Lenz's rule for determining the direction of induction current. T. Electromagnetic induction

In the experiments described in the previous paragraph, we saw that in different cases the direction of the induction current can be different: the galvanometer was thrown back sometimes in one direction, sometimes in the other. Now we will try to find a general rule that determines the direction of the induction current.

To do this, let us carefully follow the direction of the current in some induction experiment, for example, in the experiment shown in Fig. 254, a. The diagram of this experiment is shown in Fig. 261, each of coils I and II is depicted as one turn, and the arrows and indicate, respectively, the direction of the primary current in coil I and the direction of the induction current in coil II.

Fig. 261. Relationship between the direction of the primary current creating a magnetic field and the direction of the induction current: a) when the magnetic field increases; b) when the magnetic field weakens

Rice. 261,a refers to the case when the current is increased, and Fig. 261, b - to the case when it is weakened. We see that in the first case, i.e., when the magnetic field increases, and therefore, when the magnetic flux increases, the currents in coils I and II have opposite directions; on the contrary, in the case when induction occurs due to a weakening of the magnetic field, i.e., when the magnetic flux decreases, both currents have the same directions. In other words, we can say that when the cause of induction is an increase in the magnetic flux penetrating the area of ​​the circuit, then the resulting induction current is directed in such a way that it weakens the original magnetic flux. On the contrary, when induction occurs due to weakening magnetic flux, the magnetic field of the induced current strengthens the original magnetic flux.

The result we obtained can be formulated as a general rule:

An induced current always has a direction in which its magnetic field reduces (compensates) the change in magnetic flux that causes this current to occur.

This general rule is observed in all cases of induction without exception. Let us consider, in particular, the case when induction is caused by movement of a circuit or part of it relative to a magnetic field. Such an experience is depicted in Fig. 253, and its diagram is shown in Fig. 262, and the arrows on the coil indicate the direction of the current induced in the coil as it approaches the north pole of the magnet (Fig. 262, a) or as it moves away from this pole (Fig. 262, b). Using the gimlet rule (§ 124), it is easy to determine the direction of the magnetic field of the induced current and make sure that it corresponds to the rule formulated above.

Rice. 262. The direction of the induction current arising in the circuit: a) when a magnet approaches it; b) when the magnet moves away from it

Let us now pay attention to this fact. When an induced current arises in the coil, it becomes equivalent to a magnet, the position of the north and south poles of which can be determined by the gimlet rule. In Fig. 262 shows that in case a) a north pole appears at the upper end of the coil, and in case b) a south pole appears. From this figure we see that when we bring, say, the north pole of a magnet closer to the induction coil, then a north pole also appears at the end of the coil closest to it, and when we move the north pole of the magnet away from the coil, then a south pole appears at the nearest end of the coil . But, as we know, magnets facing each other with like poles repel, and opposite ones attract. Therefore, when induction occurs as a result of the magnet approaching the coil, the interaction forces between the magnet and the induced current repel the magnet from the coil, and when induction occurs when the magnet moves away from the coil, they are attracted to each other. Thus, for cases where induction occurs due to the movement of a magnet or the entire induction circuit as a whole, we can establish the following general rule, essentially equivalent to the rule formulated above, but for these cases more convenient:

The induction current always has such a direction that its interaction with the primary magnetic field counteracts the movement due to which induction occurs.

This rule is called Lenz's rule.

Lenz's rule is closely related to the law of conservation of energy. In fact, let us imagine, for example, that when the north pole of the magnet approaches the solenoid, the current in it would have a direction opposite to that required by Lenz’s rule, i.e. that at the end of the solenoid closest to the magnet, not the north, but the south pole. In this case, not repulsive forces, but attractive forces would arise between the solenoid and the magnet. The magnet would continue to spontaneously and with increasing speed approach the solenoid, creating increasingly large induction currents in it and thereby increasingly increasing the force attracting it to the solenoid. Thus, without any expenditure of external work, we would receive, on the one hand, a continuous accelerated movement of the magnet towards the solenoid, and on the other, an ever-increasing current in the solenoid, capable of producing work. It is clear that this is impossible and that the induced current cannot have a direction other than that indicated by Lenz's rule. The same can be seen by considering other cases of induction.

In Fig. 263 shows a very simple and visual experiment illustrating Lenz’s rule. An aluminum ring, serving as an induction coil, is suspended near the poles of a strong magnet or electromagnet that can be moved along a rail. Moving the magnet away from the ring, we will see that the ring follows it. On the contrary, when we move the magnet closer to the ring, we find that the ring moves away from the magnet. In both cases, when the magnet moves, the magnetic flux through the ring changes, and an induced current appears in the ring. According to Lenz's rule, this current is directed in such a way that its interaction with a moving magnet slows down the movement of the magnet; according to Newton's third law (see Volume I), counterforces are applied to the ring and cause its movement.

Rice. 263. A ring-shaped induction coil is suspended between the poles of a magnet. If the magnet is moved away from the ring, the ring follows it. If you move a magnet towards a ring, it moves away from the magnet

In Fig. 264 depicts a similar experiment in which linear motion is replaced by rotation. When magnet 1 rotates, the field, remaining constant in magnitude, rotates with it. As a result, the magnetic flux through ring 2 changes all the time and a current is induced in the ring. Applying Lenz's rule and taking into account Newton's third law, we can easily understand that a ring placed in a rotating magnetic field begins to rotate in the same direction as the field rotates.

Rice. 264. Rotation of magnet 1 creates a rotating magnetic field, which rotates ring 2

Particular attention should be paid to this experience, since it facilitates understanding of the structure of one of the most common types of electric motors.

139.1. Nearby there are two long conductors and (Fig. 265); the first of them is connected to a current source, the second to a galvanometer. If in some way, for example using a rheostat, the current strength in the first conductor is changed, then the galvanometer will detect the occurrence of an induced current in the second conductor. Explain this experience. How do the magnetic field lines go in this case and where is the induction loop located? What is the direction of the induced current when the primary current increases and decreases?

Rice. 265. For exercise 139.1

139.2. For the induction experiment shown in Fig. 258, determine, using Lenz's rule and the left-hand rule, the direction of the induced current, assuming that the magnetic field is directed from bottom to top and the conductor moves from left to right. How will the direction of the induced current change if the direction of the magnetic field or the direction of movement of the conductor is reversed? To direct the current in a conductor, formulate a similar “right-hand rule”.

139.3. An induction experiment is carried out, shown in Fig. 260. The battery pole signs are shown in the figure. Determine the direction of the current in coil II when the iron core moves in and when it moves out of coil I.

We have seen that there is always a magnetic field around a conductor carrying current.

Is it possible to create a current in a conductor using a magnetic field?

This problem was solved by M. Faraday. After intense searching, spending a lot of work and ingenuity, he came to the conclusion: only a magnetic field changing over time can generate an electric current.

Faraday's experiments consisted of the following. If a permanent magnet is pushed inside the coil to which the galvanometer is connected (Fig. 2.a), then an electric current arises in the circuit. If the magnet is pulled out of the coil, the galvanometer also shows current, but in the opposite direction (Fig. 2, b). An electric current also occurs when the magnet is stationary and the coil is moving (up or down). As soon as the movement stops, the current immediately disappears. However, not every movement of a magnet (or coil) produces an electric current. If you rotate the magnet around a vertical axis (Fig. 2, c), no current arises.

The galvanometer will show the presence of current in coil B when it and coil A move relative to the current (Fig. 3, a) at the moment of closing or opening key K or when the current strength in the circuit of coil A changes (when the rheostat motor moves, Fig. 3, b ). It is easy to see that a current in the coil occurs whenever the magnetic flux passing through the coil changes.

Emergence phenomenon EMF in a conducting circuit ( current, if the circuit is closed) when the magnetic flux passing through the circuit changes, it is called the phenomenon of electromagnetic induction. The current obtained in this way is called induced current, and the emf creating it is called induced emf.

Literature

Aksenovich L. A. Physics in secondary school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyakhavanne, 2004. - P.344-345.

1. Determine the direction of the induction current in the solid ring to which the magnet is brought (see Fig. 2.6).

2. Current strength in the conductor OO"(see Fig. 2.20) decreases. Determine the direction of the induced current in a stationary circuit ABCD and the directions of the forces acting on each side of the circuit.

3. The metal ring can move freely along the core of the coil connected to the DC circuit (Fig. 2.21). What will happen when the circuit closes and opens?

4. The magnetic flux through a conductor circuit with a resistance of 3 · 10 -2 Ohm in 2 s changed to 1.2 · 10 -2 Wb. Determine the current strength in the conductor if the flux changes uniformly.

5. An airplane flies horizontally at a speed of 900 km/h. Determine the potential difference between the ends of its wings if the module of the vertical component of the magnetic induction of the earth's magnetic field is 5 10 -5 T and the wingspan is 12 m.

6. The current in the coil changes from 1 A to 4 A in a time of 3 s. In this case, a self-inductive emf equal to 0.1 V arises. Determine the inductance of the coil and the change in the energy of the magnetic field created by the current.

7. In a coil with an inductance of 0.15 H and a very low resistance r, the current strength is 4 A. A resistor with resistance R is connected in parallel to the coil<< r. Какое количество теплоты выделится в катушке и в резисторе после быстрого отключения источника тока?

Option No. 280314

In tasks 2–5, 8, 11–14, 17, 18, 20 and 21 are written as one number, which corresponds to the number of the correct answer. Answers to tasks 1, 6, 9, 15, 19 are written as a sequence of numbers without spaces, commas or other additional characters. Answers to tasks 7, 10 and 16 are written in the form of a number, taking into account the units indicated in the answer. There is no need to indicate units of measurement in your answer.

If the option is given by the teacher, you can enter the answers to the assignments in Part C or upload them to the system in one of the graphic formats. The teacher will see the results of completing assignments in Part B and will be able to evaluate the uploaded answers to Part C. The scores assigned by the teacher will appear in your statistics. A complete correct solution to each of the problems C1-C6 must include laws and formulas, the use of which is necessary and sufficient to solve the problem, as well as mathematical transformations, calculations with a numerical answer and, if necessary, a drawing explaining the solution.

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A magnet is moved in a coil connected to a galvanometer. The magnitude of the induction current depends

The correct answer is

1) only A

2) only B

4) neither A nor B

Solution.

According to Faraday's law, the emf of magnetic induction depends only on the rate of change of the magnetic flux. Consequently, the magnitude of the induction current depends only on the speed of movement of the magnet; the direction of the current will depend on whether the magnet is brought into the coil or taken out of the coil.

Answer: 2

A magnet is moved in a coil connected to a galvanometer. The direction of the induction current depends

A. depends on whether the magnet is brought into the coil or taken out of the coil

B. on the speed of movement of the magnet

The correct answer is

1) only A

2) only B

4) neither A nor B

Solution.

The direction of the induction current depends only on whether the magnet is brought into the coil or taken out of the coil. The speed of movement of the magnet determines the magnitude of the induction current, but not the direction.

The correct answer is indicated under number 1.

Answer: 1

Coil 1 is connected to a galvanometer and inserted into coil 2, through which current is passed. Current graph I flowing in coil 2, depending on time t shown in the figure.

The induced current in coil 1 will be observed in the period of time

1) only from 0 to t 1

2) only from t 2 to t 3

3) only from t 3 to t 4

4) from 0 to t 1 and from t 2 to t 3

Solution.

According to Faraday's law, an induced current in coil 1 will be observed when the current in coil 2 changes. This will happen in the intervals from 0 to t 1 and from t 2 to t 3 .

The correct answer is indicated under number 4.

Answer: 4

The figure shows a graph of the dependence of the electric current flowing in a resistor on time. The magnetic field around the conductor occurs in the time interval(s)

1) only from 0 s to 6 s

2) only from 0 s to 1 s

3) only from 0 s to 1 s and from 4 s to 6 s

4) from 0 s to 8 s

Solution.

A magnetic field arises around a conductor carrying electric current as charges move through it. There is current only in the area from 0 s to 6 s, so the magnetic field will appear there.

The correct answer is indicated under number 1.

Answer: 1

A magnet is inserted into a coil connected to a galvanometer. The direction of the induction current depends

A. on the speed of movement of the magnet

B. depends on which pole the magnet is inserted into the coil

The correct answer is

1) only A

2) only B

4) neither A nor B

Solution.

According to Faraday's law, the direction of the induced current depends on the change in magnetic flux over time. Depending on the direction of the pole, the direction of the magnetic field depends, and, consequently, the direction of the current in the coil.

The correct answer is indicated under number 2.

Answer: 2

A frame with current is placed in a uniform horizontal magnetic field, while the normal to the plane of the frame makes a certain angle α with lines of magnetic field induction (see figure). The frame can rotate freely around its symmetry axes. What will happen to the frame after it is placed in a magnetic field?

1) the frame will remain at rest

2) the frame will begin to rotate around the vertical axis of symmetry clockwise (when viewed from above)

3) the frame will begin to rotate around the vertical axis of symmetry counterclockwise (when viewed from above)

4) the frame will begin to rotate around one of the horizontal axes of symmetry

Solution.

A current-carrying conductor in a magnetic field is acted upon by an Ampere force. Using the left-hand rule, we determine the direction of the Ampere force. The magnetic field is directed from the north pole to the south, it should enter the palm, we direct the fingers along the current, then the thumb will indicate the direction of the Ampere force. At the far end of the frame the force acts in a direction away from us, at the near end - towards us. Consequently, if viewed from above, the frame will begin to rotate around the vertical axis of symmetry counterclockwise.

The correct answer is indicated under number 3.

Answer: 3

In the first case, a strip magnet is pulled out from a solid copper ring, and in the second case, it is pulled out from a steel ring with a cut (see figure). Induction current

1) does not occur in any of the rings

2) occurs in both rings

3) occurs only in a copper ring

4) occurs only in a steel ring

Solution.

According to Faraday's law, an induced current occurs in a closed circuit when the magnetic flux that penetrates the area limited by this circuit changes. When the magnet moves out of the ring, the magnetic flux changes, but the steel ring is not closed, so the current appears only in the copper ring.

The correct answer is indicated under number 3.

Answer: 3

A coil of wire connected to a galvanometer is moved uniformly perpendicular to the induction lines B uniform magnetic field from left to right, as shown in the figure. Induction current in a turn

1) does not occur, since the coil is moved parallel to itself in a uniform magnetic field

2) does not occur, since the coil is moved evenly

3) arises because, when moving, the plane of the coil intersects the lines of magnetic field induction

4) arises since the plane of the coil is perpendicular to the lines of magnetic induction

Solution.

According to Faraday's law, an induced current occurs in a circuit if a change in magnetic flux occurs Φ , permeating this contour, in time. The flow is

Where B- module of the magnetic induction vector, S- area limited by the contour, and α - the angle between the perpendicular to the turn and the direction of the magnetic induction vector. None of these quantities change, since the field is uniform and the frame moves parallel to itself.

The correct answer is indicated under number 1.

Answer: 1

During the lesson, the teacher, using a coil closed to a galvanometer and a strip magnet (Fig. 1), sequentially carried out experiments 1 and 2 to observe the phenomenon of electromagnetic induction. A description of the teacher’s actions and galvanometer readings are presented in the table.

Which statements correspond to the results of the experimental observations? From the proposed list of statements, select two correct ones. Indicate their numbers.

1) The magnitude of the induction current depends on the geometric dimensions of the coil.

2) When the magnetic flux passing through the coil changes, an electric (induction) current arises in the coil.

3) The magnitude of the induction current depends on the rate of change of the magnetic flux passing through the coil.

4) The direction of the induction current depends on whether the magnetic flux passing through the coil increases or decreases.

5) The direction of the induction current depends on the direction of the magnetic lines of the changing magnetic flux passing through the coil.

Solution.

Let's analyze the statements.

1) The statement does not correspond to experimental data, since in both experiments the coil was the same.

2) The statement corresponds to experimental data.

3) The statement does not correspond to experimental data, since in both experiments the speed was the same.

4) The statement does not correspond to experimental data, since in both experiments a magnet was introduced into the coil, i.e., the flux was increased.

5) The statement corresponds to experimental data.

Answer: 25.

Answer: 25|52

During the lesson, the teacher, using a coil closed to a galvanometer and a strip magnet (see figure), consistently conducted experiments to observe the phenomenon of electromagnetic induction. The experimental conditions and galvanometer readings are presented in the table.

Select two statements from the proposed list that correspond to the results of the experimental observations and write down in your answer the numbers under which they are indicated.

1) The magnitude of the induction current depends on the geometric dimensions of the coil.

2) When the magnetic flux passing through the coil changes, an electric (induction) current arises in the coil.

3) The magnitude of the induction current depends on the rate of change of the magnetic flux passing through the coil.

4) The direction of the induction current depends on whether the magnetic flux passing through the coil increases or decreases.

5) The direction of the induction current depends on the direction of the magnetic lines passing through the coil.

Solution.

Let's analyze each statement.

1) Based on this experience, it is impossible to draw a conclusion about the dependence of the induction current on the size of the coil, because for such a conclusion it is necessary to change the size of the coil.

2) When a magnet is introduced into a coil, a current arises in it; therefore, we can conclude that when the magnetic flux passing through the coil changes, an electric (induction) current arises in the coil.

3) From the figure it can be seen that at a higher speed of introducing the magnet into the coil, the current strength through the coil increases, that is, the magnitude of the induction current depends on the rate of change of the magnetic field.

4) Based on this experience, it is impossible to draw a conclusion about the dependence of the direction of the induction current on the nature of the change in the magnetic flux.

5) Based on this experiment, it is impossible to draw a conclusion about the dependence of the direction of the induction current on the direction of the magnetic lines piercing the coil.

Answer: 23.

Answer: 23|32

Using two coils, one of which is connected to a current source, and the other is connected to an ammeter, the student studied the phenomenon of electromagnetic induction. Figure A shows the experimental diagram, and Figure B shows the ammeter readings for the moment of closing the circuit with coil 1 (Figure 1), for a steady-state direct current flowing through coil 1 (Figure 2), and for the moment of opening the circuit with coil 1 (Fig. 3).

From the list provided, select two statements that correspond to experimental observations. Indicate their numbers.

1) In coil 1, electric current flows only at the moment of closing and opening the circuit.

2) The direction of the induction current depends on the rate of change of the magnetic flux passing through coil 2.

3) When the magnetic field created by coil 1 changes, an induced current appears in coil 2.

4) The direction of the induction current in coil 2 depends on whether the electric current in coil 1 increases or decreases.

5) The magnitude of the induction current depends on the magnetic properties of the medium.

Solution.

1) Coil 1 is connected to a current source and current flows in it only when the circuit is closed.

Option 1

Task. a and b A) b).

Happening b

∆Ф›0

′ induced current

IN ′

Lenz's RULE . Solve the problem using the example

Option 2

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE . Solve the problem using the example

Option 3

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE . Solve the problem using the example

Option 4

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE . Solve the problem using the example

Option 5

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE . Solve the problem using the example

Option 6

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE. Solve the problem using the example

Option 7

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)

Lenz's RULE . Solve the problem using the example

Option 8

Task. Determine the direction of the induction current for the cases shown in the figuresa and b . Follow the progress of the decision in caseA) and solve it yourself for the caseb).

Happening b

Determine the direction of the induction vector B of the external field

Find the change in magnetic flux ∆Ф

∆Ф›0

Determination of induction vector B ′ induced current

IN ′

Find the direction of the induction current (using the gimlet or right hand rule)