As depicting graphically magnetic induction line. Induction of magnetic field
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The electric field is characterized by the electric field strength.
Electric field strength is a vector vector. The magnetic field is characterized by magnetic induction.
Magnetic induction is a vector magnitude, it is indicated by the letter.
Direction of magnetic induction vector
For the direction of the magnetic induction vector, a direction is taken, which shows the northern pole n of a magnetic arrow, freely installed in the magnetic field.
This direction coincides with the direction of positive normal to a closed circuit with a current.
Using a frame with a current or magnetic arrow, you can determine the direction of the magnetic induction vector at any point of the field.
In the magnetic field of the rectilinear conductor with a current, the magnetic arrow at each point is mounted on the tangent of the circumference, the plane of which is perpendicular to the wire, and its center lies on the axis of the wire.
Rule Braschik
The direction of magnetic induction vector is installed using the Brascover rule.
If the direction of the progressive motion of the bouwn coincides with the current direction in the conductor, the direction of rotation direction of the bouwn handle indicates the direction of the magnetic induction vector
Magnetic induction lines
The magnetic field can be shown using magnetic induction lines.
Magnetic induction lines Call lines tangent to which in any of their point coincide with the vector at this point point. Line of magnetic induction vector similar lines of electrostatic field strength vector.
Magnetic induction lines can be made visible by using iron sawdust.
Magnetic field of rectilinear conductor with current
For a straight conductor with a current of magnetic induction line are concentric circles lying in the plane perpendicular to this conductor with a current. The center of the circles is located on the axis of the conductor. The arrow on the lines indicate which direction the vector of magnetic induction is directed, tangent to this line.
Magnetic field coil with current (solenoid)
If the length of the solenoid is much more than its diameter, then the magnetic field inside the solenoid can be considered uniform.
Lines of magnetic induction of such a field parallel And are at equal distances from each other.
Magnetic field of land
Line of the magnetic induction of the field of the earth is similar to the magnetic induction lines of the solenoid field.
The magnetic axis of the Earth is with the axis of rotation of the Earth angle of 11.5 °.
Periodically, magnetic poles change their polarity.
Vortex field
The power lines of the electrostatic field always have sources: they begin on positive charges and ends on negative.
And the magnetic induction lines have no beginning, no end, they are always closed.
Fields with closed vector lines call vortex.
Magnetic field - vortex field.
The magnetic field does not have sources.
There are no magnetic charges of magnetic charges.
soMagnetic field is a vortex field, in each of its point the magnetic induction vector indicates a magnetic arrow, the direction of the magnetic induction vector can be determined by the rule of the rope
For a visual image of a magnetic field, use magnetic induction lines. Line magnetic induction they call such a line, at each point where the induction of the magnetic field (vector) is directed along the tangent of the curve. The direction of these lines coincides with the direction of the field. The magnetic induction line was agreed so that the number of these lines per unit area perpendicular to them would be equal to the induction module in the field of the field. Then, on the thickness of magnetic induction lines, they judge the magnetic field. Where the lines are thick, the magnetic field induction module is greater. Magnetic induction lines are always closedunlike electrostatic field strength lineswhich are open (begin and end on charges). The direction of magnetic induction lines is located according to the rule of the right screw: If the progressive movement of the screw coincides with the direction of the current, its rotation occurs in the direction of magnetic induction lines. As an example, we give the pattern of the magnetic induction of the direct current, the current perpendicular to the drawing plane from us for the drawing (Fig. 2).
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Fig. 3. Find the circulation of the induction of the magnetic field around the circle of an arbitrary radius a.which coincides with the line of magnetic induction. The field is created by current force I.flowing on an infinitely long conductor located perpendicular to the drawing plane (Fig. 3). The induction of the magnetic field is aimed at tangent to the magnetic induction line. We convert the expression, since asa \u003d 0 andcosa \u003d 1. Induction of the magnetic field created by the current currently in an infinitely long conductor is calculated by the formula: B \u003d.m0m. I /(2p. a.), T. 
Circulation of the vector for this circuit, finding the formula (3): 
M 0 M. I., as 
- circumference. So, 
It can be shown that this ratio is true for the circuit of an arbitrary shape covering the conductor with a current. If the magnetic field is created by the current system I.1, I.2, ... , I.n, then the circulation of the induction of the magnetic field along a closed contour covering these currents is equal to
(4)
Relation (4) and is a full current law: The circulation of the induction of the magnetic field along an arbitrary closed contour is equal to the product of a magnetic constant, magnetic permeability on the algebraic amount of current forces covered by this circuit.
Current strength can be found using current density j.: Where S.-Contact the cross section of the conductor. Then the full current law is written as
(5)
Magnetic stream.
By analogy with the stream of electrical field strength, a magnetic field induction flow or a magnetic flux is introduced. Magnetic flux through some surface Call the number of magnetic induction lines that permeate it. Suppose in an inhomogeneous magnetic field there is a surface area S.. To find a magnetic flux through it mentally split the surface on the elementary sections with an area ds.which can be considered flat, and the field within their limits is homogeneous (Fig. 4). Then the elementary magnetic flow dFBulls this surface is equal to: dFB. \u003d B · ds ·cOS A. \u003d B.n. ds.where B. - magnetic field induction module at the location of the site, A is the angle between the vector and the normal to the site, B.n. \u003d B ·cOS A- Projection of the induction of the magnetic field to the direction of normal. Magnetic flow F. B across the entire surface is equal to the sum of these threads dFB, i.e.
  Fig. four |
(6)
since the summation of infinitely small values \u200b\u200bis integration.
In the system of the SI, the magnetic flux is measured in Webkers (WB). 1 WB \u003d 1 T. · 1 m 2.
Gaussian Theorem for Magnetic Field
The following theorem is proved in electrodynamics: magnetic flow, permeating an arbitrary closed surface, is zero .
This ratio got a name gaussian theorems For a magnetic field. This theorem is a consequence of the fact that in nature there are no "magnetic charges" (in contrast to the electric) and magnetic induction lines are always closed (unlike the voltage lines of the electrostatic field, which begin and end on electrical charges).
Work on moving conductor with current in a magnetic field
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It is known that the power of the ampere acts on the conductor with the current in the magnetic field. If the conductor moves, then with its movement, this force makes a job. We define it for a particular case. Consider the electrical chain, one of the plots DCwhich can slide (without friction) by contacts. In this case, the chain forms a flat circuit. This circuit is located in a homogeneous magnetic field with an induction perpendicular to the contour plane pointing at us (Fig. 5). On the plot DCamp will operate
F \u003d bil ·sina. \u003d Bil., (8)
where l. - Length of the site, I. - The current current by the conductor. - angle between current and magnetic fields. (In this case \u003d 90 ° ISIN A \u003d 1). The direction of force we find on the rule of the left hand. When moving the site DCon the elementary distance dX. Elementary work is performed dAequal da \u003d F · DX. Considering (8), we get:
Da \u003d bil · dx \u003d ib · ds \u003d i · DFB, (9)
insofar as dS \u003d L · DX- area described by the conductor at its movement, dFB. \u003d B · ds- Magnetic flow through this area or changing the magnetic flux through the area of \u200b\u200ba flat closed circuit. Expression (9) is valid for an inhomogeneous magnetic field. In this way, work on the movement of a closed circuit with a constant current in a magnetic field is equal to the product of the current for the change in the magnetic flux through the area of \u200b\u200bthis circuit.
Electromagnetic induction phenomenon
The phenomenon of electromagnetic induction is as follows: With any change in the magnetic flux that penetrates the area covered by the conductive circuit, the electromotive force arises in it.. It is called E.D.S. induction . If the outline is closed, then under the action of EDs. Electric current appears, called induction .
Consider one of the experiments carried out by Faraday, on the detection of induction current, therefore, E.D.S. induction. If in a solenoid, closed on a very sensitive electrical measuring device (galvanometer) (Fig. 6), to move or extend the magnet, then when the magnet moves, the galvanometer arrow is devoting to the occurrence of the induction current. The same is observed when the solenoid movement relative to the magnet. If the magnet and the solenoid are stationary relative to each other, the induction current does not occur. Thus, with the mutual movement of these bodies, the magnetic flux created by the magnetic field of the magnet, through the solenoid turns, which leads to the appearance of an induction current caused by the resulting EDS induction.
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Lenza rule
The direction of the induction current is determined rule Lenza : Induction current always has such a direction that the magnetic field created by them prevents the change in the magnetic flux, which causes this current. From this it follows that with an increase in magnetic flux, the induction current emerging current will have such a direction so that the magnetic field generates to them is directed against the external field, counteracting the increase in the magnetic flux. Reducing the magnetic flux, on the contrary, leads to the appearance of an induction current creating a magnetic field that coincides in the direction with an external field.
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Let, for example, in a homogeneous magnetic field there is a square frame made of metal and permeated with a magnetic field (Fig. 7). Suppose that the magnetic field increases. This leads to an increase in the magnetic flux through the frame area. According to the regulation of the Lenz, the magnetic field, the emerging induction current will be directed against the external field, i.e. The vector of this field is opposite to the vector. Applying the rule of the right screw (if the screw rotate so that its translational movement coincides with the direction of the magnetic field, then its rotational motion gives the direction of current), we find the direction of the induction current II..
The law of electromagnetic induction.
The law of electromagnetic induction, which determines the resulting ED, was opened by the Faraday experienced by. However, it can be obtained based on the law of conservation of energy.
Let's return to the electrical circuit shown in Fig. 5 placed in a magnetic field. We will find the work performed by the source of the current with the ED. e.for an elementary period of time dt., when moving charge charges. From the definition of EDs. Work dAwaiting forces is equal to: dAsTOR \u003d. e · DQ.where dQ. - the magnitude of the charge flowing through the chain during the time dt.. But dQ \u003d i · dtwhere I. - Current power in the chain. Then
DA STOR \u003d. e · i · dt. (10)
The operation of the current source is spent on the allocation of a certain amount of heat dQ.and to work dA By moving the conductor DCin a magnetic field. According to the law of energy conservation, equality should be performed
DA STOR \u003d. dQ + DA.(11)
From the law of Joule - Lenz write:
DQ \u003d I.2R · dt., (12)
where R. - complete resistance of this chain, and from the expression (9)
Da \u003d i · DFB, (13)
where dFB- Change the magnetic flux through the area of \u200b\u200bthe closed contour when the conductor moves. Substituting expressions (10), (12) and (13) in formula (12), after reducing I.Receive e.· dT \u003d IR · DT + DFB. Sharing both parts of this equality on dt.Find: I. = (e -From this expression it follows the conclusion that in the chain, except E.D.S. e., there is still some kind of electromotive force eIequal
(14)
and due to a change in the magnetic flux, permeating the contour area. This ED And is E.D.S. electromagnetic induction or short E.D. induction. The ratio (14) is electromagnetic induction lawthat is formulated: e.D.S. Induction in the circuit is equal to the rate of change of the magnetic flux, which penetrates the area covered by this circuit. The minus sign in Formula (14) is a mathematical expression of Lenza rule.
We cannot see the magnetic field, but it is important for a better understanding of magnetic phenomena to learn how to portray it. This will help magnetic arrows. Each such arrow is a small permanent magnet, which is easily rotated in the horizontal plane (Fig. 2.1). About how graphically depicts a magnetic field and what physical value it characterizes, you will learn from this paragraph.
Fig. 2.2. In a magnetic field, the magnetic arrows are oriented in a certain way: the north pole arrows indicates the direction of the magnetic field induction vector at this point
We study the power characteristics of the magnetic field
If the charged particle moves in a magnetic field, the field will act on a particle with some force. The value of this force depends on the charge of the particle, directions and the values \u200b\u200bof the speed of its movement, as well as how strong is the field.
The power characteristic of the magnetic field is magnetic induction.
Magnetic induction (magnetic field induction) is a vector physical value that characterizes the power effect of the magnetic field.
Magnetic induction is denoted by the symbol of B.
A unit of magnetic induction in SI - Tesla; Named in honor of Serbian physics Nikola Tesla (1856-1943):
For the direction of the magnetic induction vector, a direction is received at this point of magnetic field, which indicates the north pole of the magnetic arrow installed at this point (Fig. 2.2).
Note! The direction of force with which the magnetic field acts on moving charged particles or to the conductor with a current, or on a magnetic arrow, does not coincide with the direction of the magnetic induction vector.
Magnetic lines:
Fig. 2.3. Line of the magnetic field of the strip magnet
Outside the magnet comes out of the north pole of the magnet and are included in the southern;
Always closed (magnetic field is a vortex field);
The most thickness is located in the magnet poles;
Never intersect
We depict a magnetic field
In fig. 2.2 We see how magnetic arrows in a magnetic field are oriented: their axes seemed to form lines, and the magnetic induction vector at each point is directed along the tangent to the line passing through this point.
With the help of magnetic lines, graphically depict magnetic fields:
1) for the direction of the magnetic induction line at this point, the direction of the magnetic induction vector is taken;
Fig. 2.4. Chains of iron sawdust reproduce the pattern of magnetic induction of the magnetic field of a horseshoe magnet
2) The greater the magnetic induction module, the closer to each other draws the magnetic lines.
Having considered the graphic image of the magnetic field of the strip magnet, you can make some conclusions (see Fig. 2.3).
Note that these findings are valid for magnetic lines of any magnet.
What direction do magnetic lines inside the bandago magnet?
The picture of magnetic lines can be reproduced using iron sawdust.
Take a horseshoe magnet, we put a plate from the plexiglass on it and we will pour iron sawdust on the plate through the corticle. In a magnetic field, each piece of iron is magnetized and turned into a small "magnetic arrow". The improvised "arrows" is centered along the magnetic lines of the magnetic field of the magnet (Fig. 2.4).
Position the magnetic lines of the magnetic field of the horseshoe magnet.
Learn about a homogeneous magnetic field
The magnetic field in some part of the space is called homogeneous if at each it point the magnetic induction vectors are the same as the module and direction (Fig. 2.5).
In areas where the magnetic field is uniform, the magnetic induction line is parallel and are located at the same distance from each other (Fig. 2.5, 2.6). The magnetic lines of a homogeneous magnetic field, directed to us, are made to depict points (Fig. 2.7, a) - we seem to see the "is the appearance of arrows" flying to us. If magnetic lines are directed from us, they are depicted with cross - we seem to see "booming booms" flying from us (Fig. 2.7, b).
In most cases, we are dealing with an inhomogeneous magnetic field - a field, in different points of which the magnetic induction vectors have different values \u200b\u200band directions. Magnetic lines of such a field are curved, and their density is different.
Fig. 2.6. Magnetic field inside a strip magnet (a) and between two magnets facing each other with multi-person poles (b), can be considered homogeneous
We study the magnetic field of the earth
To study the earth's magnetism, William Hilbert made a permanent magnet in the form of a bowl (model of the Earth). By placing a compass on a bowl, he noticed that the compass arrow behaves the same way on the surface of the earth.
Experiments allowed a scientist to assume that the Earth is a huge magnet, and its southern magnetic pole is located in the north of our planet. Further studies confirmed V. Hilbert's hypothesis.
In fig. 2.8 shows the picture of the magnetic induction of the magnetic field of the Earth.
fig. 2.7. An image of a magnetic induction lines of a homogeneous magnetic field, which are perpendicular to the plane of the pattern and are directed to us (A); directed from us (b)
Imagine that you go to the North Pole, moving exactly in the direction that the compass arrow indicates. Do you achieve destination?
The magnetic induction lines of the magnetic field of the Earth are not parallel to its surface. If you fix the magnetic arrow in the cardan suspension, that is, so that it can freely rotate both around horizontal, so
Fig. 2.8. Magnetic Line Lines Magnetic Lines Planet Earth
and around the vertical axes, the arrow will be installed at an angle to the surface of the earth (Fig. 2.9).
How the magnetic arrow will be located in the device in fig. 2.9 near the Northern Magnetic Pole of the Earth? Near the Southern Magnetic Pole of the Earth?
The magnetic field of the Earth has long been helped to navigate to travelers, sailors, military and not only to them. It has been proven that fish, marine mammals and birds during their migrations are focused on the magnetic field of the Earth. Also oriented, looking for a way home, and some animals, such as cats.
Learn about magnetic storms
Studies have shown that in any terrain the magnetic field of the Earth is periodically, every day, changes. In addition, there are small annual changes in the magnetic field of the Earth. There are, however, the sharp changes. Strong perturbations of the magnetic field of the Earth, which cover the entire planet and continue from one to several days, are called magnetic storms. Healthy people practically do not feel them, but those who have cardiovascular diseases and diseases of the nervous system, magnetic storms cause deterioration of well-being.
The magnetic field of the Earth is a kind of "shield", which protects our planet from the flying from space, mainly from the Sun ("Sunny Wind"), charged particles. Near magnetic poles, particles are quite close to the Earth's atmosphere. Under the increase in solar activity, cosmic particles fall into the upper layers of the atmosphere and ionize gas molecules - polar beams are observed on Earth (Fig. 2.10).
Let's sum up
Magnetic induction B is a vector physical value that characterizes the power action of the magnetic field. The direction of the magnetic induction vector coincides with the direction on which the north pole of the magnetic arrow indicates. Unit of magnetic induction in C - Tesla (TL).
The conditioned directional lines, at each point of which the tangent coincides with the line, along which the vector of magnetic induction is directed, is called magnetic induction lines or magnetic lines.
The magnetic induction lines are always closed, outside the magnet, they come out of the north pole of the magnet and are included in the southern, grounds are located in those areas of the magnetic field where the magnetic induction module is greater.
Planet Earth has a magnetic field. Near the Northern Geographical Pole of the Earth, its southern magnetic pole is located near the Southern Geographical Pole - the Northern Magnetic Pole.
Control questions
1. Give the definition of magnetic induction. 2. How is the magnetic induction vector directed? 3. What is the unit of magnetic induction in si? In honor of whom she is named? 4. Give the definition of magnetic induction lines. 5. What direction is taken for the direction of magnetic lines? 6. What is the depends on the thickness of magnetic lines? 7. What magnetic field is called homogeneous? 8. Prove that the earth has a magnetic field. 9. How are the magnetic poles of the earth relative to geographical? 10. What is magnetic storms? How do they affect the person?
Exercise number 2.
1. In fig. 1 depicts magnetic induction lines on a certain piece of magnetic field. For each case, A-in define: 1) what a field is homogeneous or inhomogeneous; 2) the direction of the magnetic induction vector at the points A and in the field; 3) At what point - a or in - the magnetic induction of the field is greater.
2. Why can the steel window lattice be magnetized with time?
3. In fig. 2 shows the magnetic field lines created by two identical permanent magnets facing each other poles to each other.
1) Is there a magnetic field at point a?
2) What is the direction of the magnetic induction vector at the point in? At point with?
3) At what point - a, in or C - the magnetic induction of the field the largest?
4) What is the direction of magnetic induction vectors inside the magnets?
4. Previously, during expeditions to the North Pole, difficulties arose in determining the direction of movement, because near the pole, ordinary compasses almost did not work. What do you think, why?
5. Use additional sources of information and find out what value is a magnetic field for life on our planet. What would happen if the magnetic field of the earth suddenly disappeared?
6. There are parts of the earth's surface, where the magnetic induction of the magnetic field of the Earth is much larger than in the neighboring regions. Take advantage of additional sources of information and find out about magnetic anomalies.
7. Explain why any uncharged body is always attracted to the body having an electric charge.
This is the material of the textbook
Already in the VI century. BC. In China, it was known that some ores possess the ability to attract each other and attract iron objects. Pieces of such ores were found near the city of Magnesia in Malaya Asia, so they got a name magnets.
Where does the magnet and iron objects interact? Recall why electrified bodies are attracted? Because about the electric charge is formed a peculiar form of matter - electric field. There is a similar form of matter around the magnet, but has another nature of origin (because ore is electrically neutral), it is called magnetic field.
For the study of the magnetic field, direct or horseshoe-shaped magnets are used. Certain locations of the magnet possess the greatest attractive effects, they are called poles (North and South). Multimame magnetic poles are attracted, and the same names are repelled.
For the power characteristics of the magnetic field use magnetic field induction vector b. The magnetic field is graphically depicted using power lines ( magnetic induction lines). Lines are closed, have no beginning, no end. The location from which the magnetic lines go - the North Pole (North) includes magnetic lines in the South Pole (South).
Magnetic field can be made "visible" with iron sawdust.
Magnetic Explorer field with current
And now that they discovered Hans Christian Ersted and Andre Marie Ampere In 1820 it turns out, the magnetic field exists not only around the magnet, but also any conductor with a current. Any wire, for example, a cord from the lamp, which flows electric current, is a magnet! The wire with a current interacts with a magnet (try to bring a compass to it), two wires interact with a shock with each other.
The power lines of the magnetic field of direct current are circles around the conductor.
Direction of magnetic induction vector
The direction of the magnetic field at this point can be defined as a direction that points the north pole of the compass arrow placed at this point.
The direction of magnetic induction lines depends on the direction of the current in the conductor.
The direction of the induction vector is determined by rule braschik or rule right hand.
Vector magnetic induction
This is a vector magnitude that characterizes the power action of the field.
Induction of the magnetic field of an endless rectilinear conductor with a current at a distance of R from it:
The induction of the magnetic field in the center of the thin circular turn of the radius R:
Induction of magnetic field solenoid (coil, whose turns are consistently bypassed in one direction):
Superposition principle
If the magnetic field at this point is created by several field sources, then magnetic induction - vector sum of the induction of each of the fields separately
The Earth is not only a large negative charge and source of the electric field, but at the same time, the magnetic field of our planet is like a field of a direct magnet of giant sizes.
The geographical South is located near the magnetic north, and the geographical north is close to the magnetic south. If the compass is placed in a magnetic field of the Earth, its northern arrow is focused along the magnetic induction lines in the direction of the southern magnetic pole, that is, it will indicate where the geographical north is located.
The characteristic elements of terrestrial magnetism are very slowly changed over time - century-old changes. However, from time to time, magnetic storms occur when the Earth's magnetic field is very distorted for several hours, and then gradually returns to previous values. Such a sharp change affects the well-being of people.
The magnetic field of the Earth is a "shield", covering our planet from particles penetrating from space ("solar wind"). Near magnetic poles, particles are much closer to the surface of the earth. With powerful solar flares, the magnetosphere is deformed, and these particles can move to the upper layers of the atmosphere, where they face gas molecules, polar shines are formed.
The particles of iron dioxide on the magnetic film are well magnetized during the recording process.
Trains on a magnetic cushion slide over the surface absolutely without friction. The train is able to develop speed up to 650 km / h.
The work of the brain, the pulsation of the heart is accompanied by electrical impulses. At the same time, a weak magnetic field occurs in the organs.
Themes of the EGE codifier: Magnet interaction, the magnetic field of the conductor with the current.
Magnetic properties of the substance are known to people for a long time. Magnets received their name from the ancient city of Magnesia: the mineral was spread in its surroundings (subsequently called magnetic iron or magnetite), whose pieces of iron items attracted.
Magnet interaction
On the two sides of each magnet are located north Pole and south Pole. Two magnets are attracted to each other with the varied poles and repel the same name. Magnets can act on each other even through the vacuum! All this reminds the interaction of electrical charges, however magnet interaction is not electric. This is evidenced by the following experienced facts.
Magnetic force weakens when heating a magnet. The strength of the interaction of point charges does not depend on their temperature.
Magnetic power is weakening if shaking a magnet. Nothing like electrically charged bodies occurs.
Positive electrical charges can be separated from negative (for example, when electrifying tel). But it is not possible to split the magnet poles: if you cut a magnet into two parts, then the poles also occur in the section, and the magnet disintegrates two magnets with a variety of poles at the ends (oriented in the same way as the poles of the source magnet).
Thus, magnets always bipolar, they exist only in the form dipole. Isolated magnetic poles (so-called Magnetic monopoles - Analogs of an electric charge) in case there is no way (in any case, they have not yet been detected experimentally). This is perhaps the most impressive asymmetry between electricity and magnetism.
Like electrically charged bodies, magnets act on electrical charges. However, the magnet acts only on moving charge; If the charge rests on the magnet, the actions of the magnetic force on the charge is not observed. On the contrary, the electrified body acts on any charge, regardless of whether it rests or moves.
According to modern ideas of the theory of closestream, the interaction of magnets is carried out through magnetic field. And it is that a magnet creates a magnetic field in the surrounding space, which acts on another magnet and causes a visible attraction or repulsion of these magnets.
An example of a magnet is served magnetic needle compass. With the help of a magnetic arrow, you can judge the presence of a magnetic field in this area of \u200b\u200bspace, as well as the direction of the field.
Our planet Earth is a giant magnet. Near the Northern Geographical Pole of the Earth is the Southern Magnetic Pole. Therefore, the north end of the arrow of the compass, turning to the southern magnetic pole of the Earth, indicates the geographical north. Hence, in fact, the name "North Pole" of the magnet arose.
Lines of magnetic field
The electric field, we recall, is examined using small trial charges, according to which you can judge the value and direction of the field. Analogue of a trial charge in the case of a magnetic field is a small magnetic arrow.
For example, you can get some geometric view of the magnetic field, if you place very small compass arrows at different points of space. Experience shows that the arrows will be lined up along certain lines - so called magnetic field lines. Let us give the definition of this concept in the form of the following three points.
1. Magnetic field lines, or magnetic power lines - these are directed lines in space with the following property: the small arrow of the compass placed at each point of such a line, oriented on the tangent of this line.
2. The direction of the magnetic field line is the direction of the northern ends of the compass arrows located at the points of this line.
3. The thick of the lines go, the stronger the magnetic field in this area of \u200b\u200bspace.
The role of arrows of the compass with success can perform iron sawdust: small sawdust is magnetized and behave exactly as magnetic arrows.
So, pouring iron sawders around a permanent magnet, we will see about the following picture of the magnetic field lines (Fig. 1).
Fig. 1. Field of a permanent magnet
The north pole of the magnet is denoted by blue and the letter; South Pole - Red and Letter. Please note that the field lines leave the north pole of the magnet and are included in the South Pole: after all, it is to the southern pole of a magnet that the northern end of the compass arrows will be directed.
Ersted Experience
Despite the fact that electric and magnetic phenomena were known to people from antiquity, no relationship between them has not been observed. Within a few centuries, the study of electricity and magnetism was in parallel and independently of each other.
That wonderful fact that electric and magnetic phenomena are actually connected with each other, was first discovered in 1820 - in the famous experience of Ersteda.
Ersted experience scheme is shown in Fig. 2 (image from rt.mipt.ru). Over the magnetic arrow (and - the north and south poles of the arrow) is a metal conductor connected to a current source. If you close the chain, the arrow turns perpendicular to the conductor!
This simple experience directly pointed out the relationship of electricity and magnetism. Experiments following Ersteda experience, firmly installed the following pattern: the magnetic field is generated by electrical currents and acts on current.
Fig. 2. Ersted Experience
The pattern of the magnetic field lines generated by the conductor with the current depends on the shape of the conductor.
Magnetic field of straight wire with current
The lines of the magnetic field of the rectilinear wire with the current are concentric circles. The centers of these circles lie on the wire, and their planes are perpendicular to the wire (Fig. 3).
Fig. 3. Direct wire field with current
To determine the direction of the lines of the magnetic field of the direct current, there are two alternative rules.
Rule clockwise. Field lines are counterclockwise, if you look so that the current is on us.
Rule rule (or rule Braschik, or corkscrew rule - This is who is closer ;-)). Field lines go to where you need to rotate the screw (with the usual right thread) so that it moves through the thread in the current direction.
Use the rule that you like more. It is best to get used to the right clockwise rule - you yourself subsequently make sure that it is more universally and it is easier for them (and then with gratitude, remember it in the first year when you study analytical geometry).
In fig. 3 appeared and something new: this is a vector called induction of magnetic field, or magnetic induction. The magnetic induction vector is an analogue of the electric field strength vector: it serves silence characteristic Magnetic field, determining the force with which the magnetic field acts on moving charges.
We will talk about the forces in the magnetic field later, but for now we only note that the magnitude and direction of the magnetic field is determined by the magnetic induction vector. At each point of space, the vector is directed there, where and the north end of the arrow of the compass placed at this point, namely, by tangent of the field line in the direction of this line. Magnetic induction is measured in teslah (TL).
As in the case of an electric field, for the induction of the magnetic field, the fair superposition principle. He lies in the fact that induction of magnetic fields created at this point by various currents fold vector and give the resulting magnetic induction vector:.
Magnetic field turns with shock
Consider a circular coil through which the constant current circulates. The source that creates the current, we do not show the picture.
The pattern of the fields of the field of our turn will have approximately the following form (Fig. 4).
Fig. 4. Field turn with current
It will be important for us to be able to determine which half-space (relative to the plane of the turn) is directed a magnetic field. Again we have two alternative rules.
Rule clockwise. Field lines go there, looking from where the current seems circulating counterclockwise.
Rule rule. Field lines go to where the screw will move (with the usual right thread) if you rotate it in the current direction.
As you can see, the current and the field change roles - compared with the wording of these rules for the case of direct current.
Magnetic field coil with current
Coil It turns out if it's tight, the turn to the turn, wind the wire into a rather long spiral (Fig. 5 - image from the site en.wikipedia.org). There may be several dozen, hundreds or even thousands of turns in the coil. The coil is called solenoid.
Fig. 5. Coil (solenoid)
The magnetic field of one turn, as we know, it looks not very simple. Fields? Separate coil turns are superimposed on each other, and it would seem, as a result, a completely tangled picture should be. However, this is not the case: the field of the long coil has an unexpectedly simple structure (Fig. 6).
Fig. 6. Field of coil with current
In this figure, the current in the coil comes counterclockwise, if you look at the left (it will be, if in Fig. 5 The right end of the coil to connect to the "plus" of the current source, and the left end to the "minus"). We see that the magnetic field of the coil has two characteristic properties.
1. Inside the coil away from her edge, the magnetic field is uniform: At each point, the magnetic induction vector is the same in size and direction. Field lines - parallel straight; They are twisted only near the root of the coil when they go out.
2. Outside the coil field close to zero. The more turns in the coil - the weaker the field outside it.
Note that the endlessly long coil does not release the field outside: there is no magnetic field outside the coil. Inside such a coil, the field is everywhere uniform.
Nothing reminds? The coil is a "magnetic" condenser analogue. You remember that the capacitor creates a homogeneous electric field inside itself, whose lines are twisted only near the edges of the plates, and outside the condenser the field is close to zero; The condenser with endless folds does not produce the field outwardly, and everywhere inside it the field is uniformly.
And now - the main observation. Please compare the picture of the magnetic field lines outside the coil (Fig. 6) with the magnet field lines in Fig. one . The same thing is that? And here we are approaching the question, which probably has already arisen here: if the magnetic field is generated by currents and acts on currents, then what is the cause of the magnetic field near the permanent magnet? After all, this magnet seems to be a conductor with a current!
The hypothesis of the ampere. Elementary Toki.
At first, they thought that the interaction of magnets was explained by special magnetic charges focused on the poles. But, unlike electricity, no one could isolate a magnetic charge; After all, as we have already said, it was not possible to get separately the northern and southern pole of the magnet - the poles are always present in a magnet in pairs.
Doubts about magnetic charges aggravated Ersted's experience when it turned out that the magnetic field is generated by an electric shock. Moreover, it turned out that for any magnet, you can choose a conductor with a current configuration, such that the field of this conductor coincides with the magnet field.
Ampere put forward a bold hypothesis. There are no magnetic charges. The effect of the magnet is explained by closed electrical currents inside it..
What are these currents? These elementary Toki. circulate inside atoms and molecules; They are associated with the movement of electrons in atomic orbits. The magnetic field of any body consists of magnetic fields of these elementary currents.
Elementary currents can be erratically located relative to each other. Then their fields are mutually repaid, and the body does not show magnetic properties.
But if elementary currents are coordinated, their fields, folding, strengthen each other. The body becomes a magnet (Fig. 7; The magnetic field will be directed to us; the north pole of the magnet will also be directed to us).
Fig. 7. Elementary Magnet Currents
The hypothesis of the ampere of the elementary currents clarified the properties of the magnets. The heating and shaking of the magnet destroy the order of its elementary currents, and the magnetic properties weaken. The inseparableness of the magnet poles became apparent: at the point of the magnet cut, we obtain the same elementary currents on the ends. The body's ability to magnetize in the magnetic field is explained by the agreed building of the elementary currents, "rotating" properly (about the rotation of the circular current in the magnetic field, read in the following sheet).
The hypothesis of the Ampere turned out to be fair - this showed the further development of physics. The ideas about the elementary currents became an integral part of the atom theory developed in the twentieth century - almost five years after the brilliant guess of the ampere.
