The direction of the electric field strength vector coincides with. Electric field strength. The principle of field superposition – Knowledge Hypermarket

1 .Two types of electric charges and their properties. The smallest indivisible electrical charge. Law of conservation of electric charges. Coulomb's law.

Unit of charge. Electrostatic field. Field detection method. - Tension as a characteristic of the electrostatic field. Tension vector, its direction.

Electric field strength of a point charge. Tension units. The principle of superposition of fields.

Electric charge

the quantity is invariant, i.e. does not depend on the reference frame, and therefore does not depend on whether the charge is moving or at rest.

two kinds (types) of electric charges : : positive charges and negative charges.

It has been experimentally established that like charges repel, and unlike charges attract.

An electrically neutral body must have an equal number of positive and negative charges, but their distribution throughout the volume of the body must be uniform. Law of conservation of el. charge algebraic sum of elec. charges of any closed system (a system that does not exchange charges with external heat) remains unchanged, no matter what processes occur within this system. -19 Elek. charges are not spontaneously created and do not arise, they can only be separated and transferred from one body to another.

Exists the forces of interaction between two point charges located in a homogeneous and isotropic medium are directly proportional to the product of these charges and inversely proportional to the square of the distance between them, are equal to each other and are directed in a straight line passing through these charges. g is the distance between charges q 1 and q 2, k is the proportionality coefficient, depending on the choice of the system of physical units.

m/F, a =8.85*10 -12 F/m - dielectric constant

A point charge should be understood as charges concentrated on bodies whose linear dimensions are small compared to the distances between them.

In this case, charge is measured in coulombs - the amount of electricity flowing through the cross-section of a conductor in one second at a current of 1 ampere.

The force F is directed along the straight line connecting the charges, i.e. is the central force and corresponding to attraction (F<0) в случае разноименных зарядов и отталкиванию (F>0) in the case of charges of the same name. This force is called Coulomb force.

Faraday's later studies showed that the electrical interactions between charged bodies depend on the properties of the medium in which these interactions occur.

It has long been established that electric charges do not directly affect each other. In the space surrounding all charged bodies, the action of an electric field is observed. Thus, interaction occurs between the fields located around the charges. Each field has a certain force with which it affects the charge. This ability is the main characteristic for everyone.

Determination of electric field parameters

The study of the electric field located around a charged object is carried out using the so-called test charge. As a rule, this is a point charge, the magnitude of which is very insignificant and cannot in any way noticeably affect the main charge under study.

To more accurately determine the quantitative parameters of the electric field, a special value was established. This power characteristic is named in the form of electric field strength.

Field strength is a stable physical quantity. Its value is equal to the ratio of the field strength acting on a positive test charge located at a specific point in space to the value of this test charge.

Tension vector - main characteristic

The main characteristic of the intensity is the electric field intensity vector. Thus, this characteristic is a vector physical quantity. At any spatial point, the tension vector is directed in the same direction as the force exerting influence on the positive test charge. Fixed charges, which do not change over time, have an electrostatic electric field.

In the case when an electric field created by several charged bodies at once is studied, its total force will consist of the geometric sum of the forces of each charged body acting on the test charge.

Consequently, the electric field strength vector consists of the total sum of the strength vectors of all fields created by individual charges at each point.

The electric field lines represent its visual graphic representation. The tension vector at each point is directed towards the tangent, located in relation to the lines of force. The number of power lines is proportional to the magnitude of the electric field strength vector.

Tension vector flow

If another charge is introduced into the space surrounding an electric charge, then the Coulomb force will act on it; This means that in the space surrounding electric charges, there is force field. According to the concepts of modern physics, the field really exists and, along with matter, is one of the forms of existence of matter, through which certain interactions are carried out between macroscopic bodies or particles that make up the substance. In this case, we talk about the electric field - the field through which electric charges interact. We consider electric fields that are created by stationary electric charges and are called electrostatic.

To detect and experimentally study the electrostatic field, it is used test point positive charge - such a charge that does not distort the field under study (does not cause a redistribution of charges creating the field). If in the field created by the charge Q, place a test charge Q 0, then a force acts on it F, different at different points of the field, which, according to Coulomb’s law, is proportional to the test charge Q 0 . Therefore the ratio F/ Q 0 does not depend on Q 0 and characterizes the electrostatic field at the point where the test charge is located. This quantity is called tension and is force characteristic of the electrostatic field.

Electrostatic field strength at a given point there is a physical quantity determined by the force acting on a test unit positive charge placed at this point in the field:

Field strength of a point charge in vacuum

The direction of vector E coincides with the direction of the force acting on the positive charge. If the field is created by a positive charge, then vector E is directed along the radius vector from the charge into external space (repulsion of the test positive charge); if the field is created by a negative charge, then vector E is directed towards the charge (Fig.).

The unit of electrostatic field strength is newton per coulomb (N/C): 1 N/C is the intensity of a field that acts on a point charge of 1 C with a force of 1 N; 1 N/C = 1 V/m, where V (volt) is the unit of electrostatic field potential. Graphically, the electrostatic field is represented using tension lines - lines, the tangents to which at each point coincide with the direction of vector E (Fig.).

Since at any given point in space the tension vector has only one direction, the tension lines never intersect. For uniform field(when the tension vector at any point is constant in magnitude and direction) the tension lines are parallel to the tension vector. If the field is created by a point charge, then the intensity lines are radial straight lines emerging from the charge if it is positive (Fig. A), and included in it if the charge is negative (Fig. b). Due to its great clarity, the graphical method of representing the electrostatic field is widely used in electrical engineering.

In order to use tension lines to characterize not only the direction, but also the value of the intensity of the electrostatic field, it was agreed to draw them with a certain density: the number of tension lines penetrating a unit surface area perpendicular to the tension lines must be equal to the modulus of the vector E. Then the number of tension lines , penetrating the elementary area d S, normal n which forms an angle a with the vector E, equals E d Scos a = E n d S, Where E p-vector projection E to normal n to site d S(rice.).

Value dФ E =E n dS= E dS is called tension vector flow through platform d S. Here d S=d Sn- a vector whose modulus is d S, and the direction coincides with the direction of the normal n to the site. Selecting the vector direction n(and therefore d S) is conditional, since it can be directed in any direction. The unit of flux of the electrostatic field strength vector is 1 V×m.

For an arbitrary closed surface S vector flow E through this surface

,

where the integral is taken over the closed surface S. Flow vector E is algebraic quantity: depends not only on the field configuration E, but also on the choice of direction n. For closed surfaces, the positive direction of the normal is taken to be outer normal, that is, the normal pointing outward to the area covered by the surface.

The principle of independence of force action is applied to Coulomb forces, i.e. the resulting force F acting from the field on the test charge Q 0 is equal to the vector sum of the forces Fi applied to it from each of the charges Q i: . F = Q 0 E and F i = Q 0 E i , where E is the strength of the resulting field, and E i is the strength of the field created by the charge Q i . Substituting this into the expression above, we get . This formula expresses the principle of superposition (imposition) of electrostatic fields, according to which the strength E of the resulting field created by a system of charges is equal to the geometric sum of the field strengths created at a given point by each of the charges separately.

The principle of superposition is applicable to calculate the electrostatic field of an electric dipole. An electric dipole is a system of two opposite point charges of equal magnitude (+Q, –Q), the distance l between which is significantly less than the distance to the field points under consideration. According to the principle of superposition, the strength E of the dipole field at an arbitrary point , where E+ and E– are the field strengths created by positive and negative charges, respectively.

>>Physics: Electric field strength. Principle of field superposition

It is not enough to assert that an electric field exists. It is necessary to introduce a quantitative characteristic of the field. After this, electric fields can be compared with each other and their properties can continue to be studied.
An electric field is detected by the forces acting on a charge. It can be argued that we know everything we need about the field if we know the force acting on any charge at any point in the field.
Therefore, it is necessary to introduce a characteristic of the field, knowledge of which will allow us to determine this force.
If you alternately place small charged bodies at the same point in the field and measure the forces, you will find that the force acting on the charge from the field is directly proportional to this charge. Indeed, let the field be created by a point charge q 1. According to Coulomb's law (14.2) on the charge q 2 there is a force proportional to the charge q 2. Therefore, the ratio of the force acting on a charge placed at a given point in the field to this charge for each point in the field does not depend on the charge and can be considered as a characteristic of the field. This characteristic is called electric field strength. Like force, field strength is vector quantity; it is denoted by the letter . If a charge placed in a field is denoted by q instead of q 2, then the tension will be equal to:

The field strength at a given point is equal to the ratio of the force with which the field acts on a point charge placed at this point to this charge.
Hence the force acting on the charge q from the electric field side, is equal to:

The direction of the vector coincides with the direction of the force acting on the positive charge and is opposite to the direction of the force acting on the negative charge.
Field strength of a point charge. Let's find the electric field strength created by a point charge q 0. According to Coulomb's law, this charge will act on a positive charge q with a force equal to

Field strength modulus of a point charge q 0 on distance r it is equal to:

The intensity vector at any point of the electric field is directed along the straight line connecting this point and the charge ( Fig.14.7) and coincides with the force acting on a point positive charge placed at a given point.

Principle of field superposition. If several forces act on a body, then, according to the laws of mechanics, the resulting force is equal to the geometric sum of these forces:

Electric charges are acted upon by forces from the electric field. If, when fields from several charges are superimposed, these fields do not have any influence on each other, then the resulting force from all fields must be equal to the geometric sum of the forces from each field. Experience shows that this is exactly what happens in reality. This means that the field strengths add up geometrically.
if at a given point in space various charged particles create electric fields whose strengths etc., then the resulting field strength at this point is equal to the sum of the strengths of these fields:

Moreover, the field strength created by an individual charge is determined as if there were no other charges creating the field.
Thanks to the principle of superposition, to find the field strength of a system of charged particles at any point, it is enough to know expression (14.9) for the field strength of a point charge. Figure 14.8 shows how the field strength at a point is determined A, created by two point charges q 1 And q 2 , q 1 >q 2

The introduction of an electric field allows us to divide the problem of calculating the interaction forces of charged particles into two parts. First, the field strength created by the charges is calculated, and then the forces are determined from the known strength. This division of the problem into parts usually makes force calculations easier.

???
1. What is the electric field strength called?
2. What is the field strength of a point charge?
3. How is the charge field strength q 0 directed if q 0>0 ? If q 0<0 ?
4. How is the principle of field superposition formulated?

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade

Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for the year; methodological recommendations; discussion program Integrated Lessons

If you have corrections or suggestions for this lesson,