Magnetic field around a conductor carrying current. A magnetic field

If there is a direct conductor carrying current, then detect the presence magnetic field around this conductor you can use iron filings...

Or magnetic needles.

Under the influence of the magnetic field of the current, magnetic needles or iron filings are located in concentric circles.

Magnetic lines

The magnetic field can be represented graphically using magnetic lines.
The magnetic lines of the magnetic field of the current are the lines along which the axes of small magnetic arrows are located in the magnetic field.
Magnetic lines of a current's magnetic field are closed curves that encircle a conductor.
U straight conductor with current - these are concentric expanding circles.
The direction of the magnetic line is taken to be the direction indicated by the north pole of the magnetic needle at each point in the field.

Graphic representation of the magnetic field of a straight conductor carrying current.

The direction of the magnetic lines of the magnetic field of the current is related to the direction of the current in the conductor

It is interesting to see how iron filings, attracted to the pole of a magnet, form brushes that repel each other. But they are simply located along the magnetic field lines!
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Can you draw a picture of the magnetic field lines of a current-carrying conductor folded into a figure eight?
Is this drawing similar to the one you imagined?

IS IT POSSIBLE TO SEE A MAGNETIC FIELD

You need to turn on the color TV to some still frame and bring a magnet to it. The colors of the image on the screen near the magnet will change!
The picture will shine with rainbow stains. Colored stripes thicken near the contour of the magnet, as if visualizing the magnetic field. In England, it was used in crushed form as a laxative. It is interesting to rotate the magnet, move it, or bring it closer and further away from the screen.
The picture of the magnetic field will be much more interesting than in experiments with sawdust!

Several steel needles were hung loosely from a small brass disk.

If you slowly bring a magnet from below to the needles (for example, with the south pole), then first the needles will move apart, and then, when the magnet gets very close, they will return to the vertical position again.
Why?

EXPERIMENTS WITH IRON SAWDS

Take a magnet of any shape, cover it with a piece of thin cardboard, sprinkle iron filings on top and smooth them out.
It's so interesting to observe magnetic fields!
After all, each “sawdust”, like a magnetic needle, is located along magnetic lines.
This makes the magnetic field lines of your magnet “visible”.
When the cardboard moves over the magnet (or vice versa, the magnet under the cardboard), the sawdust begins to move, changing the patterns of the magnetic field.

In previous lessons we mentioned magnetic action electric current. We can conclude that electrical and magnetic phenomena are interconnected. In this lesson, the topic of which « Magnetic field of a straight conductor. Magnetic Lines,” we will begin to confirm this conclusion.

Humanity has been collecting knowledge about magnetic phenomena for more than 4,500 years (the first mentions of electrical phenomena date back a thousand years later). In the middle of the 19th century, scientists began to pay attention to the search for relationships between the phenomena of electricity and magnetism, therefore, the previously accumulated theoretical and experimental information, separately for each phenomenon, became a good basis for creating a unified electromagnetic theory.

Most likely, the unusual properties of the natural mineral magnetite (see Fig. 1) were known in Mesopotamia back in Bronze Age, and after the emergence of iron metallurgy, it was impossible not to notice that magnetite attracts iron products.

Rice. 1. Magnetite ()

The ancient Greek philosopher Thales of Miletus thought about the reasons for such attraction, who explained it by the special animation of this mineral, therefore, it is not surprising that the word magnet also has Greek roots. An ancient Greek legend tells of a shepherd named Magnus. He once discovered that the iron tip of his stick and the nails of his boots were attracted to the black stone. This stone began to be called the “Magnus stone” or simply “magnet”, after the name of the area where it was mined iron ore(hills of Magnesia in Asia Minor).

Magnetic phenomena were of interest back in Ancient China, so Chinese sailors in the 11th century already used sea compasses.

The first description of the properties of natural magnets in Europe was made by the Frenchman Pierre de Maricourt. In 1269, he sent a friend in Picardy a document that went down in the history of science as the “Letter on the Magnet.” In this document, the Frenchman talked about his experiments with magnetite; he noticed that in each piece of this mineral there are two areas that attract iron especially strongly. Maricourt saw a parallel between these areas and the poles celestial sphere, so we now talk about the south and north magnetic pole.

In 1600, the English scientist William Gilbert published the work “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth.” In this book, Gilbert presented all the known properties of natural magnets, and also described his experiments with a magnetite ball, with the help of which he reproduced the main features of terrestrial magnetism.

After Gilbert, until the beginning of the 19th century, the science of magnetism practically did not develop.

How to explain the fact that the science of magnetism, in comparison with the science of electricity, developed very slowly? The main problem was that magnets at that time existed only in nature; they could not be obtained in laboratory conditions. This greatly limited the possibilities of the experimenters.

Electricity was in a more advantageous position - it could be received and stored. The first static charge generator was built by the burgomaster of Magdeburg, Otto von Guericke, in 1663 (see Fig. 2)

Rice. 2. German physicist Otto von Guericke and the first static electricity generator ()

In 1744, the German Ewald Georg von Kleist, and in 1745 the Dutchman Pieter van Musschenbroek invented the Leyden jar - the first electric capacitor(see Fig. 3), the first electrometers appeared at that time. As a result, by the end of the 18th century, science knew much more about electricity than about magnetism.

Rice. 3. Leyden jar ()

However, in 1800, Alessandro Volta invented the first chemical source of electric current - a galvanic battery (voltaic column) (see Fig. 4). After this, the discovery of the connection between electricity and magnetism turned out to be inevitable.

It is worth noting that the discovery of such a connection could have occurred several years after the invention of the Leyden jar, but the French scientist Laplace did not realize that parallel conductors attract when current passes through them in one direction.

Rice. 4. The first galvanic battery ()

In 1820, the Danish physicist Hans Christian Oersted, who quite consciously tried to obtain a connection between magnetic and electrical phenomena, found that a wire through which an electric current flows deflects the magnetic needle of a compass. Initially, Oersted placed the current-carrying conductor perpendicular to the arrow - the arrow remained motionless. However, during one of his lectures, he placed the conductor parallel to the arrow, and it deviated.

In order to reproduce Oersted's experiment, it is necessary to connect a conductor to the current source through a rheostat (resistance), near which a magnetic needle is located (see Fig. 5). When current flows through a conductor, a deflection of the needle is observed, this proves that the electric current in the conductor affects the magnetic needle.

Rice. 5. Oersted's experiment ()

Problem 1

Figure 13 shows the magnetic field line of a current-carrying conductor. Indicate the direction of the current.

Rice. 13 Illustration for the problem

To solve this problem, we will use the right-hand rule. Let's position our right hand so that the four bent fingers coincide with the direction of the magnetic lines, then the thumb will indicate the direction of the current in the conductor (see Fig. 14).

Rice. 14. Illustration for the problem

Answer

Current flows from a point B exactly A.

Problem 2

Indicate the poles of the electric current source that are closed by a wire (the magnetic needle is located under the wire) (see Fig. 15). Will the answer change if the same position is occupied by an arrow located above the wire?

Rice. 15. Illustration for the problem

Solution

The direction of the magnetic field lines coincides with the direction of the north pole of the magnetic needle (blue part). Therefore, according to the rule of the right hand, we position the hand so that the four bent fingers coincide with the direction of the magnetic lines and go around the wire, then the thumb will indicate the direction of the current in the conductor. The current flows from “plus” to “minus”, so the poles of the electric current source are located as in Figure 16.

Rice. 16. Illustration for the problem

If the arrow had been located above the wire, the current would have flowed in the opposite direction and the pole signs would have been different (see Fig. 17).

Rice. 17. Illustration for the problem

After the results of the experiment were announced, the French physicist and mathematician Henri Ampère decided to undertake experiments to identify magnetic properties electric current. Soon Ampere established that if an electric current flows in one direction through two parallel conductors, then such conductors attract (see Fig. 6 b); if the current flows in opposite directions, the conductors repel (see Fig. 6 a).

Rice. 6. Ampere's experiment ()

From his experiments, Ampere drew the following conclusions:

1) There is a magnetic field around a magnet, or a conductor, or an electrically charged moving particle;

2) The magnetic field acts with some force on a charged particle moving in this field;

3) Electric current is the directed movement of charged particles, therefore a magnetic field acts on a conductor with current;

4) The interaction of a conductor with current and a magnet, as well as the interaction of magnets, can be explained by assuming the existence of undamped molecular electric currents inside the magnet.

Thus, Ampere explained all magnetic phenomena by the interaction of moving charged particles. Interactions are carried out using the magnetic fields of these particles.

A magnetic field is a special form of matter that exists around moving charged particles or bodies and acts with some force on other charged particles or bodies moving in this field.

Magnetic needles (diamond-shaped magnets) have long been used to study magnetic phenomena. If placed around a magnet a large number of small magnetic needles (on stands so that the hands can rotate freely), then they will be oriented in a certain way in the magnetic field of the magnet (see Fig. 9). The axes of the magnetic needles will run along certain lines. Such lines are called magnetic field lines or magnetic lines.

The direction of the magnetic field lines is taken to be the direction towards which the north pole of the magnetic needle points (see Fig. 9).

Rice. 9. Location of magnetic arrows around a magnet ()

Using magnetic lines it is convenient to depict magnetic fields graphically (see Fig. 10)

Rice. 10. Graphically depicting magnetic lines ()

However, to determine the direction of magnetic lines it is not necessary to use magnetic arrows.

Rice. 11. Arrangement of iron filings around a current-carrying conductor ()

If iron filings are poured around a current-carrying conductor, then after some time the filings, once in the magnetic field of the conductor, will be magnetized and arranged in circles that encircle the conductor (see Fig. 11). To determine the direction of the magnetic lines in this case, you can use the gimlet rule - if you screw the gimlet in the direction of the current in the conductor, then the direction of rotation of the gimlet handle will indicate the direction of the magnetic field lines of the current. (see Fig. 12). You can also use the right hand rule - if you point the thumb of your right hand in the direction of the current in the conductor, then four bent fingers will indicate the direction of the magnetic field lines of the current (see Fig. 13).

Rice. 11.Gimlet rule ()

Rice. 12. Right hand rule ()

In this lesson we began the study of magnetism, discussed the history of the study this phenomenon and learned about magnetic field lines.

1. Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

Homework

1. P. 58, questions 1-4, p. 168, task 40 (2). Peryshkin A.V. Physics 8. - M.: Bustard, 2010.

1. Internet portal Myshared.ru ().
2. Internet portal Clck.ru ().
3. Internet portal Class-fizika.narod.ru ().

An electric current in a conductor produces a magnetic field around the conductor. Electric current and magnetic field are two inseparable parts of a single physical process. The magnetic field of permanent magnets is ultimately also generated by molecular electric currents formed by the movement of electrons in orbits and their rotation around their axes.

The magnetic field of a conductor and the direction of its lines of force can be determined using a magnetic needle. The magnetic lines of a straight conductor have the shape of concentric circles located in a plane perpendicular to the conductor. The direction of magnetic field lines depends on the direction of the current in the conductor. If the current in the conductor comes from the observer, then the lines of force are directed clockwise.

The dependence of the direction of the field on the direction of the current is determined by the gimlet rule: when the translational movement of the gimlet coincides with the direction of the current in the conductor, the direction of rotation of the handle coincides with the direction of the magnetic lines.

The gimlet rule can also be used to determine the direction of the magnetic field in the coil, but in the following formulation: if the direction of rotation of the gimlet handle is combined with the direction of the current in the turns of the coil, then forward movement The gimlet will show the direction of the field lines inside the coil (Fig. 4.4).

Inside the coil these lines go from the south pole to the north, and outside it - from north to south.

The gimlet rule can also be used to determine the direction of current if the direction of the magnetic field lines is known.

A current-carrying conductor in a magnetic field experiences a force equal to

F = I·L·B·sin

I is the current strength in the conductor; B - module of the magnetic field induction vector; L is the length of the conductor located in the magnetic field;  is the angle between the magnetic field vector and the direction of the current in the conductor.

The force acting on a current-carrying conductor in a magnetic field is called the Ampere force.

The maximum ampere force is:

F = I L B

The direction of Ampere's force is determined by the left-hand rule: if left hand positioned so that the perpendicular component of the magnetic induction vector B enters the palm, and the four extended fingers are directed in the direction of the current, then the thumb bent 90 degrees will show the direction of the force acting on the section of the conductor with the current, that is, the Ampere force.

If and lie in the same plane, then the angle between and is straight, therefore . Then the force acting on the current element is

(of course, from the side of the first conductor, exactly the same force acts on the second).

The resulting force is equal to one of these forces. If these two conductors influence the third, then their magnetic fields need to be added vectorially.

Circuit with current in a magnetic field

Rice. 4.13

Let a frame with current be placed in a uniform magnetic field (Fig. 4.13). Then the Ampere forces acting on the sides of the frame will create a torque, the magnitude of which is proportional to the magnetic induction, the current strength in the frame, and its area S and depends on the angle a between the vector and the normal to the area:

The normal direction is chosen so that the right screw moves in the normal direction when rotating in the direction of the current in the frame.

Maximum value rotational moment has when the frame is installed perpendicular to the magnetic lines of force:

This expression can also be used to determine the magnetic field induction:

A value equal to the product is called the magnetic moment of the circuit R t. The magnetic moment is a vector whose direction coincides with the direction of the normal to the contour. Then the torque can be written

At angle a = 0 the torque is zero. The value of the torque depends on the area of ​​the contour, but does not depend on its shape. Therefore, for any closed loop, through which a direct current flows, a torque acts M, which rotates it so that the magnetic moment vector is parallel to the magnetic field induction vector.

If a magnetic needle is brought close to a straight conductor carrying current, it will tend to become perpendicular to the plane passing through the axis of the conductor and the center of rotation of the needle (Fig. 67). This indicates that the needle is subject to special forces called magnetic forces. In other words, if an electric current passes through a conductor, a magnetic field appears around the conductor. A magnetic field can be considered as a special state of space surrounding current-carrying conductors.

If you pass a thick conductor through a card and pass an electric current through it, then steel filings poured onto the cardboard will be located around the conductor in concentric circles, which in this case represent the so-called magnetic lines (Fig. 68). We can move the cardboard up or down the conductor, but the location of the steel filings will not change. Consequently, a magnetic field arises around the conductor along its entire length.

If you place small magnetic arrows on the cardboard, then by changing the direction of the current in the conductor, you can see that the magnetic arrows will rotate (Fig. 69). This shows that the direction of the magnetic lines changes with the change in the direction of the current in the conductor.

The magnetic field around a current-carrying conductor has the following features: the magnetic lines of a straight conductor have the shape of concentric circles; the closer to the conductor, the denser the magnetic lines are located, the greater the magnetic induction; magnetic induction (field intensity) depends on the magnitude of the current in the conductor; The direction of the magnetic lines depends on the direction of the current in the conductor.

To show the direction of the current in the conductor shown in section, a symbol has been adopted, which we will use in the future. If you mentally place an arrow in a conductor in the direction of the current (Fig. 70), then in a conductor in which the current is directed away from us, we will see the tail of the arrow’s feathers (a cross); if the current is directed towards us, we will see the tip of an arrow (point).

The direction of magnetic lines around a current-carrying conductor can be determined by the “gimlet rule.” If a gimlet (corkscrew) with a right-hand thread moves forward in the direction of the current, then the direction of rotation of the handle will coincide with the direction of the magnetic lines around the conductor (Fig. 71).

Rice. 71. Determining the direction of magnetic lines around a current-carrying conductor using the “gimlet rule”

A magnetic needle introduced into the field of a current-carrying conductor is located along the magnetic lines. Therefore, to determine its location, you can also use the “gimlet rule” (Fig. 72).

Rice. 72. Determination of the direction of deflection of a magnetic needle brought to a conductor with current, according to the “gimlet rule”

The magnetic field is one of the most important manifestations of electric current and cannot be obtained independently and separately from the current.

In permanent magnets, the magnetic field is also caused by the movement of electrons that make up the atoms and molecules of the magnet.

The intensity of the magnetic field at each point is determined by the magnitude of magnetic induction, which is usually denoted by the letter B. Magnetic induction is a vector quantity, that is, it is characterized not only by a certain value, but also by a certain direction at each point of the magnetic field. The direction of the magnetic induction vector coincides with the tangent to the magnetic line at a given point in the field (Fig. 73).

As a result of generalizing experimental data, French scientists Biot and Savard established that magnetic induction B (magnetic field intensity) at a distance r from an infinitely long straight conductor with current is determined by the expression

where r is the radius of the circle drawn through the field point under consideration; the center of the circle is on the axis of the conductor (2πr is the circumference);

I is the amount of current flowing through the conductor.

The value μ a, which characterizes the magnetic properties of the medium, is called the absolute magnetic permeability of the medium.

For emptiness, the absolute magnetic permeability has a minimum value and is usually denoted by μ 0 and called the absolute magnetic permeability of emptiness.

1 H = 1 ohm⋅sec.

The ratio μ a / μ 0, showing how many times the absolute magnetic permeability of a given medium is greater than the absolute magnetic permeability of emptiness, is called relative magnetic permeability and is denoted by the letter μ.

IN International system units (SI) accepted units of measurement of magnetic induction B - tesla or weber on square meter(tl, wb/m2).

In engineering practice, magnetic induction is usually measured in gauss (gs): 1 t = 10 4 gs.

If at all points of the magnetic field the magnetic induction vectors are equal in magnitude and parallel to each other, then such a field is called uniform.

The product of magnetic induction B and the area S perpendicular to the direction of the field (magnetic induction vector) is called the flux of the magnetic induction vector, or simply magnetic flux, and is denoted by the letter Φ (Fig. 74):

The International System uses the weber (wb) as the unit of measurement for magnetic flux.

In engineering calculations, magnetic flux is measured in maxwells (μs):

1 vb = 10 8 μs.

When calculating magnetic fields, a quantity called magnetic field strength (denoted H) is also used. Magnetic induction B and magnetic field strength H are related by the relation

The unit of measurement for magnetic field strength is N - ampere per meter (a/m).

The magnetic field strength in a homogeneous medium, as well as magnetic induction, depends on the magnitude of the current, the number and shape of the conductors through which the current passes. But unlike magnetic induction, magnetic field strength does not take into account the influence of the magnetic properties of the medium.

When current passes through a straight conductor, a magnetic field appears around it (Fig. 26). The magnetic lines of force of this field are located in concentric circles, in the center of which there is a current-carrying conductor.

N
The direction of magnetic field lines can be determined using the gimlet rule. If the forward movement of the gimlet (Fig. 27) align with the direction of the current in the conductor, then rotation of its handle will indicate the direction of the magnetic field lines around the conductor. The greater the current passing through the conductor, the stronger the magnetic field that arises around it. When the direction of the current changes, the magnetic field also changes its direction.

As you move away from the conductor, the magnetic field lines are less frequent.

Methods of strengthening magnetic fields. To obtain strong magnetic fields at low currents, they usually increase the number of current-carrying conductors and make them in the form of a series of turns; such a device is called a coil.

With a conductor bent in the form of a coil (Fig. 28, a), the magnetic fields formed by all sections of this conductor will have the same direction inside the coil. Therefore, the intensity of the magnetic field inside the coil will be greater than around a straight conductor. When combining turns into a coil, magnetic fields, with
created by individual turns, add up (Fig. 28, b) and their lines of force are connected into a common magnetic flux. In this case, the concentration of field lines inside the coil increases, i.e., the magnetic field inside it intensifies. The greater the current passing through the coil, and the more turns there are in it, the stronger the magnetic field created by the coil.

A coil flowing with current is an artificial electric magnet. To enhance the magnetic field, a steel core is inserted inside the coil; such a device is called an electromagnet.

ABOUT

You can also determine the direction of the magnetic field created by a turn or coil using your right hand (Fig. 29) and a gimlet (Fig. 30).

18. Magnetic properties of various substances.

All substances, depending on their magnetic properties, are divided into three groups: ferromagnetic, paramagnetic and diamagnetic.

Ferromagnetic materials include iron, cobalt, nickel and their alloys. They have high magnetic permeability µ And are well attracted to magnets and electromagnets.

Paramagnetic materials include aluminum, tin, chromium, manganese, platinum, tungsten, solutions of iron salts, etc. Paramagnetic materials are attracted to magnets and electromagnets many times weaker than ferromagnetic materials.

Diamagnetic materials are not attracted to magnets, but, on the contrary, are repelled. These include copper, silver, gold, lead, zinc, resin, water, most gases, air, etc.

Magnetic properties of ferromagnetic materials. Ferromagnetic materials, due to their ability to be magnetized, are widely used in the manufacture of electrical machines, devices and other electrical installations.

Magnetization curve. The process of magnetization of a ferromagnetic material can be depicted in the form of a magnetization curve (Fig. 31), which represents the dependence of induction IN from tension N magnetic field (from magnetizing current I ).

The magnetization curve can be divided into three sections: Ooh , at which the magnetic induction increases almost proportionally to the magnetizing current; a-b , at which the growth of magnetic induction slows down, and the area of ​​magnetic saturation beyond the point b , where s addiction IN from N becomes linear again, but is characterized by a slow increase in magnetic induction with increasing field strength.

P
Remagnetization of ferromagnetic materials, hysteresis loop. Big practical significance, especially in electrical machines and AC installations, has a process of magnetization reversal of ferromagnetic materials. In Fig. Figure 32 shows a graph of changes in induction during magnetization and demagnetization of a ferromagnetic material (with a change in the magnetizing current I . As can be seen from this graph, at the same values ​​of magnetic field strength, the magnetic induction obtained by demagnetizing a ferromagnetic body (section a B C ), there will be more induction obtained during magnetization (sections Ooh And Yes ). When the magnetizing current is brought to zero, the induction in the ferromagnetic material will not decrease to zero, but will retain a certain value IN r , corresponding to the segment About . This value is called residual induction.

The phenomenon of lag, or delay, in changes in magnetic induction from corresponding changes in magnetic field strength is called magnetic hysteresis, and the preservation of a magnetic field in a ferromagnetic material after the magnetizing current has stopped flowing is called magnetic hysteresis. residual magnetism.

P
By changing the direction of the magnetizing current, you can completely demagnetize the ferromagnetic body and bring the magnetic induction in it to zero. Reverse tension N With , at which the induction in a ferromagnetic material decreases to zero is called coercive force. curve Ooh , obtained under the condition that the ferromagnetic substance has been previously demagnetized, is called the initial magnetization curve. The induction change curve is called hysteresis loop.

The influence of ferromagnetic materials on the magnetic field distribution. If you place any body made of ferromagnetic material in a magnetic field, then the magnetic lines of force will enter and exit it at right angles. In the body itself and near it, there will be a condensation of the field lines, i.e., the magnetic field induction inside the body and near it increases. If you make a ferromagnetic body in the form of a ring, then magnetic field lines will practically not penetrate into its internal cavity (Fig. 33) and the ring will serve as a magnetic shield protecting the internal cavity from the influence of the magnetic field. This property of ferromagnetic materials is the basis for the action of various screens that protect electrical measuring instruments, electrical cables and other electrical devices from the harmful effects of external magnetic fields.