a curved conductor carrying current can be determined by the formula
.
A straight conductor, if it is in a uniform field, is acted upon by the following magnetic force
.
In all the latest formulas B - magnetic induction, which is measured in teslas (T).
1 T is a magnetic induction in which a magnetic force equal to 1 N acts on a straight conductor with a current of 1A, if the lines of magnetic induction are directed perpendicular to the conductor with the current, and if the length of the conductor is 1 m.
In addition to the electric field strength and magnetic induction, the following vector quantities are considered in the theory of the electromagnetic field:
1) electrical induction D (electrical displacement), which is measured in C/m 2,
EMF vectors are functions of space and time:
Where Q- observation point, t- moment of time.
If the observation point Q is in a vacuum, then the following relations hold between the corresponding pairs of vector quantities
where is the absolute dielectric constant of vacuum (basic electrical constant), =8.85419*10 -12;
Absolute magnetic permeability of vacuum (basic magnetic constant); = 4π*10 -7 .
Control questions
1. What is electric field strength?
2. What is magnetic induction called?
3. What is the magnetic force acting on a moving charged particle?
4. What is the magnetic force acting on a current-carrying conductor?
5. What vector quantities are characterized by the electric field?
6. What vector quantities are characterized by a magnetic field?
§ 1.3. Electromagnetic field sources
Sources of EMF are electric charges, electric dipoles, moving electric charges, electric currents, magnetic dipoles.
The concepts of electric charge and electric current are given in the physics course. Electric currents are of three types:
1. Conduction currents.
2. Displacement currents.
3. Transfer currents.
Conduction current- the speed of passage of moving charges of an electrically conductive body through a certain surface.
Bias current- the rate of change of the electric displacement vector flow through a certain surface.
.
Transfer current characterized by the following expression
Where v - speed of transfer of bodies through the surface S; n - vector of the unit normal to the surface; - linear charge density of bodies flying through the surface in the direction of the normal; ρ - volume density of electric charge; ρ v - transfer current density.
Electric dipole called a pair of point charges + q And - q, located at a distance l from each other (Fig. 1).
A point electric dipole is characterized by the vector of the electric dipole moment:
Magnetic dipole called a flat circuit with electric current I. A magnetic dipole is characterized by the vector of the magnetic dipole moment
Where S - vector of the area of a flat surface stretched over a current-carrying circuit. Vector S directed perpendicular to this flat surface, and, when viewed from the end of the vector S , then movement along the contour in the direction coinciding with the direction of the current will occur counterclockwise. This means that the direction of the dipole magnetic moment vector is related to the direction of the current according to the right-hand screw rule.
Atoms and molecules of matter are electric and magnetic dipoles, therefore each point of a material type in the EMF can be characterized by the volumetric density of the electric and magnetic dipole moment:
P - electrical polarization of the substance:
M - magnetization of the substance:
Electrical polarization of matter is a vector quantity equal to the volumetric density of the electric dipole moment at some point of a real body.
Magnetization of a substance is a vector quantity equal to the volumetric density of the magnetic dipole moment at some point of a material body.
Electrical bias is a vector quantity, which for any observation point, regardless of whether it is in a vacuum or in matter, is determined from the relation:
(for vacuum or substance),
(for vacuum only).
Magnetic field strength- a vector quantity, which for any observation point, regardless of whether it is in a vacuum or in a substance, is determined from the relation:
,
where the magnetic field strength is measured in A/m.
In addition to polarization and magnetization, there are other volumetrically distributed sources of EMF:
- volumetric charge density ; ,
where the volumetric charge density is measured in C/m3;
- electric current density vector, whose normal component is equal to
More generally, the current flowing through an open surface S, is equal to the current density vector flux through this surface:
where the electric current density vector is measured in A/m 2.
Control questions
1. What are the sources of the electromagnetic field?
2. What is conduction current?
3. What is bias current?
4. What is transfer current?
5. What is an electric dipole and an electric dipole moment?
6. What is a magnetic dipole and magnetic dipole moment?
7. What is called the electrical polarization and magnetization of a substance?
8. What is called electrical displacement?
9. What is magnetic field strength called?
10. What is the volumetric density of electric charge and current density?
MATLAB Application Example
Task.
Given: Circuit with electric current I in space represents the perimeter of a triangle, the Cartesian coordinates of the vertices of which are given: x 1 , x 2 , x 3 , y 1 , y 2 , y 3 , z 1 , z 2 , z 3. Here the subscripts are the numbers of the vertices. The vertices are numbered in the direction of flow of electric current.
Required compose a MATLAB function that calculates the dipole magnetic moment vector of the loop. When compiling an m-file, it can be assumed that spatial coordinates are measured in meters, and current in amperes. Arbitrary organization of input and output parameters is allowed.
Solution
% m_dip_moment - calculation of the magnetic dipole moment of a triangular circuit with a current in space
% pm = m_dip_moment(tok,nodes)
% INPUT PARAMETERS
% tok - current in the circuit;
% nodes is a square matrix of the form ".", each row of which contains the coordinates of the corresponding vertex.
% OUTPUT PARAMETER
% pm is a row matrix of the Cartesian components of the magnetic dipole moment vector.
function pm = m_dip_moment(tok,nodes);
pm=tok*)]) det()]) det()])]/2;
% In the last statement, the triangle area vector is multiplied by the current
>> nodes=10*rand(3)
9.5013 4.8598 4.5647
2.3114 8.913 0.18504
6.0684 7.621 8.2141
>> pm=m_dip_moment(1,nodes)
13.442 20.637 -2.9692
In this case it worked P M = (13.442* 1 x + 20.637*1 y - 2.9692*1 z) A*m 2 if the current in the circuit is 1 A.
§ 1.4. Spatial differential operators in electromagnetic field theory
Gradient scalar field Φ( Q) = Φ( x, y, z) is a vector field defined by the formula:
,
Where V 1 - area containing the point Q; S 1 - closed surface bounding the area V 1 , Q 1 - point belonging to the surface S 1 ; δ - greatest distance from the point Q to points on the surface S 1 (max| Q Q 1 |).
Divergence vector field F (Q)=F (x, y, z) is called a scalar field, defined by the formula:
Rotor(vortex) vector field F (Q)=F (x, y, z) is a vector field defined by the formula:
rot F
= 
Nabla operator is a vector differential operator, which in Cartesian coordinates is defined by the formula:
Let's represent grad, div and rot through the nabla operator:
Let's write these operators in Cartesian coordinates:
; 
;
The Laplace operator in Cartesian coordinates is defined by the formula:
Second order differential operators:
Integral theorems
Gradient theorem 
;
Divergence theorem 
Rotor theorem 
In the theory of EMF, one more of the integral theorems is also used:
.
Control questions
1. What is called the scalar field gradient?
2. What is called the divergence of a vector field?
3. What is called the curl of a vector field?
4. What is the nabla operator and how are first-order differential operators expressed through it?
5. What integral theorems are true for scalar and vector fields?
MATLAB Application Example
Task.
Given: In the volume of a tetrahedron, the scalar and vector fields change according to a linear law. The coordinates of the tetrahedron vertices are specified by a matrix of the form [ x 1 , y 1 , z 1 ; x 2 , y 2 , z 2 ; x 3 , y 3 , z 3 ; x 4 , y 4 , z 4 ]. The values of the scalar field at the vertices are specified by the matrix [Ф 1 ; F 2; F 3; F 4]. The Cartesian components of the vector field at the vertices are specified by the matrix [ F 1 x, F 1y, F 1z; F 2x, F 2y, F 2z; F 3x, F 3y, F 3z; F 4x, F 4y, F 4z].
Define in the volume of the tetrahedron, the gradient of the scalar field, as well as the divergence and curl of the vector field. Write a MATLAB function for this.
Solution. Below is the text of the m-function.
% grad_div_rot - Calculate gradient, divergence and rotor... in the volume of a tetrahedron
% =grad_div_rot(nodes,scalar,vector)
% INPUT PARAMETERS
% nodes - matrix of coordinates of tetrahedron vertices:
% rows correspond to vertices, columns - coordinates;
% scalar - columnar matrix of scalar field values at the vertices;
% vector - matrix of vector field components at vertices:
% OUTPUT PARAMETERS
% grad - row matrix of Cartesian components of the gradient of the scalar field;
% div - the divergence value of the vector field in the volume of the tetrahedron;
% rot is a row matrix of the Cartesian components of the vector field rotor.
% In the calculations it is assumed that in the volume of the tetrahedron
% vector and scalar fields vary in space according to a linear law.
function =grad_div_rot(nodes,scalar,vector);
a=inv(); % Linear interpolation coefficient matrix
grad=(a(2:end,:)*scalar)."; % Gradient components of the scalar field
div=*vector(:); % Vector field divergence
rot=sum(cross(a(2:end,:),vector."),2).";
An example of running the developed m-function:
>> nodes=10*rand(4,3)
3.5287 2.0277 1.9881
8.1317 1.9872 0.15274
0.098613 6.0379 7.4679
1.3889 2.7219 4.451
>> scalar=rand(4,1)
>> vector=rand(4,3)
0.52515 0.01964 0.50281
0.20265 0.68128 0.70947
0.67214 0.37948 0.42889
0.83812 0.8318 0.30462
>> =grad_div_rot(nodes,scalar,vector)
0.16983 -0.03922 -0.17125
0.91808 0.20057 0.78844
If we assume that spatial coordinates are measured in meters, and vector and scalar fields are dimensionless, then in this example we get:
grad Ф = (-0.16983* 1 x - 0.03922*1 y - 0.17125*1 z) m -1 ;
div F = -1.0112 m -1 ;
rot F = (-0.91808*1 x + 0.20057*1 y + 0.78844*1 z) m -1 .
§ 1.5. Basic laws of electromagnetic field theory
EMF equations in integral form
Total current law:
or 
Circulation of the magnetic field strength vector along the contour l equal to the total electric current flowing through the surface S, stretched on the contour l, if the direction of the current forms a right-handed system with the direction of bypassing the circuit.
Law of electromagnetic induction:
,
Where E c is the intensity of the external electric field.
EMF electromagnetic induction e and in the circuit l equal to the rate of change of magnetic flux through the surface S, stretched on the contour l, and the direction of the rate of change of magnetic flux forms with the direction e and a left-handed screw system.
Gauss's theorem in integral form:
Electric displacement vector flow through a closed surface S equal to the sum of free electric charges in the volume limited by the surface S.
Law of continuity of magnetic induction lines:
The magnetic flux through any closed surface is zero.
Direct application of equations in integral form makes it possible to calculate the simplest electromagnetic fields. To calculate electromagnetic fields of more complex shapes, equations in differential form are used. These equations are called Maxwell's equations.
Maxwell's equations for stationary media
These equations follow directly from the corresponding equations in integral form and from the mathematical definitions of spatial differential operators.
Total current law in differential form:
,
Total electric current density,
Density of external electric current,
Conduction current density,
Bias current density: ,
Transfer current density: .
This means that the electric current is a vortex source of the vector field of magnetic field strength.
The law of electromagnetic induction in differential form:
This means that the alternating magnetic field is a vortex source for the spatial distribution of the electric field strength vector.
Equation of continuity of magnetic induction lines:
This means that the field of the magnetic induction vector has no sources, i.e. There are no magnetic charges (magnetic monopoles) in nature.
Gauss's theorem in differential form:
This means that the sources of the vector field of electric displacement are electric charges.
To ensure the uniqueness of the solution to the problem of EMF analysis, it is necessary to supplement Maxwell’s equations with equations of material connections between vectors E And D , and B And H .
Relationships between field vectors and electrical properties of the medium
It is known that
| (1) |
All dielectrics are polarized under the influence of an electric field. All magnets are magnetized under the influence of a magnetic field. The static dielectric properties of a substance can be completely described by the functional dependence of the polarization vector P from the electric field strength vector E (P =P (E )). The static magnetic properties of a substance can be completely described by the functional dependence of the magnetization vector M from the magnetic field strength vector H (M =M (H )). In the general case, such dependences are ambiguous (hysteretic) in nature. This means that the polarization or magnetization vector at a point Q is determined not only by the value of the vector E or H at this point, but also the background of the change in vector E or H at this point. It is extremely difficult to experimentally study and model these dependencies. Therefore, in practice it is often assumed that the vectors P And E , and M And H are collinear, and the electrical properties of a substance are described by scalar hysteresis functions (| P |=|P |(|E |), |M |=|M |(|H |). If the hysteresis characteristics of the above functions can be neglected, then the electrical properties are described by unambiguous functions P=P(E), M=M(H).
In many cases, these functions can be approximately considered linear, i.e.
Then, taking into account relation (1), we can write the following
| , | (4) |
Accordingly, the relative dielectric and magnetic permeability of the substance:
Absolute dielectric constant of a substance:
Absolute magnetic permeability of a substance:
Relations (2), (3), (4) characterize the dielectric and magnetic properties of the substance. The electrically conductive properties of a substance can be described by Ohm's law in differential form
where is the specific electrical conductivity of the substance, measured in S/m.
In a more general case, the relationship between the conduction current density and the electric field strength vector has a nonlinear vector-hysteresis character.
Electromagnetic field energy
The volumetric energy density of the electric field is equal to
,
Where W e is measured in J/m 3.
The volumetric energy density of the magnetic field is equal to
,
Where W m is measured in J/m 3.
The volumetric energy density of the electromagnetic field is equal to
In the case of linear electrical and magnetic properties of matter, the volumetric energy density of the EMF is equal to
This expression is valid for instantaneous values of specific energy and EMF vectors.
Specific power of heat losses from conduction currents
Power density of third party sources
Control questions
1. How is the law of total current formulated in integral form?
2. How is the law of electromagnetic induction formulated in integral form?
3. How are Gauss’s theorem and the law of magnetic flux continuity formulated in integral form?
4. How is the total current law formulated in differential form?
5. How is the law of electromagnetic induction formulated in differential form?
6. How are Gauss’s theorem and the law of continuity of magnetic induction lines formulated in integral form?
7. What relationships describe the electrical properties of a substance?
8. How is the energy of the electromagnetic field expressed through the vector quantities that determine it?
9. How is the specific power of heat losses and the specific power of third-party sources determined?
MATLAB Application Examples
Problem 1.
Given: Inside the volume of the tetrahedron, the magnetic induction and magnetization of the substance change according to a linear law. The coordinates of the vertices of the tetrahedron are given, the values of the vectors of magnetic induction and magnetization of the substance at the vertices are also given.
Calculate electric current density in the volume of the tetrahedron, using the m-function compiled when solving the problem in the previous paragraph. Perform the calculation in the MATLAB command window, assuming that spatial coordinates are measured in millimeters, magnetic induction in tesla, magnetic field strength and magnetization in kA/m.
Solution.
Let's set the initial data in a format compatible with the m-function grad_div_rot:
>> nodes=5*rand(4,3)
0.94827 2.7084 4.3001
0.96716 0.75436 4.2683
3.4111 3.4895 2.9678
1.5138 1.8919 2.4828
>> B=rand(4.3)*2.6-1.3
1.0394 0.41659 0.088605
0.83624 -0.41088 0.59049
0.37677 -0.54671 -0.49585
0.82673 -0.4129 0.88009
>> mu0=4e-4*pi % absolute magnetic permeability of vacuum, µH/mm
>> M=rand(4,3)*1800-900
122.53 -99.216 822.32
233.26 350.22 40.663
364.93 218.36 684.26
83.828 530.68 -588.68
>> =grad_div_rot(nodes,ones(4,1),B/mu0-M)
0 -3.0358e-017 0
914.2 527.76 -340.67
In this example, the vector of the total current density in the volume under consideration turned out to be equal to (-914.2* 1 x + 527.76*1 y - 340.67*1 z) A/mm 2 . To determine the modulus of the current density, we execute the following operator:
>> cur_d=sqrt(cur_dens*cur_dens.")
The calculated value of current density cannot be obtained in highly magnetized environments in real technical devices. This example is purely educational. Now let’s check the correctness of specifying the distribution of magnetic induction in the volume of the tetrahedron. To do this, we execute the following statement:
>> =grad_div_rot(nodes,ones(4,1),B)
0 -3.0358e-017 0
0.38115 0.37114 -0.55567
Here we got the div value B = -0.34415 T/mm, which cannot be in accordance with the law of continuity of magnetic induction lines in differential form. It follows from this that the distribution of magnetic induction in the volume of the tetrahedron is specified incorrectly.
Problem 2.
Let a tetrahedron, the coordinates of the vertices of which are given, be in the air (units of measurement are meters). Let the values of the electric field strength vector at its vertices be given (units of measurement - kV/m).
Required calculate the volumetric charge density inside the tetrahedron.
Solution can be done similarly:
>> nodes=3*rand(4,3)
2.9392 2.2119 0.59741
0.81434 0.40956 0.89617
0.75699 0.03527 1.9843
2.6272 2.6817 0.85323
>> eps0=8.854e-3% absolute dielectric constant of vacuum, nF/m
>> E=20*rand(4,3)
9.3845 8.4699 4.519
1.2956 10.31 11.596
19.767 6.679 15.207
11.656 8.6581 10.596
>> =grad_div_rot(nodes,ones(4,1),E*eps0)
0.076467 0.21709 -0.015323
In this example, the volumetric charge density was equal to 0.10685 µC/m 3.
§ 1.6. Boundary conditions for EMF vectors.
Law of conservation of charge. Umov-Poynting theorem
or 
Here it is indicated: H 1 - vector of magnetic field strength at the interface between media in medium No. 1; H 2 - the same in environment No. 2; H 1t- tangential (tangent) component of the magnetic field strength vector at the interface between media in medium No. 1; H 2t- the same in environment No. 2; E 1 vector of the total electric field strength at the interface between media in medium No. 1; E 2 - the same in environment No. 2; E 1 c - third-party component of the electric field strength vector at the interface between media in medium No. 1; E 2c - the same in environment No. 2; E 1t- tangential component of the electric field strength vector at the interface between media in medium No. 1; E 2t- the same in environment No. 2; E 1s t- tangential third-party component of the electric field strength vector at the interface between media in medium No. 1; E 2t- the same in environment No. 2; B 1 - vector of magnetic induction at the interface between media in medium No. 1; B 2 - the same in environment No. 2; B 1n- normal component of the magnetic induction vector at the interface between media in medium No. 1; B 2n- the same in environment No. 2; D 1 - electric displacement vector at the interface between media in medium No. 1; D 2 - the same in environment No. 2; D 1n- normal component of the electric displacement vector at the interface between media in medium No. 1; D 2n- the same in environment No. 2; σ is the surface density of the electric charge at the interface, measured in C/m2.
Law of conservation of charge
If there are no third-party current sources, then
,
and in the general case, i.e., the total current density vector has no sources, i.e., the total current lines are always closed
Umov-Poynting theoremThe volumetric power density consumed by a material point in an EMF is equal to
In accordance with identity (1)
This is the power balance equation for volume V. In the general case, in accordance with equality (3), the electromagnetic power generated by sources inside the volume V, goes to heat losses, to the accumulation of EMF energy and to radiation into the surrounding space through a closed surface that limits this volume.
The integrand in integral (2) is called the Poynting vector:
,
Where P measured in W/m2.
This vector is equal to the electromagnetic power flux density at some observation point. Equality (3) is a mathematical expression of the Umov-Poynting theorem.
Electromagnetic power emitted by the area V into the surrounding space is equal to the flux of the Poynting vector through a closed surface S, limiting the area V.
Control questions
1. What expressions describe the boundary conditions for the electromagnetic field vectors at the interfaces between media?
2. How is the law of conservation of charge formulated in differential form?
3. How is the law of conservation of charge formulated in integral form?
4. What expressions describe the boundary conditions for the current density at the interfaces?
5. What is the volumetric power density consumed by a material point in an electromagnetic field?
6. How is the electromagnetic power balance equation written for a certain volume?
7. What is a Poynting vector?
8. How is the Umov-Poynting theorem formulated?
MATLAB Application Example
Task.
Given: There is a triangular surface in space. The coordinates of the vertices are given. The values of the electric and magnetic field strength vectors at the vertices are also specified. The third-party component of the electric field strength is zero.
Required calculate the electromagnetic power passing through this triangular surface. Write a MATLAB function that performs this calculation. When calculating, assume that the positive normal vector is directed in such a way that if viewed from its end, the movement in increasing order of vertex numbers will occur counterclockwise.
Solution. Below is the text of the m-function.
% em_power_tri - calculation of electromagnetic power passing through
% triangular surface in space
% P=em_power_tri(nodes,E,H)
% INPUT PARAMETERS
% nodes is a square matrix of the form ",
% in each line of which the coordinates of the corresponding vertex are written.
% E - matrix of components of the electric field strength vector at the vertices:
% rows correspond to vertices, columns - Cartesian components.
% H - matrix of components of the magnetic field strength vector at the vertices.
% OUTPUT PARAMETER
% P - electromagnetic power passing through the triangle
% During calculations it is assumed that on the triangle
% field strength vectors change in space according to a linear law.
function P=em_power_tri(nodes,E,H);
% Calculate the double area vector of the triangle
S=)]) det()]) det()])];
P=sum(cross(E,(ones(3,3)+eye(3))*H,2))*S."/24;
An example of running the developed m-function:
>> nodes=2*rand(3,3)
0.90151 0.5462 0.4647
1.4318 0.50954 1.6097
1.7857 1.7312 1.8168
>> E=2*rand(3,3)
0.46379 0.15677 1.6877
0.47863 1.2816 0.3478
0.099509 0.38177 0.34159
>>H=2*rand(3,3)
1.9886 0.62843 1.1831
0.87958 0.73016 0.23949
0.6801 0.78648 0.076258
>> P=em_power_tri(nodes,E,H)
If we assume that spatial coordinates are measured in meters, the electric field strength vector is in volts per meter, and the magnetic field strength vector is in amperes per meter, then in this example the electromagnetic power passing through the triangle is equal to 0.18221 W.
An electromagnetic field is alternating electric and magnetic fields that generate each other.
The theory of the electromagnetic field was created by James Maxwell in 1865.
He theoretically proved that:
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.
If electric charges move with acceleration, then the electric field they create periodically changes and itself creates an alternating magnetic field in space, etc.
Sources of electromagnetic field can be:
- moving magnet;
- an electric charge moving with acceleration or oscillating (in contrast to a charge moving at a constant speed, for example, in the case of direct current in a conductor, a constant magnetic field is created here).
An electric field always exists around an electric charge, in any reference system, a magnetic field exists in the one relative to which the electric charges move.
An electromagnetic field exists in a reference frame relative to which electric charges move with acceleration.
TRY SOLVING
A piece of amber was rubbed against a cloth, and it became charged with static electricity. What kind of field can be found around motionless amber? Around a moving one?
A charged body is at rest relative to the surface of the earth. 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 a car?
What field appears around an electron if it: is at rest; moves 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 reference frame associated with one of the moving electrons?
ELECTROMAGNETIC WAVES
Electromagnetic waves are an electromagnetic field propagating in space with 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 (C = 300,000 km/s);
- these are transverse waves;
- these are traveling waves (transfer energy).
The source of electromagnetic waves are accelerated moving electrical charges.
Oscillations of electric charges are accompanied by electromagnetic radiation having a frequency equal to the frequency of charge oscillations.
ELECTROMAGNETIC WAVE SCALE
All the space around us is permeated with electromagnetic radiation. The sun, the bodies around us, and transmitter antennas emit electromagnetic waves, which, depending on their oscillation frequency, have different names.
Radio waves are electromagnetic waves (with a wavelength from more than 10000m to 0.005m), 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 wavelengths travel 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 heating radiators, 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. Infrared radiation is used for drying painted products, building walls, and wood.
Visible light includes radiation with wavelengths from approximately 770 nm to 380 nm, from red to violet light. The significance of this part of the spectrum of electromagnetic radiation in human life is extremely large, since a person receives almost all information about the world around him through vision. Light is a prerequisite for the development of green plants and, therefore, a 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 benign 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 - tanning. Discharge lamps are used as sources of ultraviolet radiation in medicine. The tubes of such lamps are made of quartz, transparent to ultraviolet rays; That's why these lamps are called quartz lamps.
X-rays (Ri) are invisible. They pass without significant absorption through significant layers of matter that are opaque to visible light. X-rays are detected by their ability to cause a certain glow in certain crystals and act on photographic film. The ability of X-rays to penetrate thick layers of substances is used to diagnose diseases of human internal organs.
Scientific and technological progress is accompanied by a sharp increase in the power of electromagnetic fields (EMF) created by man, which in some cases are hundreds and thousands of times higher than the level of natural fields.
The spectrum of electromagnetic oscillations includes waves of length from 1000 km to 0.001 µm and by frequency f from 3×10 2 to 3×10 20 Hz. The electromagnetic field is characterized by a set of vectors of electrical and magnetic components. Different ranges of electromagnetic waves have a common physical nature, but differ in energy, nature of propagation, absorption, reflection and effect on the environment and humans. The shorter the wavelength, the more energy the quantum carries.
The main characteristics of EMF are:
Electric field strength E, V/m.
Magnetic field strength N, A/m.
Energy flux density carried by electromagnetic waves I, W/m2.
The connection between them is determined by the dependence:
Energy connection I and frequencies f vibrations is defined as:
Where: f = s/l, a c = 3 × 10 8 m/s (speed of propagation of electromagnetic waves), h= 6.6 × 10 34 W/cm 2 (Planck’s constant).
In space. There are 3 zones surrounding the EMF source (Fig. 9):
A) Near zone(induction), where there is no wave propagation, no energy transfer, and therefore the electrical and magnetic components of EMF are considered independently. Zone R boundary< l/2p.
b) Intermediate zone(diffraction), where waves superimpose on each other, forming maxima and standing waves. Zone boundaries l/2p< R < 2pl. Основная характеристика зоны суммарная плотность потоков энергии волн.
V) Radiation zone(wave) with the boundary R > 2pl. There is wave propagation, therefore the characteristic of the radiation zone is the energy flux density, i.e. amount of energy incident per unit surface I(W/m2).
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Rice. 1.9. Zones of electromagnetic field existence
The electromagnetic field, as it moves away from the radiation sources, attenuates inversely proportional to the square of the distance from the source. In the induction zone, the electric field strength decreases in inverse proportion to the distance to the third power, and the magnetic field decreases in inverse proportion to the square of the distance.
Based on the nature of their impact on the human body, EMFs are divided into 5 ranges:
Power frequency electromagnetic fields (PFEMF): f < 10 000 Гц.
Electromagnetic radiation in the radio frequency range (RF EMR) f 10,000 Hz.
Electromagnetic fields of the radio frequency part of the spectrum are divided into four subranges:
1) f from 10,000 Hz to 3,000,000 Hz (3 MHz);
2) f from 3 to 30 MHz;
3) f from 30 to 300 MHz;
4) f from 300 MHz to 300,000 MHz (300 GHz).
Sources of industrial-frequency electromagnetic fields are high-voltage power lines, open distribution devices, all electrical networks and devices powered by 50 Hz alternating current. The danger of exposure to lines increases with increasing voltage due to an increase in the charge concentrated on the phase. The electric field strength in areas where high-voltage power lines pass can reach several thousand volts per meter. Waves in this range are strongly absorbed by the soil and at a distance of 50-100 m from the line, the voltage drops to several tens of volts per meter. With systematic exposure to EP, functional disturbances in the activity of the nervous and cardiovascular systems are observed. With increasing field strength in the body, persistent functional changes occur in the central nervous system. Along with the biological effect of the electric field, discharges can occur between a person and a metal object due to the body potential, which reaches several kilovolts if the person is isolated from the Earth.
Permissible levels of electric field strength at workplaces are established by GOST 12.1.002-84 “Electric fields of industrial frequency”. The maximum permissible level of EMF IF voltage is set at 25 kV/m. The permissible time spent in such a field is 10 minutes. Staying in an EMF IF with a voltage of more than 25 kV/m without protective equipment is not allowed, and staying in an EMF IF with a voltage of up to 5 kV/m is allowed throughout the entire working day. To calculate the permissible time of stay in the ED at voltages above 5 to 20 kV/m inclusive, the formula is used T = (50/E) - 2, where: T- permissible time of stay in the EMF IF, (hour); E- intensity of the electrical component of the EMF IF, (kV/m).
Sanitary standards SN 2.2.4.723-98 regulate the maximum permissible limits of the magnetic component of the EMF IF in the workplace. Magnetic component strength N should not exceed 80 A/m during an 8-hour stay in the conditions of this field.
The intensity of the electrical component of EMF IF in residential buildings and apartments is regulated by SanPiN 2971-84 “Sanitary standards and rules for protecting the population from the effects of the electric field created by overhead power lines of alternating current of industrial frequency.” According to this document, the value E should not exceed 0.5 kV/m inside residential premises and 1 kV/m in urban areas. The MPL standards for the magnetic component of EMF IF for residential and urban environments have not currently been developed.
RF EMR is used for heat treatment, metal smelting, radio communications, and medicine. The sources of EMF in industrial premises are lamp generators, in radio installations - antenna systems, in microwave ovens - energy leaks when the screen of the working chamber is damaged.
EMF RF exposure to the body causes polarization of atoms and molecules of tissues, orientation of polar molecules, the appearance of ionic currents in tissues, and heating of tissues due to the absorption of EMF energy. This disrupts the structure of electrical potentials, fluid circulation in the cells of the body, the biochemical activity of molecules, and the composition of the blood.
The biological effect of RF EMR depends on its parameters: wavelength, intensity and mode of radiation (pulsed, continuous, intermittent), the area of the irradiated surface, and the duration of irradiation. Electromagnetic energy is partially absorbed by tissues and converted into heat, local heating of tissues and cells occurs. RF EMR has an adverse effect on the central nervous system, causing disturbances in neuroendocrine regulation, changes in the blood, clouding of the lens of the eyes (exclusively 4 subbands), metabolic disorders.
Hygienic standardization of RF EMR is carried out in accordance with GOST 12.1.006-84 “Electromagnetic fields of radio frequencies. Permissible levels at workplaces and requirements for monitoring." EMF levels at workplaces are controlled by measuring the intensity of the electrical and magnetic components in the frequency range 60 kHz-300 MHz, and in the frequency range 300 MHz-300 GHz the energy flux density (PED) of EMF, taking into account the time spent in the irradiation zone.
For EMF radio frequencies from 10 kHz to 300 MHz, the strength of the electric and magnetic components of the field is regulated depending on the frequency range: the higher the frequencies, the lower the permissible value of the strength. For example, the electrical component of EMF for frequencies 10 kHz - 3 MHz is 50 V/m, and for frequencies 50 MHz - 300 MHz only 5 V/m. In the frequency range 300 MHz - 300 GHz, the radiation energy flux density and the energy load it creates are regulated, i.e. energy flow passing through a unit of irradiated surface during the action. The maximum value of energy flux density should not exceed 1000 μW/cm2. The time spent in such a field should not exceed 20 minutes. Staying in the field in a PES equal to 25 μW/cm 2 is allowed during an 8-hour work shift.
In urban and domestic environments, RF EMR regulation is carried out in accordance with SN 2.2.4/2.1.8-055-96 “Electromagnetic radiation in the radio frequency range”. In residential premises, the RF EMR PES should not exceed 10 μW/cm 2 .
In mechanical engineering, magnetic-pulse and electro-hydraulic processing of metals with a low-frequency pulse current of 5-10 kHz is widely used (cutting and crimping tubular blanks, stamping, cutting holes, cleaning castings). Sources pulse magnetic The fields at the workplace are open working inductors, electrodes, and current-carrying busbars. A pulsed magnetic field affects metabolism in brain tissue and endocrine regulatory systems.
Electrostatic field(ESP) is a field of stationary electric charges interacting with each other. ESP is characterized by tension E, that is, the ratio of the force acting in the field on a point charge to the magnitude of this charge. ESP intensity is measured in V/m. ESPs arise in power plants and in electrical processes. ESP is used in electrical gas cleaning and when applying paint and varnish coatings. ESP has a negative effect on the central nervous system; those working in the ESP zone experience headaches, sleep disturbances, etc. In ESP sources, in addition to biological effects, air ions pose a certain danger. The source of air ions is the corona that appears on the wires at voltage E>50 kV/m.
Acceptable tension levels ESPs are established by GOST 12.1.045-84 “Electrostatic fields. Permissible levels at workplaces and requirements for monitoring.” The permissible level of ESP tension is established depending on the time spent at the workplace. The ESP voltage level is set to 60 kV/m for 1 hour. When the ESP voltage is less than 20 kV/m, the time spent in the ESP is not regulated.
Main characteristics laser radiation are: wavelength l, (µm), radiation intensity, determined by the energy or power of the output beam and expressed in joules (J) or watts (W): pulse duration (sec), pulse repetition frequency (Hz) . The main criteria for the danger of a laser are its power, wavelength, pulse duration and radiation exposure.
According to the degree of danger, lasers are divided into 4 classes: 1 - output radiation is not dangerous to the eyes, 2 - direct and specularly reflected radiation is dangerous to the eyes, 3 - diffusely reflected radiation is dangerous to the eyes, 4 - diffusely reflected radiation is dangerous to the skin. .
The laser class according to the degree of danger of the generated radiation is determined by the manufacturer. When working with lasers, personnel are exposed to harmful and dangerous production factors.
The group of physical harmful and dangerous factors during laser operation includes:
Laser radiation (direct, diffuse, specular or diffusely reflected),
Increased laser power supply voltage,
Dustiness of the air in the working area with products of the interaction of laser radiation with the target, increased levels of ultraviolet and infrared radiation,
Ionizing and electromagnetic radiation in the working area, increased brightness of light from pulsed pump lamps and the risk of explosion of laser pumping systems.
Personnel servicing lasers are exposed to chemically hazardous and harmful factors, such as ozone, nitrogen oxides and other gases due to the nature of the production process.
The effect of laser radiation on the body depends on the radiation parameters (power, wavelength, pulse duration, pulse repetition rate, irradiation time and irradiated surface area), localization of the effect and characteristics of the irradiated object. Laser radiation causes organic changes in the irradiated tissues (primary effects) and specific changes in the body itself (secondary effects). When exposed to radiation, rapid heating of the irradiated tissue occurs, i.e. thermal burn. As a result of rapid heating to high temperatures, there is a sharp increase in pressure in the irradiated tissues, which leads to their mechanical damage. The effects of laser radiation on the body can cause functional disorders and even complete loss of vision. The nature of the damaged skin varies from mild to varying degrees of burns, up to necrosis. In addition to tissue changes, laser radiation causes functional changes in the body.
Maximum permissible levels of exposure are regulated by “Sanitary norms and rules for the design and operation of lasers” 2392-81. The maximum permissible levels of irradiation are differentiated taking into account the operating mode of the lasers. For each operating mode, section of the optical range, the remote control value is determined using special tables. Dosimetric monitoring of laser radiation is carried out in accordance with GOST 12.1.031-81. When monitoring, the power density of continuous radiation, the energy density of pulsed and pulse-modulated radiation and other parameters are measured.
Ultraviolet radiation - This is electromagnetic radiation invisible to the eye, occupying an intermediate position between light and x-ray radiation. The biologically active part of UV radiation is divided into three parts: A with a wavelength of 400-315 nm, B with a wavelength of 315-280 nm and C 280-200 nm. UV rays have the ability to cause a photoelectric effect, luminescence, the development of photochemical reactions, and also have significant biological activity.
UV radiation is characterized bactericidal and erythemal properties. Erythemal radiation power - this is a value that characterizes the beneficial effects of UV radiation on humans. The unit of erythemal radiation is taken to be Er, corresponding to a power of 1 W for a wavelength of 297 nm. Unit of erythemal illumination (irradiance) Er per square meter (Er/m2) or W/m2. Radiation dose Ner is measured in Er×h/m 2, i.e. This is the irradiation of a surface over a certain time. The bactericidal power of the UV radiation flux is measured in bact. Accordingly, the bactericidal irradiation is bact per m 2, and the dose is bact per hour per m 2 (bq × h/m 2).
Sources of UV radiation in production are electric arcs, autogenous flames, mercury-quartz burners and other temperature emitters.
Natural UV rays have a positive effect on the body. With a lack of sunlight, “light starvation”, vitamin D deficiency, weakened immunity, and functional disorders of the nervous system occur. At the same time, UV radiation from industrial sources can cause acute and chronic occupational eye diseases. Acute eye damage is called electroophthalmia. Erythema of the skin of the face and eyelids is often detected. Chronic lesions include chronic conjunctivitis, lens cataract, skin lesions (dermatitis, swelling with blistering).
Standardization of UV radiation carried out in accordance with “Sanitary standards for ultraviolet radiation in industrial premises” 4557-88. When normalizing, the radiation intensity is set in W/m 2. With an irradiation surface of 0.2 m2 for up to 5 minutes with a break of 30 minutes for a total duration of up to 60 minutes, the norm for UV-A is 50 W/m2, for UV-B 0.05 W/m2 and for UV -C 0.01 W/m2. With a total irradiation duration of 50% of the work shift and a single irradiation of 5 min, the norm for UV-A is 10 W/m2, for UV-B 0.01 W/m2 with an irradiation area of 0.1 m2, and irradiation UV-C is not allowed.
Details Category: Electricity and magnetism Published 06/05/2015 20:46 Views: 11962Under certain conditions, alternating electric and magnetic fields can generate each other. They form an electromagnetic field, which is not their totality at all. This is a single whole in which these two fields cannot exist without each other.
From the history
The experiment of the Danish scientist Hans Christian Oersted, carried out in 1821, showed that electric current generates a magnetic field. In turn, a changing magnetic field can generate electric current. This was proven 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”.
At that time, Newton's concept of long-range action was accepted in physics. It was believed that all bodies act on each other through the void at an infinitely high speed (almost instantly) and at any distance. It was assumed that electric charges interact in a similar way. Faraday believed that emptiness does not exist in nature, and interaction occurs at a finite speed through a certain material medium. This medium for electric charges is electromagnetic field. And it travels at a speed equal to the speed of light.
Maxwell's theory
By combining the results of previous studies, English physicist James Clerk Maxwell created in 1864 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, first one of the fields is created by a source of charges or currents. But in the future, these fields can already exist independently of such sources, causing each other to appear. That is, electric and magnetic fields are components of a single electromagnetic field. And every change in one of them causes the appearance of another. This hypothesis forms the basis of Maxwell's theory. The electric field generated by the magnetic field is a vortex. Its lines of force are closed.
This theory is phenomenological. This means that it is created based on assumptions and observations, and does not consider the cause of electric and magnetic fields.
Properties of the electromagnetic field
An electromagnetic field is a combination of electric and magnetic fields, therefore at each point in its space it is described by two main quantities: the electric field strength E and magnetic field induction IN .
Since the electromagnetic field is the process of converting an electric field into a magnetic field, and then magnetic into electric, its state is constantly changing. Propagating in space and time, it forms electromagnetic waves. Depending on the frequency and length, these waves are divided into radio waves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays.
The vectors of intensity and induction of the electromagnetic field are mutually perpendicular, and the plane in which they lie is perpendicular to the direction of propagation of the wave.
In the theory of long-range action, the speed of propagation of electromagnetic waves was considered infinitely large. However, Maxwell proved that this was not the case. In a substance, electromagnetic waves propagate at a finite speed, which depends on the dielectric and magnetic permeability of the substance. Therefore, Maxwell's Theory is called the theory of short-range action.
Maxwell's theory was 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 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 magnetic field
The flux of magnetic induction through a closed surface is zero.
The physical meaning of this law is that magnetic charges do not exist 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 changes in electrical induction generate a vortex magnetic field.
,
,
E– electric field strength;
N– magnetic field strength;
IN- magnetic induction. This is a vector quantity that shows the force with which the magnetic field acts on a charge of magnitude q moving with 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 influence of an external electric field relative to their position when there is no such field.
Δ - Operator Nabla. The action of this operator on 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– speed of light in vacuum.
The study of the electromagnetic field is a science called electrodynamics. She considers its interaction with bodies that have an electric charge. This interaction is called electromagnetic. Classical electrodynamics 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 particles-quanta that have no mass and charge. The electromagnetic field quantum is called photon .
Electromagnetic field around us
An electromagnetic field is formed around any conductor carrying alternating current. Sources of electromagnetic fields are power lines, electric motors, transformers, urban electric transport, railway transport, electrical and electronic household appliances - televisions, computers, refrigerators, irons, vacuum cleaners, radiotelephones, mobile phones, electric shavers - in a word, everything related to 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, electromagnetic fields surround us everywhere. These fields affect the environment and humans. This is not to say that this influence is always negative. Electric and magnetic fields have existed around humans for a long time, but the power of their radiation a few decades ago was hundreds of times lower than today.
Up to a certain level, electromagnetic radiation can be safe for humans. Thus, in medicine, low-intensity electromagnetic radiation is used to heal tissues, eliminate inflammatory processes, and have an analgesic effect. UHF devices relieve spasms of the smooth muscles of the intestines and stomach, improve metabolic processes in the body's cells, reducing capillary tone, and lower blood pressure.
But strong electromagnetic fields cause disruptions in the functioning of the human cardiovascular, immune, endocrine and nervous systems, and can cause insomnia, headaches, and stress. The danger is that their impact is almost invisible to humans, and disturbances occur gradually.
How can we protect ourselves from the electromagnetic radiation surrounding us? It is impossible to do this completely, so you need to try to minimize its impact. First of all, you need to arrange household appliances in such a way that they are located away from the places where we are most often. For example, don't sit too close to the TV. After all, the further the distance from the source of the electromagnetic field, the weaker it becomes. Very often we leave the device plugged in. But the electromagnetic field disappears only when the device is disconnected from the electrical network.
Human health is also affected by natural electromagnetic fields – cosmic radiation, the Earth’s magnetic field.