Let us draw a series of continuous lines in a magnetic field so that these lines coincide everywhere with the direction of the field strength (with the direction of magnetic induction). The resulting picture can serve as an image of the magnetic field.
If you move a small, freely suspended compass needle along the magnetic field line, then its axis will everywhere coincide with the nearby section of the line. On one of the lines in Fig. 2.13 shows compass arrows in four positions.
Rice. 2.13. Bar magnet magnetic field
Rice. 2.14. Magnetic field of a rectilinear current-carrying conductor. Compare with fig. 2.10
On fig. 2.13, 2.14 by means of lines the magnetic fields of a permanent magnet and a rectilinear conductor with current are shown. The arrows on the lines show the direction of the magnetic field (the direction that the north end of the compass needle would point).
In order to be able to judge the strength of the field from the figure, it was agreed to draw lines the closer to one another, the stronger the field.
From fig. 2.13 shows that the strongest field is directly near the poles of the magnet. From fig. 2.14 it can be seen that the current field is strongest near the wire, and as you move away from it, the field weakens.
In § 2.1 it was said that small iron bodies under the influence of a magnet themselves become magnets (Fig. 2.1, a).
Therefore, it is clear that if you put a permanent magnet on the board and sprinkle the board with iron filings, then they will be located as small compass needles would be located. The pictures obtained by means of sawdust give a visual representation of the field.
On fig. 2.15 shows the magnetic field of the coil. If the wire is wound into a spiral, wound like a coil, then the equally directed fields of individual turns will add to each other, strengthening the field inside the coil.
The direction of the magnetic line coincides with the axis of the coil, and the field reaches its greatest value there. The field inside the coil is approximately uniform, i.e. the field strength remains approximately the same at different points. The distances between adjacent magnetic lines having the highest density inside the coil will also be the same.
Rice. 2.15. Coil magnetic field pattern
To study the structure of the magnetic field, one uses spectrum method. Small iron filings, falling into a magnetic field, are magnetized and, interacting with each other, form chains, the arrangement of which allows one to judge the structure of the magnetic field.
As an application example spectrum method Consider an experiment with the magnetic field of a straight conductor. Let us pass a long straight conductor connected to an electric circuit through a thin dielectric plate. We will pour small iron filings onto the plate, lightly tapping on the plate. The sawdust will gather around the conductor in the form of concentric circles of various diameters (Fig. 6.10). When repeating the experiment with other conductors at other values of the current strength, we get similar patterns, which are called magnetic spectra.
Spectra can be represented on paper as lines of magnetic induction.
For a straight conductor, such an image is shown in fig. 6.11. In the images of magnetic spectra lines of magnetic induction show the direction of magnetic induction at each point. At each point of the induction line, the tangent coincides with the magnetic induction vector.
Lines tangent to which at each point show the direction of magnetic induction are called lines of magnetic induction.
Density lines of magnetic induction depends on the modulus of magnetic induction. It is larger where the module is larger, and vice versa. The direction of the lines of magnetic induction of a direct conductor is determined by the rule of the right screw.
Spectra of magnetic fields conductors of a different shape have much in common.
So, the spectrum of the magnetic field of a ring with current is similar to two combined spectra of straight conductors (Fig. 6.12). Only the density of induction lines in the center of the ring is greater (Fig. 6.13).
The magnetic spectrum of a coil with a large number of turns (solenoid) is shown in fig. 6.14. The figure shows that the lines The magnetic induction of such a coil is internally parallel and has the same density. This indicates that inside the long coil the magnetic field is uniform - at all points the magnetic induction is the same (Fig. 6.15). The lines of magnetic induction diverge only outside the coil, where the magnetic field is inhomogeneous.
If we compare the spectra of magnetic fields of conductors with current of various shapes, we can see that induction lines are always closed or with further continuation, they can close. This indicates the absence of magnetic charges. Such a field is called vortex. Vortex field has no potential.material from the site
On this page, material on the topics:
Spectra of magnetic fields GDz Reshebnik
What physical processes occur during the formation of the magnetic spectrum
Discoveries in the field of magnetic fields
Report on the topic of the magnetic field and its graphic representation
Magnetic field spectra examples
Questions about this item:
Oersted's experiment in 1820. What does the deviation of the magnetic needle indicate when the electrical circuit is closed? There is a magnetic field around a current-carrying conductor. The magnetic needle reacts to it. The source of the magnetic field are moving electric charges or currents.
Oersted's experiment in 1820. What does the fact that the magnetic needle turned on indicate? This means that the direction of the current in the conductor has changed to the opposite.
Ampère's experiment in 1820. How to explain the fact that conductors with current interact with each other? We know that a magnetic field acts on a current-carrying conductor. Therefore, the phenomenon of the interaction of currents can be explained as follows: an electric current in the first conductor generates a magnetic field that acts on the second current and vice versa ...
Unit of current strength If a current of 1 A flows through two parallel conductors 1 m long, located at a distance of 1 m from each other, then they interact with a force N.
Unit of current strength 2 A What is the strength of the current in the conductors if they interact with the force H?
What is a magnetic field and what are its properties? 1.MP is a special form of matter that exists independently of us and our knowledge about it. 2. MP is generated by moving electric charges and is detected by the action on moving electric charges. 3. With the distance from the source of MF, it weakens.
Properties of magnetic lines: 1. Magnetic lines are closed curves. What does it say? If you take a piece of a magnet and break it into two pieces, each piece will again have a "north" and a "south" pole. If you again break the resulting piece into two parts, each part will again have a "north" and a "south" pole. No matter how small the resulting pieces of magnets are, each piece will always have a "north" and a "south" pole. It is impossible to achieve a magnetic monopole ("mono" means one, monopole - one pole). At least, this is the modern point of view on this phenomenon. This suggests that there are no magnetic charges in nature. The magnetic poles cannot be separated.
2. You can detect a magnetic field by ... A) by acting on any conductor, B) by acting on a conductor through which an electric current flows, C) a charged tennis ball suspended on a thin inextensible thread, D) by moving electric charges. a) A and B, b) A and C, c) B and C, d) B and D.
7. Which statements are true? A. Electric charges exist in nature. B. There are magnetic charges in nature. Q. There are no electric charges in nature. D. There are no magnetic charges in nature. a) A and B, b) A and C, c) A and D, d) B, C and D.
10. Two parallel conductors 1 m long, located at a distance of 1 m from each other when an electric current flows through them, are attracted with a force N. This means that currents flow through the conductors ... a) opposite directions of 1 A, b ) one direction 1 A each, c) opposite directions 0.5 A each, d) one direction 0.5 A each.
23. The magnetic needle will deviate if it is placed near ... A) near the flow of electrons, B) near the flow of hydrogen atoms, C) near the flow of negative ions, D) near the flow of positive ions, E) near the flow of nuclei of the oxygen atom. a) all answers are correct b) A, B, C, and D, c) B, C, D, d) B, C, D, E
3. The figure shows a cross section of a conductor with current at point A, the electric current enters perpendicular to the plane of the figure. Which of the directions presented at point M corresponds to the direction of the vector B of the induction of the magnetic field of the current at this point? a) 1, b) 2, c) 3, 4)
Let's understand together what a magnetic field is. After all, many people live in this field all their lives and do not even think about it. Time to fix it!
A magnetic field
A magnetic field is a special kind of matter. It manifests itself in the action on moving electric charges and bodies that have their own magnetic moment (permanent magnets).
Important: a magnetic field does not act on stationary charges! A magnetic field is also created by moving electric charges, or by a time-varying electric field, or by the magnetic moments of electrons in atoms. That is, any wire through which current flows also becomes a magnet!
A body that has its own magnetic field.
A magnet has poles called north and south. The designations "northern" and "southern" are given only for convenience (as "plus" and "minus" in electricity).
The magnetic field is represented by force magnetic lines. The lines of force are continuous and closed, and their direction always coincides with the direction of the field forces. If metal shavings are scattered around a permanent magnet, the metal particles will show a clear picture of the magnetic field lines emerging from the north and entering the south pole. Graphical characteristic of the magnetic field - lines of force.
Magnetic field characteristics
The main characteristics of the magnetic field are magnetic induction, magnetic flux and magnetic permeability. But let's talk about everything in order.
Immediately, we note that all units of measurement are given in the system SI.
Magnetic induction B - vector physical quantity, which is the main power characteristic of the magnetic field. Denoted by letter B . The unit of measurement of magnetic induction - Tesla (Tl).
Magnetic induction indicates how strong a field is by determining the force with which it acts on a charge. This force is called Lorentz force.
Here q - charge, v - its speed in a magnetic field, B - induction, F is the Lorentz force with which the field acts on the charge.
F- a physical quantity equal to the product of magnetic induction by the area of the contour and the cosine between the induction vector and the normal to the plane of the contour through which the flow passes. Magnetic flux is a scalar characteristic of a magnetic field.
We can say that the magnetic flux characterizes the number of magnetic induction lines penetrating a unit area. The magnetic flux is measured in Weberach (WB).
Magnetic permeability is the coefficient that determines the magnetic properties of the medium. One of the parameters on which the magnetic induction of the field depends is the magnetic permeability.
Our planet has been a huge magnet for several billion years. The induction of the Earth's magnetic field varies depending on the coordinates. At the equator, it is about 3.1 times 10 to the minus fifth power of Tesla. In addition, there are magnetic anomalies, where the value and direction of the field differ significantly from neighboring areas. One of the largest magnetic anomalies on the planet - Kursk and Brazilian magnetic anomaly.
The origin of the Earth's magnetic field is still a mystery to scientists. It is assumed that the source of the field is the liquid metal core of the Earth. The core is moving, which means that the molten iron-nickel alloy is moving, and the movement of charged particles is the electric current that generates the magnetic field. The problem is that this theory geodynamo) does not explain how the field is kept stable.
The earth is a huge magnetic dipole. The magnetic poles do not coincide with the geographic ones, although they are in close proximity. Moreover, the Earth's magnetic poles are moving. Their displacement has been recorded since 1885. For example, over the past hundred years, the magnetic pole in the Southern Hemisphere has shifted by almost 900 kilometers and is now in the Southern Ocean. The pole of the Arctic hemisphere is moving across the Arctic Ocean towards the East Siberian magnetic anomaly, the speed of its movement (according to 2004 data) was about 60 kilometers per year. Now there is an acceleration of the movement of the poles - on average, the speed is growing by 3 kilometers per year.
What is the significance of the Earth's magnetic field for us? First of all, the Earth's magnetic field protects the planet from cosmic rays and the solar wind. Charged particles from deep space do not fall directly to the ground, but are deflected by a giant magnet and move along its lines of force. Thus, all living things are protected from harmful radiation.
During the history of the Earth, there have been several inversions(changes) of magnetic poles. Pole inversion is when they change places. The last time this phenomenon occurred about 800 thousand years ago, and there were more than 400 geomagnetic reversals in the history of the Earth. Some scientists believe that, given the observed acceleration of the movement of the magnetic poles, the next pole reversal should be expected in the next couple of thousand years.
Fortunately, no reversal of poles is expected in our century. So, you can think about the pleasant and enjoy life in the good old constant field of the Earth, having considered the main properties and characteristics of the magnetic field. And so that you can do this, there are our authors, who can be entrusted with some of the educational troubles with confidence in success! and other types of work you can order at the link.
Topics of the USE codifier: interaction of magnets, magnetic field of a conductor with current.
The magnetic properties of matter have been known to people for a long time. Magnets got their name from the ancient city of Magnesia: a mineral (later called magnetic iron ore or magnetite) was widespread in its vicinity, pieces of which attracted iron objects.
Interaction of magnets
On two sides of each magnet are located North Pole and South Pole. Two magnets are attracted to each other by opposite poles and repel by like poles. Magnets can act on each other even through a vacuum! All this is reminiscent of the interaction of electric charges, however the interaction of magnets is not electrical. This is evidenced by the following experimental facts.
The magnetic force weakens when the magnet is heated. The strength of the interaction of point charges does not depend on their temperature.
The magnetic force is weakened by shaking the magnet. Nothing similar happens with electrically charged bodies.
Positive electric charges can be separated from negative ones (for example, when bodies are electrified). But it is impossible to separate the poles of the magnet: if you cut the magnet into two parts, then poles also appear at the cut point, and the magnet breaks up into two magnets with opposite poles at the ends (oriented in exactly the same way as the poles of the original magnet).
So the magnets always bipolar, they exist only in the form dipoles. Isolated magnetic poles (so-called magnetic monopoles- analogues of electric charge) in nature do not exist (in any case, they have not yet been experimentally detected). This is perhaps the most impressive asymmetry between electricity and magnetism.
Like electrically charged bodies, magnets act on electrical charges. However, the magnet only acts on moving charge; If the charge is at rest relative to the magnet, then no magnetic force acts on the charge. On the contrary, an electrified body acts on any charge, regardless of whether it is at rest or in motion.
According to modern concepts of the theory of short-range action, the interaction of magnets is carried out through magnetic field. Namely, 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 magnetic needle compass. With the help of a magnetic needle, one can judge the presence of a magnetic field in a given region of space, as well as the direction of the field.
Our planet Earth is a giant magnet. Not far from the geographic north pole of the Earth is the south magnetic pole. Therefore, the north end of the compass needle, turning to the south magnetic pole of the Earth, points to the geographical north. Hence, in fact, the name "north pole" of the magnet arose.
Magnetic field lines
The electric field, we recall, is investigated with the help of small test charges, by the action on which one can judge the magnitude and direction of the field. An analogue of a test charge in the case of a magnetic field is a small magnetic needle.
For example, you can get some geometric idea of the magnetic field by placing very small compass needles at different points in space. Experience shows that the arrows will line up along certain lines - the so-called magnetic field lines. Let us define this concept in the form of the following three paragraphs.
1. Magnetic field lines, or magnetic lines of force, are directed lines in space that have the following property: a small compass needle placed at each point of such a line is oriented tangentially to this line.
2. The direction of the magnetic field line is the direction of the northern ends of the compass needles located at the points of this line.
3. The thicker the lines go, the stronger the magnetic field in a given region of space..
The role of compass needles can be successfully performed by iron filings: in a magnetic field, small filings are magnetized and behave exactly like magnetic needles.
So, having poured iron filings around a permanent magnet, we will see approximately the following picture of magnetic field lines (Fig. 1).
Rice. 1. Permanent magnet field
The north pole of the magnet is indicated in blue and the letter ; the south pole - in red and the letter . Note that the field lines exit the north pole of the magnet and enter the south pole, because it is to the south pole of the magnet that the north end of the compass needle will point.
Oersted's experience
Despite the fact that electrical and magnetic phenomena have been known to people since antiquity, no relationship between them has been observed for a long time. For several centuries, research on electricity and magnetism proceeded in parallel and independently of each other.
The remarkable fact that electrical and magnetic phenomena are actually related to each other was first discovered in 1820 in the famous experiment of Oersted.
The scheme of Oersted's experiment is shown in fig. 2 (image from rt.mipt.ru). Above the magnetic needle (and - the north and south poles of the arrow) is a metal conductor connected to a current source. If you close the circuit, then the arrow turns perpendicular to the conductor!
This simple experiment pointed directly to the relationship between electricity and magnetism. The experiments that followed Oersted's experience firmly established the following pattern: the magnetic field is generated by electric currents and acts on currents.
Rice. 2. Oersted's experiment
The picture of the lines of the magnetic field generated by a conductor with current depends on the shape of the conductor.
Magnetic field of a straight wire with current
The magnetic field lines of a straight wire carrying current are concentric circles. The centers of these circles lie on the wire, and their planes are perpendicular to the wire (Fig. 3).
Rice. 3. Field of a direct wire with current
There are two alternative rules for determining the direction of direct current magnetic field lines.
hour hand rule. The field lines go counterclockwise when viewed so that the current flows towards us..
screw rule(or gimlet rule, or corkscrew rule- it's closer to someone ;-)). The field lines go where the screw (with conventional right-hand thread) must be turned to move along the thread in the direction of the current.
Use whichever rule suits you best. It's better to get used to the clockwise rule - you will see for yourself later that it is more universal and easier to use (and then remember it with gratitude in your first year when you study analytic geometry).
On fig. 3, something new has also appeared: this is a vector, which is called magnetic field induction, or magnetic induction. The magnetic induction vector is an analogue of the electric field strength vector: it serves power characteristic magnetic field, determining the force with which the magnetic field acts on moving charges.
We will talk about forces in a magnetic field later, but for now we will only note that the magnitude and direction of the magnetic field is determined by the magnetic induction vector. At each point in space, the vector is directed in the same direction as the north end of the compass needle placed at this point, namely, tangent to the field line in the direction of this line. The magnetic induction is measured in teslach(Tl).
As in the case of an electric field, for the induction of a magnetic field, superposition principle. It lies in the fact that induction of magnetic fields created at a given point by various currents are added vectorially and give the resulting vector of magnetic induction:.
The magnetic field of a coil with current
Consider a circular coil through which a direct current circulates. We do not show the source that creates the current in the figure.
The picture of the lines of the field of our turn will have approximately the following form (Fig. 4).
Rice. 4. Field of the coil with current
It will be important for us to be able to determine in which half-space (relative to the plane of the coil) the magnetic field is directed. Again we have two alternative rules.
hour hand rule. The field lines go there, looking from where the current seems to be circulating counterclockwise.
screw rule. The field lines go where the screw (with conventional right hand threads) would move if rotated in the direction of the current.
As you can see, the roles of the current and the field are reversed - in comparison with the formulations of these rules for the case of direct current.
The magnetic field of a coil with current
Coil it will turn out, if tightly, coil to coil, wind the wire into a sufficiently long spiral (Fig. 5 - image from the site en.wikipedia.org). The coil may have several tens, hundreds or even thousands of turns. The coil is also called solenoid.
Rice. 5. Coil (solenoid)
The magnetic field of one turn, as we know, does not look very simple. Fields? individual turns of the coil are superimposed on each other, and it would seem that the result should be a very confusing picture. However, this is not the case: the field of a long coil has an unexpectedly simple structure (Fig. 6).
Rice. 6. coil field with current
In this figure, the current in the coil goes counterclockwise when viewed from the left (this will happen if, in Fig. 5, the right end of the coil is connected 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 its edges, the magnetic field is homogeneous: at each point, the magnetic induction vector is the same in magnitude and direction. The field lines are parallel straight lines; they bend only near the edges of the coil when they go out.
2. Outside the coil, the field is close to zero. The more turns in the coil, the weaker the field outside it.
Note that an infinitely long coil does not emit a field at all: there is no magnetic field outside the coil. Inside such a coil, the field is uniform everywhere.
Doesn't it remind you of anything? A coil is the "magnetic" counterpart of a capacitor. You remember that the capacitor creates a uniform electric field inside itself, the lines of which are curved only near the edges of the plates, and outside the capacitor the field is close to zero; a capacitor with infinite plates does not release the field at all, and the field is uniform everywhere inside it.
And now - the main observation. Compare, please, the picture of the magnetic field lines outside the coil (Fig. 6) with the field lines of the magnet in Fig. one . It's the same thing, isn't it? And now we come to a question that you probably had a long time ago: if a magnetic field is generated by currents and acts on currents, then what is the reason for the appearance of a magnetic field near a permanent magnet? After all, this magnet does not seem to be a conductor with current!
Ampère's hypothesis. Elementary currents
At first, it was thought that the interaction of magnets was due to special magnetic charges concentrated at the poles. But, unlike electricity, no one could isolate the magnetic charge; after all, as we have already said, it was not possible to obtain separately the north and south poles of the magnet - the poles are always present in the magnet in pairs.
Doubts about magnetic charges were aggravated by the experience of Oersted, when it turned out that the magnetic field is generated by an electric current. Moreover, it turned out that for any magnet it is possible to choose a conductor with a current of the appropriate configuration, such that the field of this conductor coincides with the field of the magnet.
Ampere put forward a bold hypothesis. There are no magnetic charges. The action of a magnet is explained by closed electric currents inside it..
What are these currents? These elementary currents circulate within atoms and molecules; they are associated with the movement of electrons in atomic orbits. The magnetic field of any body is made up of the magnetic fields of these elementary currents.
Elementary currents can be randomly located relative to each other. Then their fields cancel each other, and the body does not show magnetic properties.
But if elementary currents are coordinated, then their fields, adding up, reinforce each other. The body becomes a magnet (Fig. 7; the magnetic field will be directed towards us; the north pole of the magnet will also be directed towards us).
Rice. 7. Elementary magnet currents
Ampere's hypothesis about elementary currents clarified the properties of magnets. Heating and shaking a magnet destroys the arrangement of its elementary currents, and the magnetic properties weaken. The inseparability of the magnet poles became obvious: at the place where the magnet was cut, we get the same elementary currents at the ends. The ability of a body to be magnetized in a magnetic field is explained by the coordinated alignment of elementary currents that “turn” properly (read about the rotation of a circular current in a magnetic field in the next sheet).
Ampere's hypothesis turned out to be correct - this was shown by the further development of physics. The concept of elementary currents has become an integral part of the theory of the atom, developed already in the twentieth century - almost a hundred years after Ampère's brilliant conjecture.
