Inhomogeneous and homogeneous magnetic field. Magnetic field and its graphical representation
Permanent magnets N – north pole of a magnet S – south pole of a magnet Permanent magnets Permanent magnets are bodies that retain long time magnetization. Arc magnet Strip magnet N N S S Pole - the point of the magnet where the strongest effect is found
Ampere's hypothesis ++ e - SN According to Ampere's hypothesis (g), ring currents arise in atoms and molecules as a result of the movement of electrons. In 1897 The hypothesis was confirmed by the English scientist Thomson, and in 1910. The currents were measured by the American scientist Millikan. What are the causes of magnetization? When a piece of iron is introduced into an external magnetic field, all elementary magnetic fields in this iron are oriented identically in the external magnetic field, forming their own magnetic field. This is how a piece of iron becomes a magnet.
Magnetic field of permanent magnets Magnetic field component electro magnetic field, appearing in the presence of a time-varying electric field. In addition, a magnetic field can be created by a current of charged particles. An idea of the type of magnetic field can be obtained using iron filings. All you have to do is place a sheet of paper on the magnet and sprinkle iron filings on top.
Magnetic fields are represented using magnetic lines. These are imaginary lines along which magnetic needles placed in a magnetic field are located. Magnetic lines can be drawn through any point in the magnetic field, they have a direction and are always closed. Outside the magnet, magnetic lines leave the north pole of the magnet and enter the south pole, closing inside the magnet.
INHOMOGENEOUS MAGNETIC FIELD The force with which the magnetic field acts can be different both in magnitude and in direction. Such a field is called inhomogeneous. Characteristics of a non-uniform magnetic field: magnetic lines are curved; the density of magnetic lines is different; The force with which the magnetic field acts on the magnetic needle is different in magnitude and direction at different points of this field.
Where does a non-uniform magnetic field exist? Around a straight conductor carrying current. The figure shows a section of such a conductor located perpendicular to the plane of the drawing. The current is directed away from us. It can be seen that magnetic lines are concentric circles, the distance between which increases with distance from the conductor
HOMOGENEOUS MAGNETIC FIELD Characteristics of a uniform magnetic field: magnetic lines are parallel to straight lines; the density of magnetic lines is the same everywhere; The force with which the magnetic field acts on the magnetic needle is the same at all points of this field in magnitude and direction.
If a powerful flare occurs on the Sun, the solar wind intensifies. This causes a disturbance in the earth's magnetic field and leads to a magnetic storm. Solar wind particles flying past the Earth create additional magnetic fields. Magnetic storms cause serious harm: they have a strong impact on radio communications, on telecommunication lines, many measuring instruments show incorrect results. This is interesting
The Earth's magnetic field reliably protects the Earth's surface from cosmic radiation, the effect of which on living organisms is destructive. In addition to electrons and protons, cosmic radiation also includes other particles moving in space at enormous speeds. This is interesting
The result of the interaction of the solar wind with the Earth's magnetic field is the aurora. Invading the earth's atmosphere, solar wind particles (mainly electrons and protons) are guided by a magnetic field and focused in a certain way. Colliding with atoms and molecules atmospheric air, they ionize and excite them, resulting in a glow called the aurora. This is interesting
A special discipline, biometrology, studies the influence of various factors of weather conditions on the body of a healthy and sick person. Magnetic storms disrupt the functioning of the cardiovascular, respiratory and nervous system, and also change blood viscosity; in patients with atherosclerosis and thrombophlebitis, it becomes thicker and clots faster, and in healthy people, on the contrary, increases. This is interesting
1.What bodies are called permanent magnets? 2.What generates the magnetic field of a permanent magnet? 3.What are the magnetic poles of a magnet called? 4. How do homogeneous magnetic fields differ from inhomogeneous ones? 5.How do the poles of magnets interact with each other? 6.Explain why a needle attracts a paper clip? (see figure) Fastening
The topic of this lesson will be the magnetic field and its graphic image. We will discuss non-uniform and uniform magnetic field. First, let's define the magnetic field, tell you what it is associated with and what properties it has. Let's learn how to depict it on graphs. We will also learn how a non-uniform and homogeneous magnetic field is determined.
Today we will first of all repeat what a magnetic field is. A magnetic field - a force field that forms around a conductor through which electric current flows. It is associated with moving charges.
Now it is necessary to note magnetic field properties. You know that charge has several fields associated with it. In particular, electric field. But we will discuss precisely the magnetic field created by moving charges. A magnetic field has several properties. First: magnetic field is created by moving electric charges. In other words, a magnetic field is formed around a conductor through which electric current flows. The next property that tells how the magnetic field is determined. It is determined by the effect on another moving electric charge. Or, they say, to a different electric current. We can determine the presence of a magnetic field by the effect on the compass needle, on the so-called. magnetic needle.
Another property: magnetic field exerts force. Therefore they say that the magnetic field is material.
These three properties are distinctive features magnetic field. After we have decided what a magnetic field is and determined the properties of such a field, it is necessary to say how the magnetic field is studied. First of all, the magnetic field is studied using a current-carrying frame. If we take a conductor, make a round or square frame out of this conductor and pass an electric current through this frame, then in a magnetic field this frame will rotate in a certain way.
Rice. 1. The current-carrying frame rotates in an external magnetic field
By the way this frame rotates, we can judge magnetic field. There's only one thing here important condition: The frame must be very small or it must be of very small dimensions compared to the distances at which we study the magnetic field. Such a frame is called a current circuit.
We can also study the magnetic field using magnetic needles, placing them in a magnetic field and observing their behavior.
Rice. 2. The effect of a magnetic field on magnetic needles
The next thing we'll talk about is how to represent a magnetic field. As a result of research that was carried out over a long time, it became clear that the magnetic field can be conveniently represented using magnetic lines. To observe magnetic lines, let's do one experiment. For our experiment we will need a permanent magnet, metal iron filings, glass and a sheet of white paper.
Rice. 3. Iron filings line up along magnetic field lines
Cover the magnet with a glass plate and place a sheet of paper on top, White list paper Sprinkle iron filings on top of a sheet of paper. As a result, you will see how the magnetic field lines appear. What we will see are the magnetic field lines of a permanent magnet. They are also sometimes called the spectrum of magnetic lines. Notice that lines exist in all three directions, not just in the plane.
Magnetic line- an imaginary line along which the axes of the magnetic needles would line up.
Rice. 4. Schematic representation of a magnetic line
Look, the figure shows the following: the line is curved, the direction of the magnetic line is determined by the direction of the magnetic arrow. The direction is indicated by the north pole of the magnetic needle. It is very convenient to depict lines using arrows.
Rice. 5. How the direction is indicated power lines
Now let's talk about the properties of magnetic lines. First, magnetic lines have neither a beginning nor an end. These are closed lines. Since the magnetic lines are closed, then there are no magnetic charges.
Second: these are lines that do not intersect, are not interrupted, do not twist in any way. With the help of magnetic lines, we can characterize the magnetic field, imagine not only its shape, but also talk about the force effect. If we depict a greater density of such lines, then in this place, at this point in space, we will have a greater force action.
If the lines are parallel to each other, their density is the same, then in this case they say that the magnetic field is uniform. If, on the contrary, this is not fulfilled, i.e. the density is different, the lines are curved, then such a field will be called heterogeneous. At the end of the lesson, I would like to draw your attention to the following drawings.
Rice. 6. Inhomogeneous magnetic field
Firstly, we now already know that magnetic lines can be represented by arrows. And the figure represents precisely a non-uniform magnetic field. The density is different in different places, which means that the force effect of this field on the magnetic needle will be different.
The following figure shows a homogeneous field. The lines are directed in one direction, and their density is the same.
Rice. 7. Uniform magnetic field
A uniform magnetic field is a field that occurs inside a coil with a large number turns or inside a straight strip magnet. The magnetic field outside a strip magnet, or what we observed in class today, is a non-uniform field. To fully understand all this, let's look at the table.
List of additional literature:
Belkin I.K. Electric and magnetic fields // Quantum. - 1984. - No. 3. - P. 28-31. Kikoin A.K. Where does magnetism come from? // Quantum. - 1992. - No. 3. - P. 37-39.42 Leenson I. Mysteries of the magnetic needle // Quantum. - 2009. - No. 3. - P. 39-40. Elementary physics textbook. Ed. G.S. Landsberg. T. 2. - M., 1974
Magnetic field and its characteristics. When an electric current passes through a conductor, a a magnetic field. A magnetic field represents one of the types of matter. It has energy, which manifests itself in the form of electromagnetic forces acting on individual moving electric charges (electrons and ions) and on their flows, i.e. electric current. Under the influence of electromagnetic forces, moving charged particles deviate from their original path in a direction perpendicular to the field (Fig. 34). The magnetic field is formed only around moving electric charges, and its action also extends only to moving charges. Magnetic and electric fields inseparable and form together a single electromagnetic field. Any change electric field leads to the appearance of a magnetic field and, conversely, any change in the magnetic field is accompanied by the appearance of an electric field. Electromagnetic field propagates at the speed of light, i.e. 300,000 km/s.
Graphic representation of the magnetic field. Graphically, the magnetic field is represented by magnetic lines of force, which are drawn so that the direction of the field line at each point of the field coincides with the direction of the field forces; magnetic field lines are always continuous and closed. The direction of the magnetic field at each point can be determined using a magnetic needle. The north pole of the arrow is always set in the direction of the field forces. The end of a permanent magnet from which the field lines emerge (Fig. 35, a) is considered to be north pole, and the opposite end, which includes the lines of force, is south pole(field lines passing inside the magnet are not shown). The distribution of field lines between the poles of a flat magnet can be detected using steel filings sprinkled on a sheet of paper placed on the poles (Fig. 35, b). For a magnetic field in air gap between two parallel opposite poles of a permanent magnet there is a uniform distribution of magnetic force lines (Fig. 36) (the field lines passing inside the magnet are not shown).
Rice. 37. Magnetic flux penetrating the coil when its positions are perpendicular (a) and inclined (b) relative to the direction of the magnetic lines of force.
For a more visual representation of the magnetic field, the field lines are placed less frequently or denser. In those places where the magnetic field is stronger, the field lines are located closer to each other, and in places where it is weaker, they are further apart. The lines of force do not intersect anywhere.
In many cases, it is convenient to consider magnetic lines of force as some elastic stretched threads that tend to contract and also repel each other (have mutual lateral thrust). This mechanical concept of lines of force makes it possible to clearly explain the emergence of electromagnetic forces during the interaction of a magnetic field and a conductor with current, as well as two magnetic fields.
The main characteristics of a magnetic field are magnetic induction, magnetic flux, magnetic permeability and magnetic field strength.
Magnetic induction and magnetic flux. The intensity of the magnetic field, i.e. its ability to produce work, is determined by a quantity called magnetic induction. The stronger the magnetic field created by a permanent magnet or electromagnet, the greater the induction it has. Magnetic induction B can be characterized by the density of magnetic field lines, i.e., the number of field lines passing through an area of 1 m 2 or 1 cm 2 located perpendicular to the magnetic field. There are homogeneous and inhomogeneous magnetic fields. In a uniform magnetic field, the magnetic induction at each point of the field has same value and direction. The field in the air gap between the opposite poles of a magnet or electromagnet (see Fig. 36) can be considered homogeneous at some distance from its edges. Magnetic flux Ф passing through any surface is determined total number magnetic lines of force penetrating this surface, for example coil 1 (Fig. 37, a), therefore, in a uniform magnetic field
F = BS (40)
where S is the cross-sectional area of the surface through which the magnetic field lines pass. It follows that in such a field the magnetic induction is equal to the flux divided by the cross-sectional area S:
B = F/S (41)
If any surface is located obliquely with respect to the direction of the magnetic field lines (Fig. 37, b), then the flux penetrating it will be less than if it is perpendicular to its position, i.e. Ф 2 will be less than Ф 1 .
In the SI system of units, magnetic flux is measured in webers (Wb), this unit has the dimension V*s (volt-second). Magnetic induction in SI units is measured in teslas (T); 1 T = 1 Wb/m2.
Magnetic permeability. Magnetic induction depends not only on the strength of the current passing through a straight conductor or coil, but also on the properties of the medium in which the magnetic field is created. The quantity characterizing the magnetic properties of a medium is absolute magnetic permeability? A. Its unit of measurement is henry per meter (1 H/m = 1 Ohm*s/m).
In a medium with greater magnetic permeability, an electric current of a certain strength creates a magnetic field with greater induction. It has been established that the magnetic permeability of air and all substances, with the exception of ferromagnetic materials (see § 18), has approximately the same value as the magnetic permeability of vacuum. The absolute magnetic permeability of a vacuum is called the magnetic constant, ? o = 4?*10 -7 H/m. The magnetic permeability of ferromagnetic materials is thousands and even tens of thousands of times greater than the magnetic permeability of non-ferromagnetic substances. Magnetic permeability ratio? and any substance to the magnetic permeability of vacuum? o is called relative magnetic permeability:
? = ? A /? O (42)
Magnetic field strength. The intensity And does not depend on the magnetic properties of the medium, but takes into account the influence of the current strength and the shape of the conductors on the intensity of the magnetic field at a given point in space. Magnetic induction and tension are related by the relation
H = B/? a = B/(?? o) (43)
Consequently, in a medium with constant magnetic permeability, the magnetic field induction is proportional to its strength.
Magnetic field strength is measured in amperes per meter (A/m) or amperes per centimeter (A/cm).
Lesson outline plan No. 16.
Lesson topic: “Magnetic field and its graphic representation. Inhomogeneous and homogeneous magnetic field"
Goals:
Educational : establish a connection between the direction of the magnetic lines of the magnetic field of the current and the direction of the current in the conductor. Introduce the concept of inhomogeneous and homogeneous magnetic fields. In practice, obtain a picture of the magnetic field lines of a permanent magnet, solenoid, conductor through which electric current flows. Systematize knowledge on the main issues of the topic “Electromagnetic Field”, continue to teach how to solve qualitative and experimental problems.
Developmental : activate cognitive activity students in physics lessons. Develop students' cognitive activity.
Educational : to promote the formation of the idea of cognition of the world. Foster hard work and mutual understanding between students and teachers.
Tasks:
Educational : deepening and expanding knowledge about the magnetic field, justify the connection between the direction of the magnetic lines of the magnetic field of the current and the direction of the current in the conductor.
Educational : show cause-and-effect relationships when studying the magnetic field of direct current and magnetic lines, that causeless phenomena do not exist, that experience is a criterion for the truth of knowledge.
Developmental : continue to work on developing the skills to analyze and generalize knowledge about the magnetic field and its characteristics. Involving students in active practical activities when performing experiments.
Equipment: presentation,table, projector, screen, mmagnetic needles, iron filings, magnets, compass.
Lesson plan:
Organizational moment.(1-2 min)
Motivation and goal setting (1-2 min)
Studying a new topic (15-30 min)
4. Homework (1-2 min)
1. Organizational moment.
We stood up and straightened up. Hello, please sit down.
2. Motivation and goal setting.
Each of you has observed how at the end of summer, at the beginning of autumn, many birds fly away to warmer climes. Migratory birds travel great distances, fearing winter cold, and in the spring they return back. Birds navigate by the Earth's magnetic field. So that's it day we will talk about magnets, consider the properties of a magnet. Let's remember what a magnetic field is, what magnetic fields there are.
3. Studying a new topic.
The history of the magnet goes back over two and a half thousand years.
An ancient legend tells about a shepherd named Magnus. He once discovered that the iron tip of his stick and the nails of his boots were attracted to the black stone. This stone came to be called the “Magnus” stone or simply “magnet”. But another legend is known that the word “magnet” comes from the name of the area where iron ore was mined (the hills of Magnesia in Asia Minor) Slide 2 . Thus, many centuries BC. It was known that some rocks have the property of attracting pieces of iron. This was mentioned in VI in BC Greek physicist Thales. In those days, the properties of magnets seemed magical. in the same ancient Greece their strange action was directly connected with the activities of the Gods.
This is how the ancient Greek sage Socrates described the property of this stone: “This stone not only attracts an iron ring, it also bestows its power on the ring, so that it in turn can attract another ring, and thus many rings and pieces of iron can hang on each other.” ! This happens due to the power of the magnetic stone."
What are the properties of magnets and how are the properties of magnets determined? To do this, let's look at experience. Take a sheet of paper, a magnet and iron filings. What are we observing? Video
Slide 3
What if you take 2 magnets and bring them to each other with the same poles? how will they behave? What if they have opposite poles?
Why are pieces of iron filings attracted to a magnet? Just as a glass rod attracts pieces of paper, similarly a magnet attracts iron filings. There is a magnetic field around a magnet.
From the 8th grade physics course you learned that a magnetic field is generated by an electric current. It exists, for example, around metal conductor with current. In this case, the current is created by electrons moving directionally along the conductor.
Since electric current is the directed movement of charged particles, we can say thatA magnetic field is created by moving charged particles, both positive and negative.
So let's write down the definition:
A magnetic field is a special type of matter that is created around magnets by moving charged particles, both positive and negative.
Slide 5
Remember that if particles move, a magnetic field is created. We said that m.p. is a special type of matter, it is called a special type because. not perceived by the senses.
To detect m.p. Magnetic needles are used.
To visually represent the magnetic field, we use magnetic lines (they are also called magnetic field lines). Let us remind you thatmagnetic lines - these are imaginary lines along which small magnetic needles would be located, placed in a magnetic field. Slide
A magnetic line can be drawn through any point in space in which a magnetic field exists.
In Figure 86,a, b it is shown that a magnetic line (both rectilinear and curvilinear) is drawn so that at any point of this line the tangent to it coincides with the axis of the magnetic needle placed at this point. Slide 6
Magnetic lines are closed. For example, the pattern of magnetic lines of a straight current-carrying conductor consists of concentric circles lying in a plane perpendicular to the conductor.Slide 7
In those areas of space where the magnetic field is stronger, the magnetic lines are drawn closer to each other, that is, denser than in those places where the field is weaker. For example, the field shown in Figure 87 is stronger on the left than on the right.Slide 8
Thus, according tothe pattern of magnetic lines can be judged not only on the direction, but also on the magnitude of the magnetic field (i.e., at which points in space the field acts on the magnetic needle with greater force, and at which points with less).
Let's look at fig. 88 in the textbook: a conductor with current BC is shown, let's remember what electric power is. current-motion charge particles, and we said that if particles move, a magnetic field is created. Let's look at the pointNWill there be a magnetic field? Yes, it will, because current flows throughout the conductor. At which point A or M will the magnetic field be stronger? At point A because it is closer to the magnet.
There are two types of magnetic field: homogeneous and inhomogeneous. Let's look at these types of magnetic fields.
Magnetic lines have neither beginning nor end: they are either closed or go from infinity to infinity. Rice. 89
Outside a magnet, magnetic lines are most densely located at its poles. This means that the field is strongest near the poles, and as it moves away from the poles it weakens. The closer the magnetic needle is to the pole of the magnet, the greater in magnitude the force the magnetic field acts on it. Since magnetic lines are curved, the direction of the force with which the field acts on the arrow also changes from point to point.
Thus,The force with which the field of a strip magnet acts on a magnetic needle placed in this field at different points of the field can be different both in magnitude and in direction.
Slide 9
This field is calledheterogeneous. The lines of a non-uniform magnetic field are curved, their density varies from point to point.
Another example of a non-uniform magnetic field is the field around a straight conductor carrying current. Figure 90 shows a section of such a conductor located perpendicular to the plane of the drawing. The circle indicates the cross section of the conductor. From this figure it is clear that the magnetic field lines created by a straight conductor carrying current are concentric circles, the distance between which increases with distance from the conductor.
In some limited area of space you can createhomogeneous magnetic field, i.e.a field at any point of which the force on the magnetic needle is the same in magnitude and direction.
Slide 10.
Figure 91 shows a uniform field arising inside the so-called solenoid, i.e., a cylindrical wire coil with current. The field inside the solenoid can be considered uniform if the length of the solenoid is significantly greater than its diameter (outside the solenoid the field is non-uniform, its magnetic lines are located approximately the same as those of a strip magnet). From this figure we see thatmagnetic lines of a uniform magnetic field are parallel to each other and located with the same density. The field inside the permanent strip magnet in its central part is also uniform (see Fig. 89).
Slide11
To depict a magnetic field, use the following technique. If the lines of a uniform magnetic field are located perpendicular to the plane of the drawing and directed away from us behind the drawing, then they are depicted with crosses (Fig. 92), and if from behind the drawing towards us, then with dots (Fig. 93). As in the case of current, each cross is like the visible tail of an arrow flying away from us, and the point is the tip of an arrow flying towards us (in both figures the direction of the arrows coincides with the direction of the magnetic lines).
So how do birds still navigate in space when migrating? It turns out that the Earth is surrounded by a magnetic field. Inside the earth there is a large magnet that creates a huge magnetic field around the earth. And the magnet inside the earth is the iron ore from which our permanent magnets are made. Scientists say that carrier pigeons, for example, also have something like a magnet inside them, which is why they navigate space so well.
Homework.
Paragraph 43, 44. exercise 34.
Prepare messages on the topic: “M.p. Earth", "M.p. in living organisms", "Magnetic storms".
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-1.jpg" alt="> Magnetic field and its graphical representation Inhomogeneous and homogeneous"> Магнитное поле и его графическое изображение Неоднородное и однородное магнитное поле Правило буравчика Правило правой руки Правило левой руки!}
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-2.jpg" alt=">Magnetic field and its graphical representation For a visual representation"> Магнитное поле и его графическое изображение Для наглядного представления магнитного поля мы пользовались магнитными линиями. Магнитные линии – это воображаемые линии, вдоль которых расположились бы маленькие магнитные стрелки, помещенные в магнитное поле. На рисунке показано магнитная линия (как прямолинейная, так и криволинейная). По картине магнитных линий можно судить не только о направлении, но и о величине магнитного поля.!}
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-3.jpg" alt=">Inhomogeneous and uniform magnetic field The force with which the field of a strip magnet"> Неоднородное и однородное магнитное поле Сила, с которой поле полосового магнита действует на помещенную в это поле магнитную стрелку, в разных точках поля может быть различной как по модулю, так и по направлению. Такое поле называют неоднородным. Линии неоднородного магнитного поля искривлены, их густота меняется от точки к точке. В некоторой ограниченной области пространства можно создать однородное магнитное поле, т. е. поле, в любой точке которого сила действия на магнитную стрелку одинакова по модулю и направлению. Для изображения магнитного поля пользуются следующим приемом. Если линии однородного магнитного поля расположены перпендикулярно к плоскости чертежа и наплавлены от нас за чертеж, то их изображают крестиками, а если из-за чертежа к нам – то точками.!}
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-4.jpg" alt=">Gimlet rule It is known that the direction of the magnetic field lines of the current is related to"> Правило буравчика Известно, что направление линий магнитного поля тока связано с направлением тока в проводнике. Эта связь может быть выражена !} simple rule, which is called the gimlet rule. The gimlet rule is as follows: if the direction of translational movement of the gimlet coincides with the direction of the current in the conductor, then the direction of rotation of the gimlet handle coincides with the direction of the magnetic field lines of the current. Using the gimlet rule, in the direction of the current you can determine the direction of the magnetic field lines created by this current, and in the direction of the magnetic field lines - the direction of the current creating this field.
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-5.jpg" alt=">Right hand rule To determine the direction of the magnetic field lines of the solenoid it is more convenient"> Правило правой руки Для определения направления линий магнитного поля соленоида удобнее пользоваться другим правилом, которое иногда называют правилом правой руки: если обхватить соленоид ладонью правой руки, направив четыре пальца по направлению тока в витках, то отставленный большой палец покажет направление линий магнитного поля внутри соленоида. Соленоид, как и магнит, имеет полосы: тот конец соленоида, из которого магнитные линии выходят, называется северным полюсом, а тот, в который входят, - южным. Зная направления тока в соленоиде, по правилу правой руки можно определить направление магнитных линий внутри него, а значит, и его магнитные полюсы и наоборот. Правило правой руки можно применять и для определения направления линий магнитного поля в центре одиночного витка с током.!}
Src="http://present5.com/presentation/3/46060323_437197076.pdf-img/46060323_437197076.pdf-6.jpg" alt=">Right-hand rule for a current-carrying conductor If right hand"> Rule of the right hand for a conductor with current If the right hand is positioned so that the thumb is directed along the current, then the other four fingers will show the direction of the magnetic induction line