A short course of lectures on electrostatics. What can electrostatics do? Formulas for working in electrostatics
Where F- modulus of the force of interaction of two point charges of magnitude q 1 and q 2 , r- distance between charges, - dielectric constant of the medium, 0 - dielectric constant.
Electric field strength
Where
- force acting on a point charge q 0
, placed at a given point in the field.
Field strength of a point charge (modulo)
Where r- distance from charge q to the point at which tension is determined.
Field strength created by a system of point charges (principle of superposition of electric fields)
Where
- intensity at a given point of the field created by the i-th charge.
Modulus of the field strength created by an infinite uniformly charged plane:
Where 
- surface charge density.
Field strength modulus of a flat capacitor in its middle part
.
The formula is valid if the distance between the plates is much less than the linear dimensions of the capacitor plates.
Tension field created by an infinitely long uniformly charged thread (or cylinder) at a distance r from the thread or cylinder axis modulo:
,
Where 
- linear charge density.
a) through an arbitrary surface placed in a non-uniform field
,
Where
- angle between the tension vector
and normal 
to a surface element, dS- area of the surface element, E n- projection of the tension vector onto the normal;
b) through a flat surface placed in a uniform electric field:
,
c) through a closed surface:
,
where integration is carried out over the entire surface.
Gauss's theorem. Flow of a tension vector through any closed surface S q 1 , q 2 ... q n equal to the algebraic sum of charges 0 .
.
, covered by this surface, divided by
The flux of the electric displacement vector is expressed similarly to the flux of the electric field strength vector:
a) flow through a flat surface if the field is uniform
,
Where b) in the case of a non-uniform field and an arbitrary surface n D 
- vector projection dS.
to the direction of the normal to a surface element whose area is equal to Flow of a tension vector through any closed surface Gauss's theorem. q 1 , q 2 ... q n Electrical induction vector flow through a closed surface
,
Where n, covering charges
, is equal - the number of charges contained inside a closed surface (charges with their own sign). Potential energy of a system of two point charges q Q And provided that
W
,
Where r- distance between charges. Potential energy is positive when like charges interact and negative when unlike charges interact.
Electric field potential created by a point charge - the number of charges contained inside a closed surface (charges with their own sign). on distance r
=
,
Electric field potential created by a metal sphere of radius R, carrying charge - the number of charges contained inside a closed surface (charges with their own sign).:
=
(r ≤ R; field inside and on the surface of the sphere),
=
(r
>
R; field outside the sphere).
Electric field potential created by the system n point charges in accordance with the principle of superposition of electric fields is equal to the algebraic sum of potentials 1 , 2 ,…, n, created by charges q 1 , q 2 , ..., q n at a given point in the field
=
.
Relationship between potentials and tension:
a) in general 
= -qrad
or 
=
;
b) in the case of a uniform field
E
= 
,
Where d- distance between equipotential surfaces with potentials 1 And 2 along the power line;
c) in the case of a field with central or axial symmetry 
where is the derivative 
is taken along the force line.
Work done by field forces to move a charge q from point 1 to point 2
A = q( 1 - 2 ),
Where ( 1 - 2 ) is the potential difference between the starting and ending points of the field.
The potential difference and the electric field strength are related by the relations
(
1
-
2
)
=
,
Where E e- projection of the tension vector 
to the direction of movement dl.
The electrical capacity of an isolated conductor is determined by the charge ratio q on the conductor to the conductor potential .
.
Capacitance of the capacitor:
,
Where ( 1 - 2 ) = U- potential difference (voltage) between the capacitor plates; q- charge module on one capacitor plate.
Electrical capacity of a conducting ball (sphere) in SI
c = 4 0 R,
Where R- radius of the ball, - relative dielectric constant of the medium; 0 = 8.8510 -12 F/m.
Electrical capacity of a flat capacitor in the SI system:
,
Where Flow of a tension vector through any closed surface- area of one plate; d- distance between the plates.
Electrical capacity of a spherical capacitor (two concentric spheres with radii R 1 Potential energy of a system of two point charges R 2 , the space between which is filled with a dielectric, with a dielectric constant ):
.
Electrical capacity of a cylindrical capacitor (two coaxial cylinders length l and radii R 1 Potential energy of a system of two point charges R 2 , the space between which is filled with a dielectric with dielectric constant )
.
Battery capacity from n capacitors connected in series is determined by the relation
.
The last two formulas are applicable to determine the capacitance of multilayer capacitors. The arrangement of the layers parallel to the plates corresponds to the series connection of single-layer capacitors; if the boundaries of the layers are perpendicular to the plates, then it is considered that there is a parallel connection of single-layer capacitors.
Potential energy of a system of stationary point charges
.
Here i- potential of the field created at the point where the charge is located q i, all charges except i-go; n- total number of charges.
Volumetric electric field energy density (energy per unit volume):
=
=
=
,
Where b) in the case of a non-uniform field and an arbitrary surface- the magnitude of the electrical displacement vector.
Uniform field energy:
W= V.
Non-uniform field energy:
W
.
Federal Agency for Education State Educational Institution of Higher Professional Education Tula State Pedagogical University
named after L. N. Tolstoy
Yu. V. Bobylev V. A. Panin R. V. Romanov
GENERAL PHYSICS COURSE
electrodynamics
Short course of lectures
Approved by the Educational and Methodological Association
in areas of teacher education of the Ministry of Education and Science of the Russian Federation as a teaching aid
for students of higher educational institutions studying in the direction 540200 (050200)
"Physics and mathematics education"
Tula Publishing House TSPU im. L. N. Tolstoy
BBK 22.3ya73 B72
Reviewer –
Professor Yu. F. Golovnev (Tashkent State Pedagogical University named after L. N. Tolstoy)
Bobylev, Yu. V.
B72 General physics course. Electrodynamics: A short course of lectures / Yu. V. Bobylev, V. A. Panin, R. V. Romanov. – Tula: Tula Publishing House. state ped. unta im. L. N. Tolstoy, 2007.– 107 p.
This textbook is a short lecture course on electromagnetism and contains the necessary material that fully complies with the State educational standard.
The manual is intended mainly for students who, for one reason or another, cannot attend or attend classroom classes irregularly and are engaged in self-education, including distance learning.
By reducing the mathematical part, the manual can be positioned for students of non-physical specialties.
© Yu. V. Bobylev, V. A. Panin, R. V. Romanov,
© Publishing house TSPU im. L. N. Tolstoy,
Preface........................................................ ........................................... |
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Introduction........................................................ ................................................... |
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Lecture 1. Electric charge.................................................... .............. |
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Lecture 2. Coulomb's Law.................................................... ........................... |
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Lecture 4. Gauss's theorem.................................................... ........................ |
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Lecture 5. Electric field potential.................................................. |
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Lecture 6. Electric field potential (continued)................... |
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Lecture 7. Conductors in an electric field................................................. |
|
Lecture 8. Dielectrics in an electric field................................................. |
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Lecture 9. Electric capacitance. Capacitors........................ |
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Lecture 10. Electrostatic energy.................................................... |
|
Lecture 11. Direct current. Basic concepts and laws.. ............ |
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Lecture 12. Electric circuits.................................................... .............. |
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Lecture 13 Current in metals.................................................... ........................ |
|
Lecture 14. Current in a vacuum.................................................... ......................... |
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Lecture 15. Current in gases. ........................................................ ........................ |
|
Lecture 16. Current in electrolytes. ........................................................ ......... |
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Lecture 17. Basic laws of magnetism. ......................................... |
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Lecture 18. Basic laws of magnetism (continued)................ |
|
Lecture 19. Movement of charged particles in a magnetic field.......... |
|
Lecture 20 Electromagnetic induction. ............................................ |
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Lecture 21. Electric oscillatory circuit.................................... |
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Lecture 22. Alternating current.................................................... .................... |
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Lecture 23. Electric field.................................................... .............. |
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Lecture 24. Maxwell's equations.................................................... .......... |
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Lecture 25. Electromagnetic waves.................................................... .... |
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Conclusion................................................. ............................................ |
|
Literature................................................. ........................................... |
Preface
The authors of this manual work at the Faculty of Mathematics, Physics and Computer Science of Tula State Pedagogical University. L.N. Tolstoy and have already repeatedly taught various disciplines and special courses related to electromagnetic processes, including phenomena in nonequilibrium material media, as part of courses in general and theoretical physics.
Teaching experience, formed by significant work experience (from 20 to 25 years), suggested the concept of creating a single end-to-end course in electrodynamics. It should include, without duplication or repetition, which is quite important, all the topics studied in the courses of general and theoretical physics, such as “Electricity and Magnetism”, “Electrodynamics and the Fundamentals of SRT”, “Electrodynamics of Continuum Media” and so on.
Such a course will allow you to maintain a unified style of presentation and design, the same notation, a unified system of units, and a similar use of mathematical apparatus, which will certainly simplify the perception of this difficult material by students.
It should be noted that the scientific interests of the authors lie in the areas of electrodynamics of highly nonequilibrium plasma, nonlinear phenomena in electrodynamic systems and structures of various natures, certain issues of plasma electronics and radiophysics, which, of course, makes this manual as close as possible to modern scientific achievements.
The implementation of this concept began in 2002 with the release of a textbook for the course “Electricity and magnetism: a course of lectures. Part 1. Electrostatics", which was approved by the Ministry of Education as a teaching aid for students of physics and mathematics.
Teaching using this manual has shown its undoubted effectiveness and demand among students. In 2004, a collection of problems for the course “Electricity and Magnetism” was published. The preparation of these materials in WEB document format made it possible to use them not only for full-time students, but also for distance learning.
In this manual, a more concise “telegraphic” style of presentation is used, and the language, generally speaking, is far from academic and is as close as possible to colloquial, as, in fact, it should be, since the material is a record of what the student heard and saw at the lecture.
A large number of drawings are used, which, however, are schematic and simplified. Some complex formulas are given with detailed conclusions, which will be especially valuable for students graduating from rural schools. In addition, according to the authors, the manual contains a significant number of examples of problem solutions that make it easier to understand
theoretical material and contributing to the development of practical skills of the future teacher.
IN The International System of Units (SI) is used as the main one.
IN In general, the material corresponds to the minimum specified in the State Educational Standard for Higher Professional Education and the curriculum.
The authors believe that this textbook on electromagnetism will help students who, for one reason or another (we will consider it valid) cannot attend or attend classroom classes irregularly and are engaged in self-education. There are more and more such students, but getting them to read traditional textbooks and scrupulously select the necessary information from them, taking into account the realities of the present time, is very problematic. This manual contains the necessary already selected material that fully complies with the State Educational Standard, so that the average student receives a positive mark on the exam without the use of additional literature.
For students who want to gain deeper knowledge and plan to continue their studies in a master's program, at the end of this manual there is a fairly comprehensive list of useful literature.
You should not think that this manual is only suitable for lagging students. It is intended for all students, the only difference being that a student who attended the lecture and a student who missed the lecture will have to work with this manual in different ways.
Moreover, in the context of the transition to two-level education and in the conditions of increasing penetration and implementation of the basic ideas of the Bologna process, such manuals, which on the one hand are sufficiently unified to meet the strict requirements of the state standard, and on the other hand, have an undoubted “stamp” of individuality and creative views authors will be more and more in demand on the “student market”.
It should also be noted that this manual, while shortening the mathematical part, can be positioned for students of non-physical specialties.
Tula, April 2007
Introduction
1. Electrodynamics as a science
Definition: Electrodynamics– a science that studies the behavior of the electromagnetic field that interacts between electric charges.
2. Historical background
Here you can cite almost the entire course on the history of physics, to which we refer you.
3. Theory of long- and short-range action
For a long time, physics was dominated by the theory of long-range action, which, based on mathematical laws, described the interaction of bodies without indicating the mechanism of this interaction. This is due to the fact that Newton’s well-formulated laws perfectly described all mechanical phenomena, without themselves being subject to any explanation. The mechanical approach extended to other branches of physics (Coulomb's law). The works of Ostrogradsky, Gauss, Laplace, etc. this theory acquired a complete mathematical form. At the same time, scientists were concerned about the question of how and with what help the interaction is transmitted. Faraday introduced the concept of a field, which is the carrier of interaction. For a long time, theories existed equally.
In quasi-static fields they lead to the same results. And only after the experiments of Hertz and Popov with rapidly varying fields was the question clearly resolved in favor of the theory of short-range action. It is believed that interactions between charges are carried out using an electromagnetic field that propagates in space. In a vacuum the field propagates at a speed
c=299792458 m/s≈3.00·108 m/s.
Electric charge
1. General concepts
Definition: Electric charge is a physical quantity that determines the electromagnetic field through which the interaction between charges occurs.
Despite the various ways to obtain a charge, there are only two types of electricity: “glass” and “resin” (“+” and “–”). Although there is an opinion that in fact this is an excess or lack of electricity of one kind, namely negative. In nature, the amount of positive electricity is approximately equal to the amount of negative electricity.
2. Methods for obtaining electrified bodies
3. Charge measurement
Definition: A test charge is a charge that does not introduce distortion into the existing field.
Let there be some electric field. We place a test charge at some point in the field. The field will act on it with some force.
We introduce another test charge into this field. If the forces are directed in one direction, then the charges are of the same name; if not, then they are opposite.
F 1 = F 2 q 1 q 2
F 1 = const = q 1 F 2 q 2
Knowing the ratio of forces, we also know the ratio of charges, and, taking one of the charges as a standard, we indicate the fundamental method for measuring charges.
4. Charge unit
Definition: 1 Coulomb is an SI unit of electric charge equal to the charge flowing through the cross-section of a conductor in 1 s at a constant current of 1 A.
5. Law of conservation of charge
If an energetic photon falls on a closed system, a paired electric charge can be created. In total, the charge of the system will not change. All experiments show that charge has the inherent property of being conserved, so this position is elevated to the rank of a postulate.
Law: In a closed system, electric charge is a constant quantity.
∑ qi = const.
i= 1
6. Charge the Earth
The Earth's charge is negative.
q = − 6 105 C .
7. Charge invariance
Basically, charges are measured by comparing forces. Force is an invariant, i.e. it is the same in different reference systems. Therefore, the charge ratio is also invariant. And if the charge standard is the same, then we can say that the charge has the same quantitative value in different reference systems.
8. Charge discreteness
Any charge can be represented in the form
q = N e , N = 0, ± 1, ± 2, ...
|e| = 1.6021892(46)·10-19 C - elementary charge
Electric charge is said to be discrete or quantized, i.e. There is a certain minimum portion of charge that cannot be divided further.
9. Models of charged bodies
As a rule, it is believed that the charge is continuously “smeared” over the body and the concepts of physically infinitesimal charge and volume are introduced.
<< dV
< |
10− 27 |
÷ 10 |
− 30 m 3 ; |
<< dq << Q ; |
||||||||||||
Bulk Density |
Superficial |
Linear density |
||||||||||||||
density |
||||||||||||||||
ρ = |
= ρ(x, y, z) |
σ = dq |
τ = dq |
|||||||||||||
Q = ∫ ρ (x, y, z) dV |
Q = ∫ σ dS |
Q = ∫ τ dl |
||||||||||||||
V body |
S body |
L body |
||||||||||||||
10. Point charge
Definition: Point charge is called a material point that has a charge.
The point charge density can be written as a formula;
ρ (r) = q δ (r − r 0 ).
Here r 0 is the radius vector that determines the position of the point charge; δ (r − r 0 )
– Dirac delta function.
11. Delta function or Dirac function.
In the one-dimensional case, this function is defined as follows:
0, x ≠ 0 |
||
∫ δ (x) dx = 1 |
||
δ(x) = ∞, x = 0 |
||
It also follows that
Electrostatic (or Coulomb) repulsion occurs between similarly charged bodies, and electrostatic attraction occurs between oppositely charged bodies. The phenomenon of repulsion of like charges underlies the creation of an electroscope - a device for detecting electric charges.
Electrostatics is based on Coulomb's law. This law describes the interaction of point electric charges.
The foundation of electrostatics was laid by the work of Coulomb (although ten years before him, the same results, even with even greater accuracy, were obtained by Cavendish. The results of Cavendish’s work were kept in the family archive and were published only a hundred years later); the law of electrical interactions discovered by the latter made it possible for Green, Gauss and Poisson to create a mathematically elegant theory. The most essential part of electrostatics is the potential theory created by Green and Gauss. A lot of experimental research on electrostatics was carried out by Rees, whose books in the past constituted the main guide for the study of these phenomena.
Faraday's experiments, carried out in the first half of the thirties of the 19th century, should have entailed a radical change in the basic principles of the doctrine of electrical phenomena. These experiments indicated that what was considered to be completely passively related to electricity, namely, insulating substances or, as Faraday called them, dielectrics, is of decisive importance in all electrical processes and, in particular, in the electrification of conductors itself. These experiments revealed that the substance of the insulating layer between the two surfaces of the capacitor plays an important role in the value of the electrical capacitance of that capacitor. Replacing air, as an insulating layer between the surfaces of a capacitor, with some other liquid or solid insulator has the same effect on the electrical capacity of the capacitor as a corresponding reduction in the distance between these surfaces while maintaining air as an insulator. When replacing a layer of air with a layer of another liquid or solid dielectric, the electrical capacity of the capacitor increases by K times. This value of K is called by Faraday the inductive capacity of a given dielectric. Today, the value K is usually called the dielectric constant of this insulating substance.
The same change in electrical capacitance occurs in each individual conducting body when this body is transferred from air to another insulating medium. But a change in the electrical capacity of a body entails a change in the amount of charge on this body at a given potential on it, and also vice versa, a change in the potential of the body at a given charge. At the same time, it changes the electrical energy of the body. So, the importance of the insulating medium in which the electrified bodies are placed or which separates the surfaces of the capacitor is extremely significant. The insulating substance not only retains the electrical charge on the surface of the body, it affects the electrical state of the latter itself. This is the conclusion to which Faraday's experiments led. This conclusion was quite consistent with Faraday's basic view of electrical actions.
According to Coulomb's hypothesis, electrical actions between bodies were considered as actions that occur at a distance. It was assumed that two charges q and q", mentally concentrated at two points separated from each other by a distance r, repel or attract each other along the direction of the line connecting these two points, with a force determined by the formula
Moreover, the coefficient C depends solely on the units used to measure the quantities q, r and f. The nature of the medium within which these two points with charges q and q are located was assumed to be of no importance and does not affect the value of f. Faraday had a completely different view of this. In his opinion, an electrified body only exerts an apparent effect on another body , located at some distance from it; in fact, the electrified body only causes special changes in the insulating medium in contact with it, which are transmitted in this medium from layer to layer, finally reaching the layer directly adjacent to the other body under consideration and producing something there. , which seems to be the direct action of the first body on the second through the medium separating them. With this view of electrical actions, Coulomb’s law, expressed by the above formula, can only serve to describe what observation gives, and does not in any way express the true process occurring in this case. It becomes clear that in general electrical actions change when the insulating medium changes, since in this case the deformations that occur in the space between two electrified bodies apparently acting on each other should also change. Coulomb's law, so to speak, which describes the phenomenon externally, must be replaced by another, which includes a characteristic of the nature of the insulating medium. For an isotropic and homogeneous medium, Coulomb’s law, as further research has shown, can be expressed by the following formula:
Here K denotes what is called above the dielectric constant of a given insulating medium. The value of K for air is equal to unity, i.e. for air, the interaction between two points with charges q and q" is expressed as Coulomb accepted it.
According to Faraday’s basic idea, the surrounding insulating medium or, better, those changes (polarization of the medium) that appear in the ether filling this medium under the influence of the process that brings bodies into an electrical state represent the cause of all the electrical actions we observe. According to Faraday, the very electrification of conductors on their surface is only a consequence of the influence of a polarized environment on them. The insulating medium is in a stressed state. Based on very simple experiments, Faraday came to the conclusion that when electric polarization is excited in any medium, when an electric field, as they say now, is excited, in this medium there should be tension along the lines of force (a line of force is a line to which the tangents coincide with the directions of the electric forces experienced by the positive electricity imagined at points located on this line) and there must be pressure in directions perpendicular to the lines of force. Such a stressed state can only be caused in insulators. Conductors are not capable of experiencing such a change in their state; no disturbance occurs in them; and only on the surface of such conducting bodies, i.e., at the boundary between the conductor and the insulator, the polarized state of the insulating medium becomes noticeable; it is expressed in the apparent distribution of electricity on the surface of the conductors. So, the electrified conductor is, as it were, connected to the surrounding insulating medium. From the surface of this electrified conductor, lines of force seem to spread, and these lines end on the surface of another conductor, which visibly appears to be covered with electricity of opposite sign. This is the picture that Faraday painted for himself to explain the phenomena of electrification.
Faraday's teachings were not quickly accepted by physicists. Faraday's experiments were considered even in the sixties as not giving the right to assume any significant role of insulators in the processes of electrification of conductors. Only later, after the advent of Maxwell's remarkable works, Faraday's ideas began to spread more and more among scientists and were finally recognized as fully consistent with the facts.
It is appropriate to note here that back in the sixties prof. F. N. Shvedov, on the basis of his experiments, very ardently and convincingly proved the correctness of Faraday’s basic principles regarding the role of insulators. In fact, however, many years before Faraday's work, the effect of insulators on electrical processes had already been discovered. Back in the early 70s of the 18th century, Cavendish observed and very carefully studied the significance of the nature of the insulating layer in a capacitor. Cavendish's experiments, as well as Faraday's subsequent experiments, showed an increase in the electrical capacity of a capacitor when the layer of air in this capacitor is replaced by a layer of some solid dielectric of the same thickness. These experiments even make it possible to determine the numerical values of the dielectric constants of some insulating substances, and these values turn out to be relatively slightly different from those found recently with the use of more advanced measuring instruments. But this work of Cavendish, as well as his other research on electricity, which led him to the establishment of the law of electrical interactions, identical with the law published in 1785 by Coulomb, remained unknown until 1879. Only this year were Cavendish’s memoirs made public by Maxwell, who repeated almost all of Cavendish's experiments and who made many, very valuable instructions about them.
Potential
As mentioned above, the basis of electrostatics, until the appearance of Maxwell’s works, was based on Coulomb’s law:Assuming C = 1, i.e., when expressing the amount of electricity in the so-called absolute electrostatic unit of the CGS system, this Coulomb law receives the expression:
Hence the potential function or, more simply, the potential at a point whose coordinates are (x, y, z), is determined by the formula:
In which the integral extends to all electric charges in a given space, and r denotes the distance of the charge element dq to the point (x, y, z). Denoting the surface density of electricity on electrified bodies by σ, and the volumetric density of electricity in them by ρ, we have
Here dS denotes the body surface element, (ζ, η, ξ) - the coordinates of the body volume element. Projections on the coordinate axes of the electric force F experienced by a unit of positive electricity at the point (x, y, z) are found according to the formulas:
Surfaces at all points of which V = constant are called equipotential surfaces or, more simply, level surfaces. Lines orthogonal to these surfaces are electric lines of force. The space in which electric forces can be detected, i.e. in which lines of force can be constructed, is called the electric field. The force experienced by a unit of electricity at any point in this field is called the electric field voltage at that point. The function V has the following properties: it is unambiguous, finite, and continuous. It can also be set so that it becomes 0 at points located at an infinite distance from a given distribution of electricity. The potential retains the same value at all points of any conducting body. For all points on the globe, as well as for all conductors metallic connected to the ground, the function V is equal to 0 (at the same time, no attention is paid to the Volta phenomenon, which was reported in the article Electrification). Denoting by F the magnitude of the electric force experienced by a unit of positive electricity at some point on the surface S, enclosing a part of space, and by ε the angle formed by the direction of this force with the outer normal to the surface S at the same point, we have
In this formula, the integral extends over the entire surface S, and Q denotes the algebraic sum of the quantities of electricity contained within the closed surface S. Equality (4) expresses a theorem known as Gauss's theorem. Simultaneously with Gauss, the same equality was obtained by Green, which is why some authors call this theorem Green’s theorem. From Gauss's theorem can be derived as corollaries,
here ρ denotes the volumetric density of electricity at the point (x, y, z);
this equation applies to all points where there is no electricity
Here Δ is the Laplace operator, n1 and n2 denote the normals at a point on any surface at which the surface density of electricity is σ, the normals drawn in one direction or the other from the surface. From Poisson’s theorem it follows that for a conducting body in which V = constant at all points, there must be ρ = 0. Therefore, the expression for the potential takes the form
From the formula expressing the boundary condition, i.e. from formula (7), it follows that on the surface of the conductor
Moreover, n denotes the normal to this surface, directed from the conductor into the insulating medium adjacent to this conductor. From the same formula it is deduced
Here Fn denotes the force experienced by a unit of positive electricity located at a point infinitely close to the surface of the conductor, having at that location a surface density of electricity equal to σ. The force Fn is directed normal to the surface at this location. The force experienced by a unit of positive electricity located in the electrical layer itself on the surface of the conductor and directed along the outer normal to this surface is expressed through
Hence, the electrical pressure experienced in the direction of the outer normal by each unit of the surface of an electrified conductor is expressed by the formula
The above equations and formulas make it possible to draw many conclusions related to the issues considered in E. But all of them can be replaced by even more general ones if we use what is contained in the theory of electrostatics given by Maxwell.
Maxwell's electrostatics
As mentioned above, Maxwell was the interpreter of Faraday's ideas. He put these ideas into mathematical form. The basis of Maxwell's theory lies not in Coulomb's law, but in the acceptance of a hypothesis, which is expressed in the following equality:Here the integral extends over any closed surface S, F denotes the magnitude of the electric force experienced by a unit of electricity at the center of the element of this surface dS, ε denotes the angle formed by this force with the outer normal to the surface element dS, K denotes the dielectric coefficient of the medium adjacent to element dS, and Q denotes the algebraic sum of the quantities of electricity contained within the surface S. The consequences of expression (13) are the following equations:
These equations are more general than equations (5) and (7). They apply to the case of any isotropic insulating media. Function V, which is the general integral of equation (14) and satisfies at the same time equation (15) for any surface that separates two dielectric media with dielectric coefficients K 1 and K 2, as well as the condition V = constant. for each conductor located in the electric field under consideration, represents the potential at the point (x, y, z). From expression (13) it also follows that the apparent interaction of two charges q and q 1 located at two points located in a homogeneous isotropic dielectric medium at a distance r from each other can be represented by the formula
That is, this interaction is inversely proportional to the square of the distance, as it should be according to Coulomb’s law. From equation (15) we obtain for the conductor:
These formulas are more general than the above (9), (10) and (12).
is an expression of the flow of electrical induction through the dS element. By drawing lines through all points of the contour of the dS element, coinciding with the directions of F at these points, we obtain (for an isotropic dielectric medium) an induction tube. For all cross sections of such an induction tube, which does not contain electricity within itself, it should be, as follows from equation (14),
KFCos ε dS = constant
It is not difficult to prove that if in any system of bodies electric charges are in equilibrium when the densities of electricity are, respectively, σ1 and ρ1 or σ 2 and ρ 2, then the charges will be in equilibrium even when the densities are σ = σ 1 + σ 2 and ρ = ρ 1 + ρ 2 (the principle of adding charges that are in equilibrium). It is equally easy to prove that under given conditions there can be only one distribution of electricity in the bodies that make up any system.
The property of a conductive closed surface in connection with the ground turns out to be very important. Such a closed surface is a screen, protection for the entire space enclosed within it, from the influence of any electrical charges located on the outside of the surface. As a result, electrometers and other electrical measuring instruments are usually surrounded by metal cases connected to the ground. Experiments show that for such electric There is no need to use solid metal for screens; it is quite enough to construct these screens from metal mesh or even metal gratings.
A system of electrified bodies has energy, that is, it has the ability to perform a certain amount of work upon complete loss of its electrical state. In electrostatics, the following expression is derived for the energy of a system of electrified bodies:
In this formula, Q and V denote, respectively, any amount of electricity in a given system and the potential in the place where this amount is located; the sign ∑ indicates that we must take the sum of the products VQ for all quantities Q of a given system. If a system of bodies is a system of conductors, then for each such conductor the potential has the same value at all points of this conductor, and therefore in this case the expression for energy takes the form:
Here 1, 2.. n are the icons of different conductors that make up the system. This expression can be replaced by others, namely, the electrical energy of a system of conducting bodies can be represented either depending on the charges of these bodies, or depending on their potentials, i.e. for this energy the expressions can be applied:
In these expressions, the various coefficients α and β depend on the parameters that determine the positions of conducting bodies in a given system, as well as their shapes and sizes. In this case, coefficients β with two identical icons, such as β11, β22, β33, etc., represent the electrical capacity (see Electrical capacity) of bodies marked with these icons, coefficients β with two different icons, such as β12, β23, β24, etc., represent the coefficients of mutual induction of two bodies, the icons of which are next to this coefficient. Having an expression for electrical energy, we obtain an expression for the force experienced by any body, whose symbol is i, and from the action of which the parameter si, which serves to determine the position of this body, receives an increase. The expression of this force will be
Electrical energy can be represented in another way, namely, through
In this formula, the integration extends over the entire infinite space, F denotes the magnitude of the electric force experienced by a unit of positive electricity at a point (x, y, z), i.e., the electric field voltage at that point, and K denotes the dielectric coefficient at the same point . With this expression of the electrical energy of a system of conducting bodies, this energy can be considered distributed only in insulating media, and the share of the dielectric element dxdyds accounts for the energy 
Expression (26) is fully consistent with the views on electrical processes that were developed by Faraday and Maxwell.
An extremely important formula in electrostatics is Green's formula, namely:In this formula, both triple integrals extend to the entire volume of any space A, double integrals to all surfaces bounding this space, ∆V and ∆U denote the sums of the second derivatives of the functions V and U with respect to x, y, z; n is the normal to the element dS of the bounding surface, directed inside the space A.
Examples
Example 1As a special case of Green's formula, we obtain a formula expressing the above Gauss theorem. In the Encyclopedic Dictionary it is not appropriate to touch upon questions about the laws of distribution of electricity on various bodies. These questions represent very difficult problems of mathematical physics, and various methods are used to solve such problems. We present here only for one body, namely, for an ellipsoid with semi-axes a, b, c, the expression for the surface density of electricity σ at the point (x, y, z). We find:
Here Q denotes the entire amount of electricity located on the surface of this ellipsoid. The potential of such an ellipsoid at some point on its surface, when there is a homogeneous isotropic insulating medium with dielectric coefficient K around the ellipsoid, is expressed through
The electrical capacity of the ellipsoid is obtained from the formula
Example 2
Using equation (14), assuming only ρ = 0 and K = constant in it, and formula (17), we can find an expression for the electrical capacitance of a flat capacitor with a guard ring and a guard box, the insulating layer in which has a dielectric coefficient K. This is the expression looks like
Here S denotes the size of the collecting surface of the capacitor, D is the thickness of its insulating layer. For a capacitor without a guard ring and a guard box, formula (28) will only give an approximate expression of the electrical capacity. For the electrical capacity of such a capacitor, Kirchhoff’s formula is given. And even for a capacitor with a guard ring and a box, formula (29) does not represent a completely strict expression of the electrical capacity. Maxwell indicated the correction that must be made to this formula in order to obtain a more rigorous result.
The energy of a flat capacitor (with guard ring and box) is expressed through
Here V1 and V2 are the potentials of the conducting surfaces of the capacitor.
Example 3
For a spherical capacitor, the expression for electrical capacity is obtained:
In which R 1 and R 2 denote the radii of the inner and outer conducting surface of the capacitor, respectively. Using the expression for electrical energy (formula 22), the theory of absolute and quadrant electrometers is easily established
Finding the value of the dielectric coefficient K of any substance, a coefficient included in almost all formulas that one has to deal with in electrostatics, can be done in very different ways. The most commonly used methods are the following.
1) Comparison of the electrical capacitances of two capacitors that have the same size and shape, but in which the insulating layer of one is a layer of air, and the other is a layer of the dielectric being tested.
2) Comparison of attractions between the surfaces of a capacitor, when these surfaces are given a certain potential difference, but in one case there is air between them (attractive force = F 0), in the other case - the test liquid insulator (attractive force = F). The dielectric coefficient is found by the formula:
3) Observations of electric waves (see Electrical vibrations) propagating along wires. According to Maxwell's theory, the speed of propagation of electric waves along wires is expressed by the formula
In which K denotes the dielectric coefficient of the medium surrounding the wire, μ denotes the magnetic permeability of this medium. We can put μ = 1 for the vast majority of bodies, and therefore it turns out
Usually, the lengths of standing electric waves that arise in parts of the same wire located in the air and in the test dielectric (liquid) are compared. Having determined these lengths λ 0 and λ, we obtain K = λ 0 2 / λ 2. According to Maxwell’s theory, it follows that when an electric field is excited in any insulating substance, special deformations occur inside this substance. Along the induction tubes, the insulating medium is polarized. Electrical displacements arise in it, which can be likened to the movements of positive electricity along the axes of these tubes, and through each cross section of the tube an amount of electricity passes equal to
Maxwell's theory makes it possible to find expressions for those internal forces (forces of tension and pressure) that appear in dielectrics when an electric field is excited in them. This question was first considered by Maxwell himself, and later in more detail by Helmholtz. Further development of the theory of this issue and the closely connected theory of electrostriction (i.e., the theory that considers phenomena that depend on the occurrence of special voltages in dielectrics when an electric field is excited in them) belongs to the works of Lorberg, Kirchhoff, Duhem, N. N. Schiller and some others
Border conditions
Let us complete our brief presentation of the most significant aspects of electrostriction by considering the issue of refraction of induction tubes. Let us imagine two dielectrics in an electric field, separated from each other by some surface S, with dielectric coefficients K 1 and K 2. Let at points P 1 and P 2 located infinitely close to the surface S on either side of it, the magnitudes of the potentials are expressed through V 1 and V 2 , and the magnitudes of the forces experienced by a unit of positive electricity placed at these points through F 1 and F 2. Then for a point P lying on the surface S itself, there must be V 1 = V 2,
if ds represents an infinitesimal displacement along the line of intersection of the tangent plane to the surface S at point P with the plane passing through the normal to the surface at this point and through the direction of the electric force in it. On the other hand, it should be
Let us denote by ε 2 the angle made by the force F 2 with the normal n 2 (inside the second dielectric), and by ε 1 the angle made by the force F 1 with the same normal n 2 Then, using formulas (31) and (30), we find
So, on the surface separating two dielectrics from each other, the electric force undergoes a change in its direction, like a light ray entering from one medium into another. This consequence of the theory is justified by experience.
Material from Wikipedia - the free encyclopedia
Electrostatics is a branch of physics where the properties and interactions of electrically charged bodies or particles that have an electric charge that are stationary relative to an inertial reference frame are studied.
Electric charge is a physical quantity that characterizes the property of bodies or particles to enter into electromagnetic interactions and determines the values of forces and energies during these interactions. In the International System of Units, the unit of electric charge is the coulomb (C).
There are two types of electric charges:
- positive;
- negative.
A body is electrically neutral if the total charge of negatively charged particles that make up the body is equal to the total charge of positively charged particles.
Stable carriers of electric charges are elementary particles and antiparticles.
Positive charge carriers are proton and positron, and negative charge carriers are electron and antiproton.
The total electric charge of the system is equal to the algebraic sum of the charges of the bodies included in the system, i.e.:
Law of conservation of charge: in a closed, electrically isolated system, the total electrical charge remains unchanged, no matter what processes occur within the system.
Isolated system- this is a system into which electrically charged particles or any bodies do not penetrate from the external environment through its boundaries.
Law of conservation of charge- this is a consequence of the conservation of the number of particles; a redistribution of particles occurs in space.
Conductors- these are bodies with electrical charges that can move freely over significant distances.
Examples of conductors: metals in solid and liquid states, ionized gases, electrolyte solutions.
Dielectrics- these are bodies with charges that cannot move from one part of the body to another, i.e. bound charges.
Examples of dielectrics: quartz, amber, ebonite, gases under normal conditions.
Electrification- this is a process as a result of which bodies acquire the ability to take part in electromagnetic interaction, that is, they acquire an electric charge.
Electrification of bodies- this is a process of redistribution of electrical charges located in bodies, as a result of which the charges of the bodies become of opposite signs.
Types of electrification:
- Electrification due to electrical conductivity. When two metal bodies come into contact, one charged and the other neutral, a certain number of free electrons transfer from the charged body to the neutral one if the charge of the body was negative, and vice versa if the charge of the body is positive.
As a result, in the first case, the neutral body will receive a negative charge, in the second - a positive one.
- Electrification by friction. As a result of contact by friction of some neutral bodies, electrons are transferred from one body to another. Electrification by friction is the cause of static electricity, discharges of which can be noticed, for example, if you comb your hair with a plastic comb or take off a synthetic shirt or sweater.
- Electrification through influence occurs if a charged body is brought to the end of a neutral metal rod, and a violation of the uniform distribution of positive and negative charges occurs in it. Their distribution occurs in a peculiar way: an excess negative charge appears in one part of the rod, and a positive one in the other. Such charges are called induced, the occurrence of which is explained by the movement of free electrons in the metal under the influence of the electric field of a charged body brought to it.
Point charge- this is a charged body, the dimensions of which can be neglected under given conditions.
Point charge is a material point that has an electric charge.
Charged bodies interact with each other in the following way: oppositely charged bodies attract, similarly charged bodies repel.
Coulomb's law: the force of interaction between two stationary point charges q1 and q2 in a vacuum is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them:
The main property of the electric field- this is that the electric field affects electric charges with some force. The electric field is a special case of the electromagnetic field.
Electrostatic field is the electric field of stationary charges. Electric field strength is a vector quantity characterizing the electric field at a given point. The field strength at a given point is determined by the ratio of the force acting on a point charge placed at a given point in the field to the magnitude of this charge:
Tension- this is the force characteristic of the electric field; it allows you to calculate the force acting on this charge: F = qE.
In the International System of Units, the unit of voltage is the volt per meter. Voltage lines are imaginary lines needed to use a graphical representation of the electric field. The tension lines are drawn so that the tangents to them at each point in space coincide in direction with the field strength vector at a given point.
The principle of field superposition: the field strength from several sources is equal to the vector sum of the field strengths of each of them.
Electric dipole- this is a collection of two equal in modulus opposite point charges (+q and –q), located at a certain distance from each other.
Dipole (electric) moment is a vector physical quantity that is the main characteristic of a dipole.
In the International System of Units, the unit of dipole moment is the coulomb meter (C/m).
Types of dielectrics:
- Polar, which include molecules in which the centers of distribution of positive and negative charges do not coincide (electric dipoles).
- Non-polar, in the molecules and atoms of which the centers of distribution of positive and negative charges coincide.
Polarization is a process that occurs when dielectrics are placed in an electric field.
Polarization of dielectrics is the process of displacement of associated positive and negative charges of a dielectric in opposite directions under the influence of an external electric field.
The dielectric constant is a physical quantity that characterizes the electrical properties of a dielectric and is determined by the ratio of the modulus of the electric field strength in a vacuum to the modulus of the intensity of this field inside a homogeneous dielectric.
Dielectric constant is a dimensionless quantity and is expressed in dimensionless units.
Ferroelectrics- this is a group of crystalline dielectrics that do not have an external electric field and instead a spontaneous orientation of the dipole moments of the particles occurs.
Piezoelectric effect- this is an effect during mechanical deformations of some crystals in certain directions, where electrical charges of opposite types appear on their faces.
Electric field potential. Electrical capacity
Electrostatic potential is a physical quantity that characterizes the electrostatic field at a given point, it is determined by the ratio of the potential energy of interaction of a charge with the field to the value of the charge placed at a given point in the field: 
The unit of measurement in the International System of Units is the volt (V).
The field potential of a point charge is determined by: 
Under the conditions if q > 0, then k > 0; if q
The principle of field superposition for potential: if an electrostatic field is created by several sources, then its potential at a given point in space is defined as an algebraic sum of potentials:
The potential difference between two points of the electric field is a physical quantity determined by the ratio of the work of electrostatic forces to move a positive charge from the starting point to the final point to this charge: 
Equipotential surfaces- this is the geometric region of points of the electrostatic field where the potential values are the same.
Electrical capacity is a physical quantity that characterizes the electrical properties of a conductor, a quantitative measure of its ability to hold an electric charge.
The electrical capacitance of an isolated conductor is determined by the ratio of the charge of the conductor to its potential, and we will assume that the field potential of the conductor is taken equal to zero at the point at infinity:
Ohm's law
Homogeneous chain section- this is a section of the circuit that does not have a current source. The voltage in such a section will be determined by the potential difference at its ends, i.e.: 
In 1826, the German scientist G. Ohm discovered a law that determines the relationship between the current strength in a homogeneous section of the circuit and the voltage across it: the current strength in a conductor is directly proportional to the voltage across it. , where G is the proportionality coefficient, which is called in this law the electrical conductivity or conductivity of the conductor, which is determined by the formula.
Conductor conductivity is a physical quantity that is the reciprocal of its resistance.
In the International System of Units, the unit of electrical conductivity is Siemens (Cm).
The physical meaning of Siemens: 1 cm is the conductivity of a conductor with a resistance of 1 ohm.
To obtain Ohm's law for a section of a circuit, it is necessary to substitute resistance R into the formula given above instead of electrical conductivity, then:
Ohm's law for a circuit section: The current strength in a section of a circuit is directly proportional to the voltage across it and inversely proportional to the resistance of a section of the circuit.
Ohm's law for a complete circuit: the current strength in an unbranched closed circuit, including a current source, is directly proportional to the electromotive force of this source and inversely proportional to the sum of the external and internal resistances of this circuit:
Sign Rules:
- If, when bypassing the circuit in the selected direction, the current inside the source goes in the direction of the bypass, then the EMF of this source is considered positive.
- If, when bypassing the circuit in the selected direction, the current inside the source flows in the opposite direction, then the emf of this source is considered negative.
Electromotive force (EMF) is a physical quantity that characterizes the action of external forces in current sources; it is an energy characteristic of the current source. For a closed loop, EMF is defined as the ratio of the work done by external forces to move a positive charge along a closed loop to this charge: 
In the International System of Units, the unit of EMF is the volt. When the circuit is open, the emf of the current source is equal to the electrical voltage at its terminals.
Joule-Lenz law: the amount of heat generated by a current-carrying conductor is determined by the product of the square of the current, the resistance of the conductor and the time the current passes through the conductor: 
When moving the electric field of a charge along a section of the circuit, it does work, which is determined by the product of the charge and the voltage at the ends of this section of the circuit:
DC power is a physical quantity that characterizes the rate of work done by the field to move charged particles along a conductor and is determined by the ratio of the work done by the current over time to this period of time: 
Kirchhoff's rules, which are used to calculate branched DC circuits, the essence of which is to find, based on the given resistances of sections of the circuit and the EMF applied to them, the current strengths in each section.
The first rule is the node rule: the algebraic sum of the currents that converge at a node is the point at which there are more than two possible current directions, it is equal to zero
The second rule is the rule of contours: in any closed circuit, in a branched electrical circuit, the algebraic sum of the products of current strengths and the resistance of the corresponding sections of this circuit is determined by the algebraic sum of the emf applied in it: 
A magnetic field- this is one of the forms of manifestation of the electromagnetic field, the specificity of which is that this field affects only moving particles and bodies with an electric charge, as well as magnetized bodies, regardless of the state of their motion.
Magnetic induction vector is a vector quantity that characterizes the magnetic field at any point in space, determining the ratio of the force acting from the magnetic field on a conductor element with electric current to the product of the current strength and the length of the conductor element, equal in modulus to the ratio of the magnetic flux through the cross-section of the area to the area of this cross section.
In the International System of Units, the unit of induction is the tesla (T).
Magnetic circuit is a collection of bodies or regions of space where a magnetic field is concentrated.
Magnetic flux (magnetic induction flux) is a physical quantity that is determined by the product of the magnitude of the magnetic induction vector by the area of the flat surface and by the cosine of the angle between the normal vectors to the flat surface / the angle between the normal vector and the direction of the induction vector.
In the International System of Units, the unit of magnetic flux is the weber (Wb).
Ostrogradsky-Gauss theorem for magnetic induction flux: magnetic flux through an arbitrary closed surface is zero:
Ohm's law for a closed magnetic circuit: 
Magnetic permeability is a physical quantity that characterizes the magnetic properties of a substance, which is determined by the ratio of the modulus of the magnetic induction vector in the medium to the modulus of the induction vector at the same point in space in a vacuum:
Magnetic field strength is a vector quantity that defines and characterizes the magnetic field and is equal to: 
Ampere power- this is the force that acts from the magnetic field on a conductor carrying current. The elementary Ampere force is determined by the relation:
Ampere's law: modulus of force acting on a small segment of a conductor through which current flows, from the side of a uniform magnetic field with induction making an angle with the element
Superposition principle: when at a given point in space, diverse sources form magnetic fields, the inductions of which are B1, B2, .., then the resulting field induction at this point is equal to: 
The gimlet rule or the right screw rule: if the direction of translational movement of the tip of the gimlet when screwing in coincides with the direction of the current in space, then the direction of the rotational movement of the gimlet at each point coincides with the direction of the magnetic induction vector.
Biot-Savart-Laplace Law: determines the magnitude and direction of the magnetic induction vector at any point of the magnetic field created in a vacuum by a conductor element of a certain length with current: 
Movement of charged particles in electric and magnetic fields The Lorentz force is a force influencing a moving particle from the magnetic field: 
Left hand rule:
- It is necessary to position the left hand so that the lines of magnetic induction enter the palm, and the extended four fingers are aligned with the current, then the thumb bent 90° will indicate the direction of the Ampere force.
- It is necessary to position the left hand so that the lines of magnetic induction enter the palm, and the four extended fingers coincide with the direction of the particle speed with a positive charge of the particle or are directed in the direction opposite to the speed of the particle with a negative charge of the particle, then the thumb bent 90° will show the direction Lorentz force acting on a charged particle.
If there is a joint action on a moving charge of electric and magnetic fields, then the resulting force will be determined by: 
Mass spectrographs and mass spectrometers- These are instruments that are designed specifically for accurate measurements of the relative atomic masses of elements.
Faraday's law. Lenz's rule
Electromagnetic induction- this is a phenomenon that consists in the fact that an induced emf occurs in a conducting circuit located in an alternating magnetic field.
Faraday's law: The electromagnetic induction emf in the circuit is numerically equal and opposite in sign to the rate of change of the magnetic flux F through the surface bounded by this circuit: 
Induction current- this is the current that is formed if charges begin to move under the influence of Lorentz forces.
Lenz's rule: the induced current appearing in a closed circuit always has such a direction that the magnetic flux it creates through the area limited by the circuit tends to compensate for the change in the external magnetic field that caused this current.
The procedure for using Lenz's rule to determine the direction of the induced current:
Vortex field- this is a field in which the tension lines are closed lines, the cause of which is the generation of an electric field by a magnetic field.
The work of a vortex electric field when moving a single positive charge along a closed stationary conductor is numerically equal to the induced emf in this conductor.
Toki Fuko- these are large induction currents that appear in massive conductors due to the fact that their resistance is low. The amount of heat released per unit time by eddy currents is directly proportional to the square of the frequency of change of the magnetic field.
Self-induction. Inductance
Self-induction- this is a phenomenon consisting in the fact that a changing magnetic field induces an emf in the very conductor through which the current flows, forming this field.
Magnetic flux Ф of a circuit with current I is determined:
Ф = L, where L is the self-inductance coefficient (current inductance).
Inductance- this is a physical quantity that is a characteristic of the self-inductive emf that appears in the circuit when the current strength changes, determined by the ratio of the magnetic flux through the surface bounded by the conductor to the direct current strength in the circuit:
In the International System of Units, the unit of inductance is the henry (H).
The self-induction emf is determined by: 
The magnetic field energy is determined by:
The volumetric energy density of a magnetic field in an isotropic and non-ferromagnetic medium is determined by: 
