Change in magnetic flux. Magnetic field induction flux Magnetic flux changes

The relationship between electric and magnetic fields has been noticed for a very long time. This connection was discovered in the 19th century by the English physicist Faraday and gave it a name. It appears at the moment when the magnetic flux penetrates the surface of a closed circuit. After a change in the magnetic flux occurs for a certain time, an electric current appears in this circuit.

The relationship of electromagnetic induction and magnetic flux

The essence of the magnetic flux is displayed by the well-known formula: Ф = BS cos α. In it, F is a magnetic flux, S is the surface of the contour (area), B is the vector of magnetic induction. The angle α is formed due to the direction of the magnetic induction vector and the normal to the contour surface. It follows that the magnetic flux will reach the maximum threshold at cos α = 1, and the minimum threshold at cos α = 0.

In the second variant, the vector B will be perpendicular to the normal. It turns out that the flow lines do not cross the contour, but only slide along its plane. Therefore, the characteristics will be determined by the lines of the vector B that intersect the surface of the contour. For calculation, Weber is used as a unit of measurement: 1 wb \u003d 1v x 1s (volt-second). Another, smaller unit of measure is the maxwell (µs). It is: 1 wb \u003d 108 μs, that is, 1 μs \u003d 10-8 wb.

For research by Faraday, two wire spirals were used, isolated from each other and placed on a wooden coil. One of them was connected to an energy source, and the other to a galvanometer designed to record small currents. At that moment, when the circuit of the original spiral closed and opened, in the other circuit the arrow of the measuring device deviated.

Conducting research on the phenomenon of induction

In the first series of experiments, Michael Faraday inserted a magnetized metal bar into a coil connected to a current, and then pulled it out (Fig. 1, 2).

1 2

When a magnet is placed in a coil connected to a measuring device, an inductive current begins to flow in the circuit. If the magnetic bar is removed from the coil, the induction current still appears, but its direction is already reversed. Consequently, the parameters of the induction current will be changed in the direction of the bar and depending on the pole with which it is placed in the coil. The strength of the current is affected by the speed of movement of the magnet.

In the second series of experiments, a phenomenon is confirmed in which a changing current in one coil causes an induction current in another coil (Fig. 3, 4, 5). This happens at the moments of closing and opening the circuit. The direction of the current will depend on whether the electrical circuit closes or opens. In addition, these actions are nothing more than ways to change the magnetic flux. When the circuit is closed, it will increase, and when it is opened, it will decrease, simultaneously penetrating the first coil.

3 4

5

As a result of the experiments, it was found that the occurrence of an electric current inside a closed conducting circuit is possible only when they are placed in an alternating magnetic field. At the same time, the flow can change in time by any means.

The electric current that appears under the influence of electromagnetic induction is called induction, although this will not be a current in the conventional sense. When a closed circuit is in a magnetic field, an EMF is generated with an exact value, and not a current depending on different resistances.

This phenomenon is called the EMF of induction, which is reflected by the formula: Eind = - ∆F / ∆t. Its value coincides with the rate of change of the magnetic flux penetrating the surface of a closed loop, taken with a negative value. The minus present in this expression is a reflection of Lenz's rule.

Lenz's rule for magnetic flux

A well-known rule was derived after a series of studies in the 30s of the 19th century. It is formulated in the following way:

The direction of the induction current, excited in a closed circuit by a changing magnetic flux, affects the magnetic field it creates in such a way that it, in turn, creates an obstacle to the magnetic flux that causes the appearance of the inductive current.

When the magnetic flux increases, that is, it becomes Ф > 0, and the induction EMF decreases and becomes Eind< 0, в результате этого появляется электроток с такой направленностью, при которой под влиянием его магнитного поля происходит изменение потока в сторону уменьшения при его прохождении через плоскость замкнутого контура.

If the flow decreases, then the reverse process occurs when F< 0 и Еинд >0, that is, the action of the magnetic field of the induction current, there is an increase in the magnetic flux passing through the circuit.

The physical meaning of Lenz's rule is to reflect the law of conservation of energy, when when one quantity decreases, the other increases, and, conversely, when one quantity increases, the other will decrease. Various factors also affect the induction emf. When a strong and weak magnet is alternately inserted into the coil, the device will respectively show a higher value in the first case, and a lower value in the second. The same thing happens when the speed of the magnet changes.

The figure below shows how the direction of the induction current is determined using the Lenz rule. The blue color corresponds to the lines of force of the magnetic fields of the induction current and the permanent magnet. They are located in the direction of the north-south poles that are present in every magnet.

The changing magnetic flux leads to the emergence of an inductive electric current, the direction of which causes opposition from its magnetic field, which prevents changes in the magnetic flux. In this regard, the lines of force of the magnetic field of the coil are directed in the direction opposite to the lines of force of the permanent magnet, since its movement occurs in the direction of this coil.

To determine the direction of the current, it is used with a right-hand thread. It must be screwed in in such a way that the direction of its forward movement coincides with the direction of the induction lines of the coil. In this case, the directions of the induction current and the rotation of the gimlet handle will coincide.

Electrical And magnetic fields are generated by the same sources - electric charges, so we can assume that there is a certain connection between these fields. This assumption found experimental confirmation in 1831 in the experiments of the outstanding English physicist M. Faraday. He opened phenomenon of electromagnetic induction.

The phenomenon of electromagnetic induction underlies the operation of induction electric current generators, which account for all the electricity generated in the world.

• magnetic flux

Closed circuit placed in a uniform magnetic field

The quantitative characteristic of the process of changing the magnetic field through a closed loop is a physical quantity called magnetic flux. Magnetic flux (F) through a closed loop area (S) is a physical quantity equal to the product of the modulus of the magnetic induction vector (B) by the loop area (S) and the cosine of the angle betweenvector B and normal to the surface: Φ = BS cos α. The unit of magnetic flux is F - weber (Wb): 1 Wb \u003d 1 T 1 m 2.

perpendicular maximum.

If the magnetic induction vector parallel contour area, then the magnetic flux equals zero.

• Law of electromagnetic induction

Empirically, the law of electromagnetic induction was established: The EMF of induction in a closed circuit is equal in absolute value to the rate of change of the magnetic flux through the surface bounded by the circuit: This formula is called Faraday's law .

Faraday's first experiment is a classic demonstration of the basic law of electromagnetic induction. In it, the faster the magnet is moved through the turns of the coil, the more induction current appears in it, and hence the induction EMF.

• Lenz's rule

The dependence of the direction of the induction current on the nature of the change in the magnetic field through a closed circuit in 1833 was experimentally established by the Russian physicist E.Kh. Lenz. According to Lenz's rule , arising in a closed circuit, the induction current with its magnetic field counteracts the change in the magnetic flux, which it called. More briefly, this rule can be formulated as follows: induced current is directed so as to prevent the reason that causes it. Lenz's rule reflects the experimental fact that they always have opposite signs (the minus sign in Faraday formula).

Lenz designed a device consisting of two aluminum rings, solid and cut, mounted on an aluminum crossbar. They could rotate around an axis, like a rocker. When a magnet was introduced into a solid ring, it began to "run away" from the magnet, turning the rocker accordingly. When taking the magnet out of the ring, it tried to "catch up" with the magnet. When the magnet moved inside the cut ring, no movement occurred. Lenz explained the experiment by the fact that the magnetic field of the induction current sought to compensate for the change in the external magnetic flux.

Lenz's rule has a deep physical meaning - it expresses law of energy conservation.

Questions.

1. What determines the magnetic flux penetrating the area of ​​a flat circuit placed in a uniform magnetic field?

From the vector of magnetic induction B, the area of ​​\u200b\u200bthe contour S, and its orientation.

2. How does the magnetic flux change with an increase in magnetic induction by a factor of n, if neither the area nor the orientation of the circuit change?

Increases n times.

3. At what orientation of the circuit with respect to the lines of magnetic induction is the magnetic flux penetrating the area of ​​this circuit maximum? equal to zero?

The magnetic flux is maximum if the contour plane is perpendicular to the lines of magnetic induction and is zero when parallel.

4. Does the magnetic flux change with such a rotation of the circuit, when the lines of magnetic induction then penetrate it. then slide along its plane?

Yes. In the case when the angle of inclination of the magnetic lines relative to the plane of the circuit changes, the magnetic flux also changes.

Exercises.

1. A wire coil K, with a steel core, is connected to a DC source circuit in series with a rheostat R and a key K (Fig. 125). An electric current flowing through the turns of the coil K 1 creates a magnetic field in the space around it. In the field of the coil K 1 is the same coil K 2. How can you change the magnetic flux penetrating the coil K 2? Consider all possible options.

The magnetic flux penetrating the coil K 2 can be changed: 1) by changing the current strength I with a rheostat; 2) closing-opening of the key; 3) changing the orientation of coil K 2.

If there is a closed conducting circuit in the magnetic field that does not contain current sources, then when the magnetic field changes, an electric current arises in the circuit. This phenomenon is called electromagnetic induction. The appearance of a current indicates the occurrence of an electric field in the circuit, which can provide a closed movement of electric charges or, in other words, the occurrence of an EMF. The electric field, which arises when the magnetic field changes and whose work is not equal to zero when moving charges along a closed circuit, has closed lines of force and is called vortex.

For a quantitative description of electromagnetic induction, the concept of magnetic flux (or magnetic induction vector flux) through a closed loop is introduced. For a flat circuit located in a uniform magnetic field (and only such situations can be encountered by schoolchildren at a unified state exam), the magnetic flux is defined as

where is the field induction, is the contour area, is the angle between the induction vector and the normal (perpendicular) to the contour plane (see figure; the perpendicular to the contour plane is shown by a dotted line). The unit of magnetic flux in the international SI system of units is Weber (Wb), which is defined as the magnetic flux through a contour of area 1 m 2 of a uniform magnetic field with an induction of 1 T, perpendicular to the plane of the contour.

The value of the EMF of induction that occurs in the circuit when the magnetic flux through this circuit changes is equal to the rate of change of the magnetic flux

Here is the change in the magnetic flux through the circuit over a small time interval. An important property of the law of electromagnetic induction (23.2) is its universality with respect to the reasons for changing the magnetic flux: the magnetic flux through the circuit can change due to a change in the magnetic field induction, a change in the area of ​​the circuit, or a change in the angle between the induction vector and the normal, which occurs when the circuit rotates in field. In all these cases, according to the law (23.2), induction EMF and induction current will appear in the circuit.

The minus sign in formula (23.2) is "responsible" for the direction of the current resulting from electromagnetic induction (Lenz's rule). However, it is not so easy to understand in the language of the law (23.2) which direction of the induction current this sign will lead to with this or that change in the magnetic flux through the circuit. But it is easy enough to remember the result: the induction current will be directed in such a way that the magnetic field created by it will “tend” to compensate for the change in the external magnetic field that generated this current. For example, with an increase in the flow of an external magnetic field through a circuit, an induction current will appear in it, the magnetic field of which will be directed opposite to the external magnetic field so as to reduce the external field and thus maintain the original value of the magnetic field. With a decrease in the field flux through the circuit, the induction current field will be directed in the same way as the external magnetic field.

If, for some reason, the current changes in a circuit with a current, then the magnetic flux through the circuit of the magnetic field that is created by this current itself also changes. Then, according to the law (23.2), induction EMF should appear in the circuit. The phenomenon of the occurrence of an EMF of induction in a certain electrical circuit as a result of a change in current in this circuit itself is called self-induction. To find the EMF of self-induction in some electrical circuit, it is necessary to calculate the flux of the magnetic field created by this circuit through itself. Such a calculation is a difficult problem due to the inhomogeneity of the magnetic field. However, one property of this flow is obvious. Since the magnetic field created by the current in the circuit is proportional to the magnitude of the current, then the magnetic flux of the own field through the circuit is proportional to the current in this circuit

where is the current strength in the circuit, is the proportionality factor, which characterizes the "geometry" of the circuit, but does not depend on the current in it and is called the inductance of this circuit. The unit of inductance in the international SI system of units is Henry (H). 1 H is defined as the inductance of such a circuit, the flux of induction of its own magnetic field through which is 1 Wb at a current strength of 1 A. Taking into account the definition of inductance (23.3) from the law of electromagnetic induction (23.2), we obtain for the EMF of self-induction

Due to the phenomenon of self-induction, the current in any electrical circuit has a certain "inertia" and, therefore, energy. Indeed, to create a current in the circuit, it is necessary to do work to overcome the self-induction EMF. The energy of the circuit with current and is equal to this work. It is necessary to remember the formula for the energy of the circuit with current

where is the inductance of the circuit, is the current in it.

The phenomenon of electromagnetic induction is widely used in technology. It is based on the creation of electric current in electric generators and power plants. Thanks to the law of electromagnetic induction, mechanical vibrations are converted into electrical vibrations in microphones. On the basis of the law of electromagnetic induction, in particular, an electric circuit, which is called an oscillatory circuit (see the next chapter), and which is the basis of any radio transmitting or radio receiving equipment, works.

Consider now the tasks.

Of those listed in task 23.1.1 phenomena, there is only one consequence of the law of electromagnetic induction - the appearance of a current in the ring when a permanent magnet is passed through it (the answer 3 ). Everything else is the result of the magnetic interaction of currents.

As indicated in the introduction to this chapter, the phenomenon of electromagnetic induction underlies the operation of an alternator ( task 23.1.2), i.e. device that creates alternating current, a given frequency (the answer 2 ).

The induction of the magnetic field created by a permanent magnet decreases with increasing distance from it. Therefore, when the magnet approaches the ring ( task 23.1.3) the induction flux of the magnetic field of the magnet through the ring changes, and an induction current appears in the ring. Obviously, this will happen when the magnet approaches the ring with both the north and south poles. But the direction of the induction current in these cases will be different. This is due to the fact that when the magnet approaches the ring with different poles, the field in the plane of the ring in one case will be directed opposite to the field in the other. Therefore, to compensate for these changes in the external field, the magnetic field of the inductive current must be directed differently in these cases. Therefore, the directions of the induction currents in the ring will be opposite (the answer is 4 ).

For the occurrence of induction EMF in the ring, it is necessary that the magnetic flux through the ring changes. And since the magnetic induction of the magnet field depends on the distance to it, then in the considered case task 23.1.4 case, the flow through the ring will change, an induction current will appear in the ring (the answer is 1 ).

When rotating frame 1 ( task 23.1.5) the angle between the lines of magnetic induction (and, therefore, the induction vector) and the plane of the frame at any time is equal to zero. Consequently, the magnetic flux through the frame 1 does not change (see formula (23.1)), and the induction current does not occur in it. In frame 2, an induction current will occur: in the position shown in the figure, the magnetic flux through it is zero, when the frame turns a quarter of a turn, it will be equal to , where is the induction, is the area of ​​\u200b\u200bthe frame. After another quarter of a turn, the flow will again be zero, and so on. Therefore, the flux of magnetic induction through frame 2 changes during its rotation, therefore, an induction current arises in it (the answer is 2 ).

IN task 23.1.6 induction current occurs only in case 2 (answer 2 ). Indeed, in case 1, the frame remains at the same distance from the conductor during movement, and, consequently, the magnetic field created by this conductor in the plane of the frame does not change. When the frame moves away from the conductor, the magnetic induction of the conductor field in the frame area changes, the magnetic flux through the frame changes, and an induction current arises

The law of electromagnetic induction states that the inductive current in the ring will flow at such moments in time when the magnetic flux through this ring changes. Therefore, while the magnet is at rest near the ring ( task 23.1.7) the inductive current in the ring will not flow. So the correct answer for this problem is 2 .

According to the law of electromagnetic induction (23.2), the induction EMF in the frame is determined by the rate of change of the magnetic flux through it. And since by condition tasks 23.1.8 the induction of the magnetic field in the region of the frame changes uniformly, the rate of its change is constant, the magnitude of the induction emf does not change during the experiment (the answer is 3 ).

IN task 23.1.9 The induction emf that occurs in the frame in the second case is four times greater than the induction emf that occurs in the first (the answer is 4 ). This is due to a fourfold increase in the frame area and, accordingly, the magnetic flux through it in the second case.

IN task 23.1.10 in the second case, the rate of change of the magnetic flux doubles (the field induction changes by the same amount, but in half the time). Therefore, the EMF of electromagnetic induction that occurs in the frame in the second case is twice as large as in the first (the answer is 1 ).

When the current in a closed conductor doubles ( task 23.2.1), the value of the induction of the magnetic field will increase at each point in space twice, without changing in direction. Therefore, the magnetic flux through any small area and, accordingly, the entire conductor will change exactly twice (the answer is 1 ). But the ratio of the magnetic flux through the conductor to the current in this conductor, which is the inductance of the conductor , while not changing ( task 23.2.2- answer 3 ).

Using formula (23.3) we find in task 32.2.3 gn (answer 4 ).

Relationship between the units of measurement of magnetic flux, magnetic induction and inductance ( task 23.2.4) follows from the definition of inductance (23.3): a unit of magnetic flux (Wb) is equal to the product of a unit of current (A) per unit of inductance (H) - the answer 3 .

According to formula (23.5), with a twofold increase in the inductance of the coil and a twofold decrease in the current in it ( task 23.2.5) the energy of the magnetic field of the coil will decrease by 2 times (the answer 2 ).

When the frame rotates in a uniform magnetic field, the magnetic flux through the frame changes due to a change in the angle between the perpendicular to the plane of the frame and the magnetic field vector. And since in the first and second cases in task 23.2.6 this angle changes according to the same law (by condition, the frequency of rotation of the frames is the same), then the EMF of induction changes according to the same law, and, therefore, the ratio of the amplitude values ​​of the EMF of induction within the framework is equal to one (the answer 2 ).

The magnetic field created by a conductor with current in the region of the frame ( task 23.2.7), sent "from us" (see the solution of problems in Chapter 22). The value of the wire field induction in the frame area will decrease as it moves away from the wire. Therefore, the induction current in the frame must create a magnetic field directed inside the frame "away from us". Now using the gimlet rule to find the direction of magnetic induction, we conclude that the induction current in the loop will be directed clockwise (the answer is 1 ).

With an increase in current in the wire, the magnetic field created by it will increase and an induction current will appear in the frame ( task 23.2.8). As a result, there will be an interaction of the induction current in the loop and the current in the conductor. To find the direction of this interaction (attraction or repulsion), you can find the direction of the induction current, and then, using the Ampère formula, the force of interaction between the frame and the wire. But you can do it differently, using the Lenz rule. All inductive phenomena must have such a direction as to compensate for the cause that causes them. And since the reason is an increase in the current in the loop, the force of interaction between the inductive current and the wire should tend to reduce the magnetic flux of the wire field through the loop. And since the magnetic induction of the wire field decreases with increasing distance to it, this force will repel the frame from the wire (answer 2 ). If the current in the wire decreased, then the frame would be attracted to the wire.

Task 23.2.9 also related to the direction of induction phenomena and Lenz's rule. When a magnet approaches a conducting ring, an induction current will appear in it, and its direction will be such as to compensate for the cause that causes it. And since this reason is the approach of a magnet, the ring will repel from it (answer 2 ). If the magnet is moved away from the ring, then for the same reasons there would be an attraction of the ring to the magnet.

Task 23.2.10 is the only computational problem in this chapter. To find the EMF of induction, you need to find the change in the magnetic flux through the circuit . It can be done like this. Let at some point in time the jumper was in the position shown in the figure, and let a small time interval pass. During this time interval, the jumper will move by the value . This will increase the contour area by the amount . Therefore, the change in the magnetic flux through the circuit will be equal, and the magnitude of the induction emf (answer 4 ).

In the lesson, we will learn about a new concept for us - magnetic flux - and consider how it is characterized.

Recall that when the parameters of the magnetic field change near a closed conductor, a current arises in it. This current is called the induction current, and the phenomenon is the phenomenon of electromagnetic induction.

However, the question remains, what specific parameters of the magnetic field do we need in order to obtain this effect. Let's start with an experiment:

To carry it out, we need: a coil with a large number of turns and an ammeter connected to it. During the experiment, pay attention to the behavior of the ammeter needle (Fig. 1).

Rice. 1. Faraday's experiments

As we can see, when lowering and removing the bar magnet from the coil, an induction current is formed in it.

Let us analyze which parameter change led to the observed effect. As the magnet approaches and moves away from the coil, the strength of the magnetic field in it changes.

Thus, the value that affects the formation of the induction current in the coil is the strength of the magnetic field.

Recall that it is described by such a quantity as magnetic induction. It is a vector and is denoted and measured in T.

A closed wire ring placed perpendicular to the magnetic field is compressed from several sides so that it changes its shape (Fig. 2).

Rice. 2. Illustration for experience

In this case, during the deformation process, an induction current appears in the ring. What did we change this time?

Now the area of ​​the ring has been changed. Of course, instead of a ring, you can experiment with any closed conductor.

The circuit is a closed conductor (Fig. 3).

Rice. 3. Contour

Rice. 4. Generator

Its main elements are (Fig. 4):

• a coil that can rotate around its axis;
• a permanent magnet around the coil.

When the coil rotates in a magnetic field, you can see that the light bulb lights up (i.e., an induction current appears in the circuit).

From this experience, we can conclude that the phenomenon of electromagnetic induction also manifests itself when a coil or a conducting frame is rotated in a magnetic field (Fig. 5), i.e., when the angle between the magnetic lines and the plane of the conductor changes.

Rice. 5. Illustration for experience

All three parameters, changes in which affect the magnitude of the induction current, are united by a physical quantity called magnetic flux.

IN - field magnetic induction module

S- contour area

Characterizes the location of the contour plane relative to the magnetic line.

Magnetic flux is measured in Weber (Wb) and denoted by the letter F.

Thus, the magnetic flux is proportional to the modulus of the magnetic induction of the field, the area of ​​the circuit and depends on the location of the circuit plane relative to the magnetic line.

The task of analyzing the parameters of the magnetic flux

In order to learn how to draw conclusions about the change in the magnetic flux in the elements of various electrical circuits, which can lead to the presence of unwanted induction currents, consider the problem.

A wire coil with a steel core is connected to the DC circuit in series with a rheostat and a key (Fig. 6).

Rice. 6. Illustration for the problem

The electric current flowing through the branches of the coil creates a magnetic field in the space around it (Fig. 7). In the field of the coil is the same coil.

Rice. 7. Illustration for the problem

How can you change the magnetic flux penetrating the coil? Consider all possible options.

Let us recall which parameters change leads to a change in the magnetic flux.

Let's start by changing the induction of the magnetic field of the coil. This can be achieved by changing the strength of the current that generates its magnetic field. You can change the current in the circuit shown in 2 ways:

1. Moving the rheostat slider

2. Key on/off

It is worth noting that the change in the current value will be the largest from maximum to zero, which will lead to the largest change in the magnetic flux in the coil.

The next parameter, the change of which will affect the value of the magnetic flux, is the area of ​​the circuit. In our case, coils But we cannot change the cross-sectional area of ​​\u200b\u200bthe coil. Therefore, the option is out.

The last option for changing the magnetic flux is to rotate the coil relative to the magnetic lines of the coil. To achieve the maximum result of the change, it is necessary to turn the coil by 90 (Fig. 8).

Rice. 8. Illustration for the problem

What is described by magnetic flux?

As we have already noted, it depends:

• From the strength of the magnetic field
• From the area of ​​\u200b\u200bthe contour through which these magnetic lines pass
• From the angle of location between the contour and the magnetic lines

Thus, magnetic flux characterizes the number of magnetic lines penetrating a limited contour.

This is easy to check.

1. Let's compare the number of lines that cross the same contour, but in magnetic fields of different strength (Fig. 9).

In a stronger field, the outline cuts through more lines.

Rice. 9. Illustration for the problem

2. If we compare the number of lines that in the same uniform magnetic field penetrate contours of different area, then there are obviously more of them through a larger contour (Fig. 10).

Rice. 10. Illustration for the problem

3. If we compare the rotation of the contour in a magnetic field at an angle to the magnetic lines and its location along the lines, then in the first case their number through the plane of the contour will be maximum. And in the second, the magnetic lines will slide along the contour and not penetrate it at all (Fig. 11).

In these examples, a greater number of lines through the circuit corresponded to a greater magnetic flux.

As a result, we note that since the magnitude of the induction current depends on the change in magnetic induction, the area of ​​\u200b\u200bthe circuit and on its orientation in space, it is customary to say that it depends on changes in the magnetic flux.

In addition, Faraday's experiments showed that the rate of change of the magnetic flux is important. The faster you change these values, the greater the value of the induction current will be.

Thus, it can be argued that the phenomenon of electromagnetic induction is characterized by the rate of change of the magnetic flux.

The task of determining the conditions for the occurrence of induction current

In order to understand the relationship between the magnetic flux through the circuit and the phenomenon of electromagnetic induction in it, consider the problem:

A small coil is moved forward in a uniform magnetic field. Is there an induced current in the coil? Justify the answer.

Rice. 12. Illustration for the problem

It may seem that due to the movement of the coil there may be changes, the consequence of which will be the occurrence of an induction current in its turns (Fig. 12).

Recall that a prerequisite for the occurrence of an induction current is a change in the magnetic flux through the turns of the coil. This requires a change in magnetic induction through the circuit of the coil. What is not observed, because by condition the field is homogeneous.

In addition, it is possible to change the cross-sectional area of ​​the coil, which is also not observed.

The last possible option is to change the angle of rotation of the coil plane to the magnetic field lines, which, obviously, also does not occur, since the movement is translational, which means that no coil rotations are observed.

Therefore, we conclude that the magnetic flux will not change, respectively, no induction current will be formed in the turns of the coil either.

Comparison of magnetic flux with water flow

The name of the new physical quantity of the magnetic flux studied by us is not accidental. The fact is that the magnetic flux through the circuit can be compared with the flow of water through the ring, which is placed in a pipe (Fig. 13). (1)

The greater the speed of water, the more it passes through the ring per unit time. (2)

The larger the area of ​​the ring, the more water will flow through it in the observed time. (3)

If the ring is rotated when it is transverse to the water flow, the maximum amount of water will flow through the plane of the ring. (4)

If you start to turn it at an acute angle to the flow, then less and less water will flow. (5)

Rice. 13. Comparison of magnetic flux with water flow

And when turning along the outflow, the water will not pass through the ring at all, but will slide along it. (6)

We have considered similar properties for the magnetic flux.

In the lesson, we explained which parameters of the magnetic field and circuit must be changed to observe the phenomenon of electromagnetic induction. We have combined this into the concept of "magnetic flux".

Bibliography

1. Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education.
2. Yavorsky B.M., Pinsky A.A., Fundamentals of Physics, v.2., - M. Fizmatlit., 2003.
3. Elementary textbook of physics. Ed. G.S. Landsberg, T. 3. - M., 1974.

1. Festival.1september.ru ().
2. Nvtc.ee ().
3. Сlass-fizika.narod.ru ().

Homework

1. What determines the magnetic flux penetrating the area of ​​a flat circuit placed in a uniform magnetic field?
2. How does the magnetic flux change with an increase in magnetic induction by n times, if neither the area nor the orientation of the circuit change?
3. Does the magnetic flux change with such a rotation of the circuit, when the lines of magnetic induction then penetrate it. then slide along its plane?