The topic of this lesson will be the magnetic field and its graphical representation. 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 - force field that forms around a conductor through which flow electricity. It is associated with moving charges.
Now it is necessary to note properties magnetic field . You know that charge has several fields associated with it. In particular, the 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 a 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 is the direction of field lines indicated?
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 curl 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
We know that a current-carrying conductor creates a magnetic field around itself. A permanent magnet also creates a magnetic field. Will the fields they create be different? Undoubtedly they will. The difference between them can be seen clearly if you create graphical images of magnetic fields. The magnetic field lines will be directed differently.
Uniform magnetic fields
When current carrying conductor magnetic lines form closed concentric circles around a conductor. If we look at a cross-section of a current-carrying conductor and the magnetic field it creates, we will see a set of circles of different diameters. The figure on the left shows just a conductor carrying current.
The closer you are to the conductor, the stronger the effect of the magnetic field. As you move away from the conductor, the action and, accordingly, the strength of the magnetic field will decrease.
When permanent magnet we have lines coming out of the south pole of the magnet, passing along the body of the magnet itself and entering its north pole.
Having sketched such a magnet and the magnetic lines of the magnetic field formed by it graphically, we will see that the effect of the magnetic field will be strongest near the poles, where the magnetic lines are most densely located. The picture on the left with two magnets just depicts the magnetic field of permanent magnets.
We will see a similar picture of the location of magnetic lines in the case of a solenoid or coil with current. The magnetic lines will have the greatest intensity at the two ends or ends of the coil. In all the above cases we had a non-uniform magnetic field. The magnetic lines had different direction, and their density was different.
Can a magnetic field be uniform?
If we look closely at the graphical representation of the solenoid, we will see that the magnetic lines are parallel and have the same density in only one place inside the solenoid.
The same picture will be observed inside the body of a permanent magnet. And if in the case of a permanent magnet we cannot “climb” inside its body without destroying it, then in the case of a coil without a core or solenoid, we get a uniform magnetic field inside them.
Such a field may be required by humans in a number of technological processes, so solenoids can be constructed of a sufficient size to allow the necessary processes to be carried out within them.
Graphically, we are accustomed to depicting magnetic lines as circles or segments, that is, we seem to see them from the side or along. But what if the drawing is created in such a way that these lines are directed towards us or at reverse side from U.S? Then they are drawn in the form of a dot or a cross.
If they are directed at us, then they are depicted as a point, as if it were the tip of an arrow flying towards us. In the opposite case, when they are directed away from us, they are drawn in the form of a cross, as if it were the tail of an arrow moving away from us.
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 forward movement 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 !} thumb will show the direction of the magnetic field lines inside the solenoid. A solenoid, like a magnet, has stripes: the end of the solenoid from which the magnetic lines come out is called the north pole, and the end into which the magnetic lines enter is called the south pole. Knowing the direction of the current in the solenoid, using the right hand rule, you can determine the direction of the magnetic lines inside it, and therefore its magnetic poles and vice versa. The right-hand rule can also be used to determine the direction of the magnetic field lines in the center of a single current-carrying coil.
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
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 vast distances, fearing the 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 talks 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 a metal conductor carrying 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 on 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, a picture of magnetic lines straight conductor with current represents 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 - movement of charger. 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 straight conductor with 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".
“Magnetic field and its graphic representation. Inhomogeneous and homogeneous magnetic fields"
The purpose of the lesson: providing conditions for students to gain knowledge about the magnetic fieldcmethodahegographic image
Tasks:
educational:
identify the existence of a magnetic field in the process of solving a given situation;
give a definition of magnetic field;
investigate the dependence of the magnitude of the magnetic field of a magnet on the distance to it;
investigate the interaction of the poles of two magnets;
find out the properties of the magnetic field;
get acquainted with the image of a magnetic field through lines of force.
developing: development logical thinking; ability to analyze, compare, systematize information;
educational: develop skills in working in groups;
create responsibility in implementation educational task.
Lesson type: learning new material.
Equipment: magnets (strip, arc-shaped) according to the number of students, iron filings, white sheet.
During the classes
1) Organizational stage. The motto of our lesson will be the words of R. Descartes: “...In order to improve the mind, you need to think more than memorize.”
2) Setting the goals and objectives of the lesson. Motivation for students' learning activities.
Situation. This was many centuries ago. In search of a sheep, the shepherd went into unfamiliar places, into the mountains. There were black stones all around. He noticed with amazement that his stick with an iron tip was being pulled towards itself by the stones, as if some invisible hand was grabbing and holding it. Struck by the miraculous power of the stones, the shepherd brought them to the nearest city. Here everyone could be convinced that the shepherd’s story was not fiction - amazing stones attracted iron things to themselves! Moreover, it was necessary to rub the blade of a knife with such a stone, and it itself began to attract iron objects: nails, arrowheads. It was as if some kind of power, mysterious, of course, flowed into them from a stone brought from the mountains.
Loving Stone” - this is the poetic name the Chinese gave to this stone. A loving stone (tshu-shi), the Chinese say, attracts iron, just as a tender mother attracts her children.
Teacher. What stone are we talking about in the legend? (About the magnet.)
bodies, long time retaining magnetization are called permanent magnets or just magnets.
Teacher. You have magnets on your desks. I suggest taking the magnets and bringing them to each other without touching. What are you observing? How do you explain? Why does magnet interaction occur? It turns out there is something between the magnets that we cannot see and cannot touch with our hands. Then this is called a special form of matter - a field. Magnetic field. We find out the topic of the lesson and set the goal of the lesson - the study of the magnetic field. Not just the concept of a magnetic field, but its properties.
3 ) Primary assimilation of new knowledge.
So we write down the topic in our notebook. Magnetic field and its graphic representation. Inhomogeneous and homogeneous magnetic fields. The purpose of our lesson: to identify the basic properties of the magnetic field and ways to depict it
So a little about magnets (INFOUROCK website, Magnetic field)
(while watching the film, we write down definitions, field properties, and make sketches)
A magnetic field - special form of matter ( force field) that forms around moving charged particles)
1. The magnetic field is generated only by moving charges.
2. The magnetic field is invisible, but material. It can only be detected by the effect it has.
3. A magnetic field can be detected by its effect on a magnetic needle and other moving bodies.
You can depict a magnetic field using magnetic lines.
Magnetic lines are imaginary lines along which small magnetic needles would be located when placed in a magnetic field.
We can see them by doing an experiment with iron filings.
Experiment: Slowly sprinkle iron filings onto a white sheet under which there is a magnet. The sawdust lines up along the magnetic field lines.
Please note that in those areas where the magnetic field is stronger - at the poles, the magnetic lines are located closer to each other, i.e. thicker. Than in those places where the field is weaker.
Features of magnetic lines (write down)
1. Magnetic lines can be drawn through any point in space.
2. They are closed and do not intersect. The middle line goes on forever.
3. The magnetic line is drawn so that the tangent at each point of the line coincides with the axis of the magnetic needle placed at this point.
4. The direction of the magnetic line is taken to be the direction of the north pole of the compass needles located along this line.
5. A stronger magnetic field is represented by a higher concentration.
Consider the power lines of a coil with current. We have been familiar with the concept of solenoid since 8th grade. .
Solenoid- this is a coil in the form of an insulated conductor wound on a cylindrical surface through which an electric current flows (show)
Arrow rule (draw in notebook)
Uniform field (draw in a notebook)
Inhomogeneous field (draw in a notebook)
4 ) Initial check of understanding fill out the tables
| The result is a graphical representation of magnetic field lines |
|
| Strip magnet | |
| Arc magnet |
| Inhomogeneous magnetic field | Uniform magnetic field |
|
| Line layout | Curved, their thickness varies | Parallel, their density is the same |
| Density of lines | not the same | Same |
| not the same | is the same |
5 ) Primary consolidation. Independent work with mutual verification.
1. The rotation of a magnetic needle near a current-carrying conductor is explained by the fact that it is acted upon by...
A. ...magnetic field created by charges moving in a conductor.
B. ...electric field created by the charges of a conductor.
V. ... electric field created by charges moving in a conductor.
2. Magnetic fields are created...
A. ...both stationary and moving electric charges.
B. ... stationary electric charges.
B. ...moving electric charges.
3. Magnetic field lines are...
A. ...lines that coincide with the shape of the magnet.
B. ... lines along which a positive charge moves when entering a magnetic field.
B. ...imaginary lines along which small magnetic arrows would be located, placed in a magnetic field.
4. Magnetic field lines in space outside a permanent magnet...
A. ...start at the north pole of the magnet and end at infinity.
B. ... begin at the north pole of the magnet and end at the south.
V. ... begin at the pole of the magnet and end at infinity.
G. ...start at the south pole of the magnet and end at the north.
5. The configurations of the magnetic field lines of the solenoid are similar to the pattern of field lines...
A. ... strip magnet.
B. ...horseshoe magnet.
IN. … straight wire with current.
Standard testing and self-assessment:
3 correct answers – score 3,
4 correct answers – score 4,
5 correct answers – score 5.
6) Information about homework, instructions on its implementation
7) Reflexion (summarizing the lesson)
Choose the beginning of a phrase and continue the sentence.
I will try…
I was surprised...
gave me a lesson for life...
today I found out...
it was interesting…
it was difficult…
I completed tasks...
I realized that...
Now I can…
I felt that...
I purchased...
I learned…
I managed …
