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.

By modern ideas 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 analog of the intensity vector electric field: he 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 points in the same direction as the north end of the compass needle placed in given 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. 1 . It's the same thing, isn't it? And now we come to a question that you probably had a long time ago: if the 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).

Ampère's hypothesis turned out to be correct - it showed further development 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.

In this lesson, the topic of which is: “The magnetic field of a constant electric current”, we will learn what a magnet is, how it interacts with other magnets, write down the definitions of the magnetic field and the magnetic induction vector, and also use the gimlet rule to determine the direction of the magnetic induction vector.

Each of you held a magnet in your hands and knows its amazing property: it interacts at a distance with another magnet or with a piece of iron. What is it about a magnet that gives it these amazing properties? Can you make your own magnet? It is possible, and what is needed for this - you will learn from our lesson. Let's get ahead of ourselves: if we take a simple iron nail, it will not have magnetic properties, but if we wrap it with wire and connect it to a battery, we get a magnet (see Fig. 1).

Rice. 1. A nail wrapped in wire and connected to a battery

It turns out that to get a magnet, you need an electric current - the movement of an electric charge. The properties of permanent magnets, such as fridge magnets, are also associated with the movement of an electric charge. A certain magnetic charge, like an electric one, does not exist in nature. It is not needed, enough moving electric charges.

Before investigating the magnetic field of a direct electric current, it is necessary to agree on how to quantitatively describe the magnetic field. For a quantitative description of magnetic phenomena, it is necessary to introduce the force characteristic of the magnetic field. The vector quantity that quantitatively characterizes the magnetic field is called magnetic induction. It is usually denoted by a capital Latin letter B, measured in Tesla.

Magnetic induction - vector quantity, which is the force characteristic of the magnetic field at a given point in space. The direction of the magnetic field is determined by analogy with the model of electrostatics, in which the field is characterized by the action on a trial charge at rest. Only here a magnetic needle (an elongated permanent magnet) is used as a "trial element". You saw such an arrow in a compass. The direction of the magnetic field at some point is taken to be the direction that will indicate the north pole N of the magnetic needle after reorientation (see Fig. 2).

A complete and clear picture of the magnetic field can be obtained by constructing the so-called magnetic field lines (see Fig. 3).

Rice. 3. Field lines of the magnetic field of a permanent magnet

These are lines showing the direction of the magnetic induction vector (that is, the direction of the N pole of the magnetic needle) at each point in space. With the help of a magnetic needle, one can thus obtain a picture of the lines of force of various magnetic fields. Here, for example, is a picture of the magnetic field lines of a permanent magnet (see Fig. 4).

Rice. 4. Field lines of the magnetic field of a permanent magnet

A magnetic field exists at every point, but we draw lines at some distance from each other. This is just a way of depicting a magnetic field, similarly we did with the electric field strength (see Fig. 5).

Rice. 5. Electric field strength lines

The more densely the lines are drawn, the greater the modulus of magnetic induction in a given region of space. As you can see (see Fig. 4), the lines of force exit the north pole of the magnet and enter the south pole. Inside the magnet, the field lines also continue. Unlike electric field lines, which start at positive charges and end at negative charges, magnetic field lines are closed (see Fig. 6).

Rice. 6. Magnetic field lines are closed

A field whose lines of force are closed is called a vortex vector field. The electrostatic field is not vortex, it is potential. The fundamental difference between vortex and potential fields is that the work of a potential field on any closed path is zero, but this is not the case for a vortex field. The earth is also a huge magnet, it has a magnetic field that we detect with a compass needle. Read more about the Earth's magnetic field in the branch.

Our planet Earth is a large magnet, the poles of which are located near the intersection of the surface with the axis of rotation. Geographically, these are the South and North Poles. That is why the arrow in the compass, which is also a magnet, interacts with the Earth. It is oriented in such a way that one end points to the North Pole, and the other to the South (see Fig. 7). Fig.7. The arrow in the compass interacts with the Earth The one that points to the North Pole of the Earth was designated N, which means North - translated from English as "North". And the one that points to the South Pole of the Earth - S, which means South - translated from English "South". Since opposite poles of magnets are attracted, the north pole of the arrow points to the South magnetic pole of the Earth (see Fig. 8). Rice. 8. Interaction of the compass and the magnetic poles of the Earth It turns out that the South magnetic pole is located at the North geographic. And vice versa, the North magnetic is located at the South geographic pole of the Earth.

Now, having become acquainted with the model of the magnetic field, we examine the field of a conductor with direct current. Back in the 19th century, the Danish scientist Oersted discovered that a magnetic needle interacts with a conductor through which an electric current flows (see Fig. 9).

Rice. 9. Interaction of a magnetic needle with a conductor

Practice shows that in the magnetic field of a rectilinear conductor with current, the magnetic needle at each point will be set tangentially to a certain circle. The plane of this circle is perpendicular to the conductor with current, and its center lies on the axis of the conductor (see Fig. 10).

Rice. 10. The location of the magnetic needle in the magnetic field of a straight conductor

If you change the direction of current flow through the conductor, then the magnetic needle at each point will turn in opposite side(see fig. 11).

Rice. 11. When changing the direction of the flow of electric current

That is, the direction of the magnetic field depends on the direction of current flow through the conductor. This dependence can be described using a simple experimentally established method - gimlet rules:

if the direction of the translational movement of the gimlet coincides with the direction of the current in the conductor, then the direction of rotation of its handle coincides with the direction of the magnetic field created by this conductor (see Fig. 12).

So, the magnetic field of a conductor with current is directed at each point tangentially to a circle lying in a plane perpendicular to the conductor. The center of the circle coincides with the axis of the conductor. The direction of the magnetic field vector at each point is related to the direction of the current in the conductor by the gimlet rule. Empirically, when changing the current strength and the distance from the conductor, it was found that the modulus of the magnetic induction vector is proportional to the current and inversely proportional to the distance from the conductor. The modulus of the magnetic induction vector of the field created by an infinite current-carrying conductor is equal to:

where is the coefficient of proportionality, which is often found in magnetism. It is called the magnetic permeability of the vacuum. Numerically equal to:

For magnetic fields, as well as for electric ones, the principle of superposition is valid. Magnetic fields created by different sources at one point in space add up (see Fig. 13).

Rice. 13. Magnetic fields from different sources add up

The total power characteristic of such a field will be the vector sum of the power characteristics of the fields of each of the sources. The magnitude of the magnetic induction of the field created by the current at a certain point can be increased by bending the conductor into a circle. This will be clear if we consider the magnetic fields of small segments of such a coil of wire at a point inside this coil. For example, in the center.

The segment marked , according to the gimlet rule, creates an upward field in it (see Fig. 14).

Rice. 14. Magnetic field of the segments

The segment similarly creates a magnetic field at this point directed there. The same is true for other segments. Then the total force characteristic (that is, the magnetic induction vector B) at this point will be a superposition of the force characteristics of the magnetic fields of all small segments at this point and will be directed upwards (see Fig. 15).

Rice. 15. Total power characteristic in the center of the coil

For an arbitrary coil, not necessarily in the shape of a circle, for example, for a square frame (see Fig. 16), the value of the vector inside the coil will naturally depend on the shape, size of the coil and the current strength in it, but the direction of the magnetic induction vector will always be determined in the same way (as a superposition of fields created by small segments).

Rice. 16. Magnetic field of square frame segments

We have described in detail the determination of the direction of the field inside the coil, but in the general case it can be found much easier, according to a slightly modified gimlet rule:

if you rotate the handle of the gimlet in the direction where the current flows in the coil, then the tip of the gimlet will indicate the direction of the magnetic induction vector inside the coil (see Fig. 17).

That is, now the rotation of the handle corresponds to the direction of the current, and the movement of the gimlet corresponds to the direction of the field. And not vice versa, as was the case with a straight conductor. If a long conductor, through which current flows, is coiled into a spring, then this device will be a set of turns. The magnetic fields of each turn of the coil will add up according to the principle of superposition. Thus, the field created by the coil at some point will be the sum of the fields created by each of the turns at that point. The picture of the field lines of the field of such a coil you see in Fig. 18.

Rice. 18. Power lines of the coil

Such a device is called a coil, solenoid or electromagnet. It is easy to see that the magnetic properties of the coil will be the same as those of a permanent magnet (see Fig. 19).

Rice. 19. Magnetic properties of the coil and permanent magnet

One side of the coil (which is in the picture above) plays the role of the north pole of the magnet, and the other side - the south pole. Such a device is widely used in technology, because it can be controlled: it becomes a magnet only when the current in the coil is turned on. Note that the magnetic field lines inside the coil are nearly parallel and dense. The field inside the solenoid is very strong and uniform. The field outside the coil is non-uniform, it is much weaker than the field inside and is directed in the opposite direction. The direction of the magnetic field inside the coil is determined by the gimlet rule as for the field inside one turn. For the direction of rotation of the handle, we take the direction of the current that flows through the coil, and the movement of the gimlet indicates the direction of the magnetic field inside it (see Fig. 20).

Rice. 20. Rule of the gimlet for the reel

If you place a current-carrying coil in a magnetic field, it will reorient itself like a magnetic needle. The moment of force causing the rotation is related to the modulus of the magnetic induction vector at a given point, the area of ​​the coil and the current strength in it by the following relationship:

Now it becomes clear to us where the magnetic properties of a permanent magnet come from: an electron moving in an atom along a closed path is like a coil with current, and, like a coil, it has a magnetic field. And, as we saw with the example of a coil, many turns of current, ordered in a certain way, have a strong magnetic field.

The field created by permanent magnets is the result of the movement of charges inside them. And these charges are electrons in atoms (see Fig. 21). Rice. 21. Movement of electrons in atoms Let us explain the mechanism of its occurrence at a qualitative level. As you know, electrons in an atom are in motion. So, each electron, in each atom, creates its own magnetic field, thus, a huge number of magnets the size of an atom is obtained. In most substances, these magnets and their magnetic fields are randomly oriented. Therefore, the total magnetic field created by the body is zero. But there are substances in which the magnetic fields created by individual electrons are oriented in the same way (see Fig. 22). Rice. 22. Magnetic fields are oriented the same Therefore, the magnetic fields created by each electron add up. As a result, a body made of such a substance has a magnetic field and is a permanent magnet. In an external magnetic field, individual atoms or groups of atoms, which, as we found out, have their own magnetic field, turn like a compass needle (see Fig. 23). Rice. 23. Rotation of atoms in an external magnetic field If before that they were not oriented in one direction and did not form a strong total magnetic field, then after the ordering of elementary magnets, their magnetic fields will add up. And if, after the action of an external field, the order is preserved, the substance will remain a magnet. The described process is called magnetization.

Designate the poles of the current source feeding the solenoid at the indicated in fig. 24 interactions. Let's reason: a solenoid in which a direct current flows behaves like a magnet.

Rice. 24. Current source

According to fig. 24 shows that the magnetic needle is oriented with the south pole towards the solenoid. Like poles of magnets repel each other, while opposite poles attract. It follows from this that the left pole of the solenoid itself is the north one (see Fig. 25).

Rice. 25. Left pole of the solenoid north

The lines of magnetic induction leave the north pole and enter the south. This means that the field inside the solenoid is directed to the left (see Fig. 26).

Rice. 26. The field inside the solenoid is directed to the left

Well, the direction of the field inside the solenoid is determined by the gimlet rule. We know that the field is directed to the left, so let's imagine that the gimlet is screwed in this direction. Then its handle will indicate the direction of the current in the solenoid - from right to left (see Fig. 27).

The direction of the current is determined by the direction of movement of the positive charge. A positive charge moves from a point with a large potential (the positive pole of the source) to a point with a smaller one (the negative pole of the source). Therefore, the source pole located on the right is positive, and on the left is negative (see Fig. 28).

Rice. 28. Determination of source poles

Task 2

A frame with an area of ​​400 is placed in a uniform magnetic field with an induction of 0.1 T so that the normal of the frame is perpendicular to the lines of induction. At what current strength will torque 20 act on the frame (see Fig. 29)?

Rice. 29. Drawing for problem 2

Let's reason: the moment of force causing the rotation is related to the modulus of the magnetic induction vector at a given point, the area of ​​the coil and the current strength in it by the following relationship:

In our case, all the necessary data is available. It remains to express the desired current strength and calculate the answer:

Problem solved.

Bibliography

1. Sokolovich Yu.A., Bogdanova G.S. Physics: Handbook with examples of problem solving. - 2nd edition redistribution. - X .: Vesta: Publishing house "Ranok", 2005. - 464 p.
2. Myakishev G.Ya. Physics: Proc. for 11 cells. general education institutions. - M.: Education, 2010.

1. Internet portal "Knowledge Hypermarket" ()
2. Internet portal "Unified collection of DER" ()

Homework

Just as an electric charge at rest acts on another charge through an electric field, an electric current acts on another current through magnetic field. The action of a magnetic field on permanent magnets is reduced to its action on charges moving in the atoms of a substance and creating microscopic circular currents.

Doctrine of electromagnetism based on two assumptions:

• the magnetic field acts on moving charges and currents;
• a magnetic field arises around currents and moving charges.

Interaction of magnets

Permanent magnet(or magnetic needle) is oriented along the magnetic meridian of the Earth. The end pointing north is called north pole (N) and the opposite end is south pole(S). Approaching two magnets to each other, we note that their poles of the same name repel, and their opposite poles attract ( rice. 1 ).

If we separate the poles by cutting the permanent magnet into two parts, then we will find that each of them will also have two poles, i.e. will be a permanent magnet ( rice. 2 ). Both poles - north and south - are inseparable from each other, equal.

The magnetic field created by the Earth or permanent magnets is depicted, like the electric field, by magnetic lines of force. A picture of the magnetic field lines of any magnet can be obtained by placing a sheet of paper over it, on which iron filings are poured in a uniform layer. Getting into a magnetic field, the sawdust is magnetized - each of them has a north and south poles. Opposite poles tend to approach each other, but this is prevented by the friction of sawdust on paper. If you tap the paper with your finger, the friction will decrease and the filings will be attracted to each other, forming chains that represent the lines of a magnetic field.

On rice. 3 shows the location in the field of a direct magnet of sawdust and small magnetic arrows indicating the direction of the magnetic field lines. For this direction, the direction of the north pole of the magnetic needle is taken.

Oersted's experience. Magnetic field current

IN early XIX V. Danish scientist Oersted made an important discovery by discovering action of electric current on permanent magnets . He placed a long wire near the magnetic needle. When a current was passed through the wire, the arrow turned, trying to be perpendicular to it ( rice. 4 ). This could be explained by the appearance of a magnetic field around the conductor.

The magnetic lines of force of the field created by a direct conductor with current are concentric circles located in a plane perpendicular to it, with centers at the point through which the current passes ( rice. 5 ). The direction of the lines is determined by the right screw rule:

If the screw is rotated in the direction of the field lines, it will move in the direction of the current in the conductor .

The force characteristic of the magnetic field is magnetic induction vector B . At each point, it is directed tangentially to the field line. Electric field lines start on positive charges and end on negative ones, and the force acting in this field on a charge is directed tangentially to the line at each of its points. Unlike the electric field, the lines of the magnetic field are closed, which is due to the absence of "magnetic charges" in nature.

The magnetic field of the current is fundamentally no different from the field created by a permanent magnet. In this sense, an analogue of a flat magnet is a long solenoid - a coil of wire, the length of which is much greater than its diameter. The diagram of the lines of the magnetic field he created, depicted in rice. 6 , similar to that for a flat magnet ( rice. 3 ). The circles indicate the sections of the wire forming the solenoid winding. The currents flowing through the wire from the observer are indicated by crosses, and the currents in the opposite direction - towards the observer - are indicated by dots. The same designations are accepted for magnetic field lines when they are perpendicular to the plane of the drawing ( rice. 7 a, b).

The direction of the current in the solenoid winding and the direction of the magnetic field lines inside it are also related by the right screw rule, which in this case is formulated as follows:

If you look along the axis of the solenoid, then the current flowing in the clockwise direction creates a magnetic field in it, the direction of which coincides with the direction of movement of the right screw ( rice. 8 )

Based on this rule, it is easy to figure out that the solenoid shown in rice. 6 , its right end is the north pole, and its left end is the south pole.

The magnetic field inside the solenoid is homogeneous - the magnetic induction vector has a constant value there (B = const). In this respect, the solenoid is similar to a flat capacitor, inside which a uniform electric field is created.

The force acting in a magnetic field on a conductor with current

It was experimentally established that a force acts on a current-carrying conductor in a magnetic field. In a uniform field, a rectilinear conductor of length l, through which current I flows, located perpendicular to the field vector B, experiences the force: F = I l B .

The direction of the force is determined left hand rule:

If the four outstretched fingers of the left hand are placed in the direction of the current in the conductor, and the palm is perpendicular to the vector B, then the retracted thumb will indicate the direction of the force acting on the conductor (rice. 9 ).

It should be noted that the force acting on a conductor with current in a magnetic field is not directed tangentially to its lines of force, like an electric force, but perpendicular to them. A conductor located along the lines of force is not affected by the magnetic force.

The equation F = IlB lets give quantitative characteristic magnetic field induction.

Attitude does not depend on the properties of the conductor and characterizes the magnetic field itself.

Modulus of magnetic induction vector B numerically equal to strength acting on a conductor of unit length located perpendicular to it, through which a current of one ampere flows.

In the SI system, the unit of magnetic field induction is tesla (T):

A magnetic field. Tables, diagrams, formulas

(Interaction of magnets, Oersted's experiment, magnetic induction vector, vector direction, superposition principle. Graphic representation of magnetic fields, magnetic induction lines. magnetic flux, energy characteristic of the field. Magnetic forces, Ampere force, Lorentz force. Movement of charged particles in a magnetic field. Magnetic properties of matter, Ampere's hypothesis)

"Determination of the magnetic field" - According to the data obtained during the experiments, fill in the table. J. Verne. When we bring a magnet to the magnetic needle, it turns. Graphic representation of magnetic fields. Hans Christian Oersted. Electric field. The magnet has two poles: north and south. The stage of generalization and systematization of knowledge.

"Magnetic field and its graphic representation" - Non-uniform magnetic field. Coils with current. magnetic lines. Ampère's hypothesis. Inside the bar magnet. Opposite magnetic poles. Polar Lights. The magnetic field of a permanent magnet. A magnetic field. Earth's magnetic field. Magnetic poles. Biometrology. concentric circles. Uniform magnetic field.

"Magnetic field energy" - Scalar value. Calculation of inductance. Permanent magnetic fields. Relaxation time. Definition of inductance. coil energy. Extracurrents in a circuit with inductance. Transition processes. Energy density. Electrodynamics. Oscillatory circuit. Pulsed magnetic field. Self-induction. Magnetic field energy density.

"Characteristics of the magnetic field" - Lines of magnetic induction. Gimlet's rule. Rotate along the lines of force. Computer model of the Earth's magnetic field. Magnetic constant. Magnetic induction. The number of charge carriers. Three ways to set the magnetic induction vector. Magnetic field of electric current. Physicist William Hilbert.

"Properties of the magnetic field" - Type of substance. Magnetic induction of a magnetic field. Magnetic induction. Permanent magnet. Some values ​​of magnetic induction. Magnetic needle. Speaker. Modulus of magnetic induction vector. Lines of magnetic induction are always closed. Interaction of currents. Torque. Magnetic properties of matter.

"Motion of particles in a magnetic field" - Spectrograph. Manifestation of the action of the Lorentz force. Lorentz force. Cyclotron. Determination of the magnitude of the Lorentz force. Control questions. Directions of the Lorentz force. Interstellar matter. The task of the experiment. Change settings. A magnetic field. Mass spectrograph. Movement of particles in a magnetic field. Cathode-ray tube.

In total there are 20 presentations in the topic

Lecture: Oersted's experience. The magnetic field of a current-carrying conductor. The pattern of the field lines of a long straight conductor and a closed ring conductor, a coil with current

Oersted's experience

The magnetic properties of some substances have been known to people for a long time. However, a not so old discovery was that magnetic and electrical nature substances are related. This connection was shown Oersted who conducted experiments with electric current. Quite by chance, next to the conductor through which the current ran, there is a magnet. It changed its direction rather sharply at the time when the current ran through the wires, and returned to its original position when the circuit key was open.

From this experience, it was concluded that a magnetic field is formed around the conductor through which the current runs. That is, you can do conclusion: the electric field is caused by all charges, and the magnetic field is caused only around charges that have a directed movement.

Conductor magnetic field

If we consider the cross section of a conductor with current, then its magnetic lines will have circles of different diameters around the conductor.

To determine the direction of current or magnetic field lines around a conductor, use the rule right screw:

If you grab the conductor with your right hand and point your thumb along it in the direction of the current, then the bent fingers will show the direction of the magnetic field lines.

The power characteristic of a magnetic field is magnetic induction. Sometimes magnetic field lines are called induction lines.

Induction is designated and measured as follows: [V] = 1 T.

As you may recall, the principle of superpositions was valid for the force characteristic of the electric field, the same can be said for the magnetic field. That is, the resulting field induction is equal to the sum of the induction vectors at each point.

coil with current

As you know, conductors can have different shape, including consisting of several turns. A magnetic field is also formed around such a conductor. To determine it, use gimlet rule:

If you clasp the coils with your hand so that 4 bent fingers clasp them, then the thumb will show the direction of the magnetic field.