Why does the magnetic flux penetrating the coil change 1. The phenomenon of electromagnetic induction. Instructions for performing laboratory work

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Lab #9

Phenomenon study electromagnetic induction

Goal of the work: to study the conditions for the occurrence of induction current, induction EMF.

Equipment: coil, two bar magnets, milliammeter.

Theory

The mutual connection of electric and magnetic fields was established by the outstanding English physicist M. Faraday in 1831. He discovered the phenomenon electromagnetic induction.

Numerous experiments by Faraday show that with the help of magnetic field you can get an electric current in the conductor.

The phenomenon of electromagnetic inductionlies in the occurrence electric current in a closed circuit with a change in the magnetic flux penetrating the circuit.

The current that occurs during the phenomenon of electromagnetic induction is called induction.

IN electrical circuit(Figure 1) an induction current occurs if there is movement of the magnet relative to the coil, or vice versa. The direction of the induction current depends both on the direction of movement of the magnet and on the location of its poles. There is no induction current if there is no relative movement of the coil and magnet.

Picture 1.

Strictly speaking, when the circuit moves in a magnetic field, not a certain current is generated, but a certain e. d.s.

Figure 2.

Faraday experimentally found that when the magnetic flux changes in the conducting circuit, an EMF of induction E ind arises, equal to the rate of change of the magnetic flux through the surface bounded by the circuit, taken with a minus sign:

This formula expresses Faraday's law:e. d.s. induction is equal to the rate of change of the magnetic flux through the surface bounded by the contour.

The minus sign in the formula reflects Lenz's rule.

In 1833, Lenz experimentally proved a statement called Lenz's rule: the induction current excited in a closed circuit when the magnetic flux changes is always directed so that the magnetic field it creates prevents a change in the magnetic flux that causes the induction current.

With increasing magnetic fluxФ>0, and ε ind< 0, т.е. э. д. с. индукции вызывает ток такого направления, при котором его маг­нитное поле уменьшает магнитный поток через контур.

With decreasing magnetic flux F<0, а ε инд >0, i.e. the magnetic field of the inductive current increases the decreasing magnetic flux through the circuit.

Lenz's rule has a deep physical meaning – it expresses the law of conservation of energy: if the magnetic field through the circuit increases, then the current in the circuit is directed so that its magnetic field is directed against the external one, and if the external magnetic field through the circuit decreases, then the current is directed so that its magnetic field supports this decreasing magnetic field.

The induction emf depends on various reasons. If you push into the coil once strong magnet, and in the other - weak, then the readings of the device in the first case will be higher. They will also be higher when the magnet is moving fast. In each of the experiments carried out in this work, the direction of the induction current is determined by the Lenz rule. The procedure for determining the direction of the induction current is shown in Figure 2.

In the figure, the lines of force of the magnetic field of the permanent magnet and the lines of the magnetic field of the induction current are indicated in blue. The magnetic field lines are always directed from N to S - from north pole to the south pole of the magnet.

According to Lenz's rule, the inductive electric current in the conductor, which occurs when the magnetic flux changes, is directed in such a way that its magnetic field counteracts the change in the magnetic flux. Therefore, in the coil, the direction of the magnetic field lines is opposite to the lines of force of the permanent magnet, because the magnet moves towards the coil. We find the direction of the current according to the rule of the gimlet: if the gimlet (with the right thread) is screwed in so that its translational movement coincides with the direction of the induction lines in the coil, then the direction of rotation of the gimlet handle coincides with the direction of the induction current.

Therefore, the current through the milliammeter flows from left to right, as shown in Figure 1 by the red arrow. In the case when the magnet moves away from the coil, the magnetic field lines of the inductive current will coincide in direction with the lines of force of the permanent magnet, and the current will flow from right to left.

Progress.

Prepare a table for the report and fill it in as the experiments are carried out.

Lesson plan

Lesson topic: Laboratory work: "Studying the phenomenon of electromagnetic induction"

Type of occupation - mixed.

Lesson type combined.

Learning objectives of the lesson: to study the phenomenon of electromagnetic induction

Lesson objectives:

Educational:study the phenomenon of electromagnetic induction

Developing. To develop the ability to observe, form an idea of ​​the process of scientific knowledge.

Educational. Develop cognitive interest to the subject, to develop the ability to listen and be heard.

Planned educational outcomes: to contribute to strengthening the practical orientation in teaching physics, the formation of skills to apply the acquired knowledge in various situations.

Personality: with contribute to the emotional perception of physical objects, the ability to listen, clearly and accurately express their thoughts, develop initiative and activity in solving physical problems, form the ability to work in groups.

Metasubject: pdevelop the ability to understand and use visual aids (drawings, models, diagrams). Development of an understanding of the essence of algorithmic prescriptions and the ability to act in accordance with the proposed algorithm.

subject: about know the physical language, the ability to recognize parallel and serial connections, the ability to navigate in an electrical circuit, to assemble circuits. Ability to generalize and draw conclusions.

Lesson progress:

1. Organization of the beginning of the lesson (marking absentees, checking students' readiness for the lesson, answering students' questions on homework) - 2-5 min.

The teacher tells the students the topic of the lesson, formulates the objectives of the lesson and introduces the students to the lesson plan. Students write the topic of the lesson in their notebooks. The teacher creates conditions for the motivation of learning activities.

Mastering new material:

Theory. The phenomenon of electromagnetic inductionconsists in the occurrence of an electric current in a conducting circuit, which either rests in an alternating magnetic field, or moves in a constant magnetic field in such a way that the number of magnetic induction lines penetrating the circuit changes.

The magnetic field at each point in space is characterized by the magnetic induction vector B. Let a closed conductor (circuit) be placed in a uniform magnetic field (see Fig. 1.)

Picture 1.

Normal to the plane of the conductor makes an anglewith the direction of the magnetic induction vector.

magnetic fluxФ through a surface with an area S is called a value equal to the product of the modulus of the magnetic induction vector B and the area S and the cosine of the anglebetween vectors And .

Ф=В S cos α (1)

The direction of the inductive current that occurs in a closed circuit when the magnetic flux through it changes is determined by Lenz's rule: the inductive current arising in a closed circuit counteracts with its magnetic field the change in the magnetic flux by which it is caused.

Apply Lenz's rule as follows:

1. Set the direction of the lines of magnetic induction B of the external magnetic field.

2. Find out if the magnetic induction flux of this field increases through the surface bounded by the contour ( F 0), or decreases ( F 0).

3. Set the direction of the lines of magnetic induction B "magnetic field

inductive current Iusing the gimlet rule.

When the magnetic flux changes through the surface bounded by the contour, external forces appear in the latter, the action of which is characterized by the EMF, called EMF of induction.

According to the law of electromagnetic induction, the EMF of induction in a closed loop is equal in absolute value to the rate of change of the magnetic flux through the surface bounded by the loop:

Devices and equipment:galvanometer, power supply, core coils, arched magnet, key, connecting wires, rheostat.

Work order:

1. Obtaining an induction current. For this you need:

1.1. Using Figure 1.1., assemble a circuit consisting of 2 coils, one of which is connected to a DC source through a rheostat and a key, and the second, located above the first, is connected to a sensitive galvanometer. (see fig. 1.1.)

Figure 1.1.

1.2. Close and open the circuit.

1.3. Make sure that the induction current occurs in one of the coils at the moment of closing the electrical circuit of the coil, which is stationary relative to the first, while observing the direction of deviation of the galvanometer needle.

1.4. Set in motion a coil connected to a galvanometer relative to a coil connected to a direct current source.

1.5. Make sure that the galvanometer detects the occurrence of an electric current in the second coil with any movement of it, while the direction of the arrow of the galvanometer will change.

1.6. Perform an experiment with a coil connected to a galvanometer (see Fig. 1.2.)

Figure 1.2.

1.7. Make sure that the induction current occurs when the permanent magnet moves relative to the coil.

1.8. Make a conclusion about the cause of the induction current in the experiments performed.

2. Checking the fulfillment of the Lenz rule.

2.1. Repeat the experiment from paragraph 1.6. (Fig. 1.2.)

2.2. For each of the 4 cases of this experiment, draw diagrams (4 diagrams).

Figure 2.3.

2.3. Check the fulfillment of the Lenz rule in each case and fill in Table 2.1 according to these data.

Table 2.1.

3. Make a conclusion about the laboratory work done.

4. Answer security questions.

Control questions:

1. How should a closed circuit move in a uniform magnetic field, translationally or rotationally, so that an inductive current arises in it?

2. Explain why the inductive current in the circuit has such a direction that its magnetic field prevents a change in the magnetic flux of its cause?

3. Why is there a "-" sign in the law of electromagnetic induction?

4. A magnetized steel bar falls through a magnetized ring along its axis, the axis of which is perpendicular to the plane of the ring. How will the current in the ring change?

Admission to laboratory work 11

1. What is the name of the power characteristic of the magnetic field? Its graphic meaning.

2. How is the modulus of the magnetic induction vector determined?

3. Give the definition of the unit of measurement of the magnetic field induction.

4. How is the direction of the magnetic induction vector determined?

5. Formulate the gimlet rule.

6. Write down the formula for calculating the magnetic flux. What is its graphic meaning?

7. Define the unit of measure for magnetic flux.

8. What is the phenomenon of electromagnetic induction?

9. What is the reason for the separation of charges in a conductor moving in a magnetic field?

10. What is the reason for the separation of charges in a stationary conductor in an alternating magnetic field?

11. Formulate the law of electromagnetic induction. Write down the formula.

12. Formulate Lenz's rule.

13. Explain Lenz's rule based on the law of conservation of energy.

In this lesson, we will conduct laboratory work No. 4 "Studying the phenomenon of electromagnetic induction." The purpose of this lesson will be to study the phenomenon of electromagnetic induction. Using the necessary equipment, we will conduct laboratory work, at the end of which we will learn how to properly study and determine this phenomenon.

The goal is to study phenomena of electromagnetic induction.

Equipment:

1. Milliammeter.

2. Magnet.

3. Coil-coil.

4. Current source.

5. Rheostat.

6. Key.

7. Coil from an electromagnet.

8. Connecting wires.

Rice. 1. Experimental equipment

Let's start the lab by collecting the setup. To assemble the circuit that we will use in the lab, we will attach a coil to a milliammeter and use a magnet that we will move closer or further away from the coil. At the same time, we must remember what will happen when the induction current appears.

Rice. 2. Experiment 1

Think about how to explain the phenomenon we are observing. How does the magnetic flux affect what we see, in particular the origin of the electric current. To do this, look at the auxiliary figure.

Rice. 3. Magnetic field lines of a permanent bar magnet

Please note that the lines of magnetic induction come out of the north pole, enter into South Pole. At the same time, the number of these lines, their density is different in different parts of the magnet. Note that the direction of the magnetic field also changes from point to point. Therefore, we can say that a change in the magnetic flux leads to the fact that an electric current arises in a closed conductor, but only when the magnet moves, therefore, the magnetic flux penetrating the area limited by the turns of this coil changes.

The next stage of our study of electromagnetic induction is connected with the definition direction of induction current. We can judge the direction of the induction current by the direction in which the arrow of the milliammeter deviates. Let's use an arcuate magnet and we will see that when the magnet approaches, the arrow will deviate in one direction. If now the magnet is moved in the other direction, the arrow will deviate in the other direction. As a result of the experiment, we can say that the direction of the induction current also depends on the direction of movement of the magnet. We also note that the direction of the induction current also depends on the pole of the magnet.

Please note that the magnitude of the induction current depends on the speed of movement of the magnet, and at the same time on the rate of change of the magnetic flux.

The second part of our laboratory work associated with another experiment. Let's look at the scheme of this experiment and discuss what we will do now.

Rice. 4. Experiment 2

In the second circuit, in principle, nothing has changed regarding the measurement of the inductive current. The same milliammeter attached to the coil. Everything remains as it was in the first case. But now we will get a change in the magnetic flux not due to the movement of a permanent magnet, but due to a change in the current strength in the second coil.

In the first part, we will investigate the presence induction current when closing and opening the circuit. So, the first part of the experiment: we close the key. Pay attention, the current increases in the circuit, the arrow deviated to one side, but pay attention, now the key is closed, and the milliammeter does not show electric current. The fact is that there is no change in the magnetic flux, we have already talked about this. If the key is now opened, the milliammeter will show that the direction of the current has changed.

In the second experiment, we will see how induction current when the electric current in the second circuit changes.

The next part of the experiment will be to follow how the induction current will change if the current in the circuit is changed due to the rheostat. You know that if we change electrical resistance in the circuit, then, following Ohm's law, the electric current will also change. As the electric current changes, the magnetic field will change. At the moment of moving the sliding contact of the rheostat, the magnetic field changes, which leads to the appearance of an induction current.

To conclude the lab, we should look at how an inductive electric current is created in an electric current generator.

Rice. 5. Electric current generator

Its main part is a magnet, and inside these magnets there is a coil with a certain number of wound turns. If we now rotate the wheel of this generator, an induction electric current will be induced in the coil winding. From the experiment it can be seen that an increase in the number of revolutions leads to the fact that the bulb starts to burn brighter.

List of additional literature:

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Proc. allowance for institutions providing general. environments, education / L.A. Aksenovich, N.N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsy i vykhavanne, 2004. - C. 347-348. Myakishev G.Ya. Physics: Electrodynamics. 10-11 grades. Textbook for in-depth study of physics / G.Ya. Myakishev, A.3. Sinyakov, V.A. Slobodskov. - M.: Bustard, 2005. - 476 p. Purysheva N.S. Physics. Grade 9 Textbook. / Purysheva N.S., Vazheevskaya N.E., Charugin V.M. 2nd ed., stereotype. - M.: Bustard, 2007.

Goal of the work: experimental study of the phenomenon of magnetic induction verification of Lenz's rule.
Theoretical part: The phenomenon of electromagnetic induction consists in the occurrence of an electric current in a conducting circuit, which either rests in a magnetic field that changes in time, or moves in a constant magnetic field in such a way that the number of magnetic induction lines penetrating the circuit changes. In our case, it would be more reasonable to change the magnetic field in time, since it is created by a moving (freely) magnet. According to Lenz's rule, the inductive current that occurs in a closed circuit counteracts with its magnetic field the change in the magnetic flux by which it is caused. In this case, we can observe this by the deviation of the milliammeter needle.
Equipment: Milliammeter, power supply, coils with cores, arcuate magnet, push-button switch, connecting wires, magnetic needle (compass), rheostat.

Work order

1. Connect the coil-coil to the clamps of the milliammeter.
2. Observing the readings of the milliammeter, note whether an induction current occurred if:

* insert a magnet into the fixed coil,
* remove the magnet from the fixed coil,
* place the magnet inside the coil, leaving it motionless.

3. Find out how the magnetic flux Ф, penetrating the coil, changed in each case. Make a conclusion about the condition under which the inductive current appeared in the coil.
II. Study of the direction of the induction current.

1. The direction of the current in the coil can be judged by the direction in which the milliammeter needle deviates from zero division.
Check whether the direction of the induction current will be the same if:
* insert into the coil and remove the magnet with the north pole;
* insert the magnet into the magnet coil with the north pole and the south pole.
2. Find out what changed in each case. Make a conclusion about what determines the direction of the induction current. III. The study of the magnitude of the induction current.

1. Move the magnet closer to the fixed coil slowly and with greater speed, noting how many divisions (N 1 , N 2 ) the arrow of the milliammeter deviates.

2. Bring the magnet closer to the coil with the north pole. Note how many divisions N 1 the needle of the milliammeter deviates.

Attach the north pole of the bar magnet to the north pole of the arcuate magnet. Find out how many divisions N 2, the arrow of the milliammeter deviates when two magnets approach simultaneously.

3. Find out how the magnetic flux changed in each case. Make a conclusion on what the magnitude of the induction current depends.

Answer the questions:

1. First quickly, then slowly push the magnet into the coil of copper wire. Is it the same electric charge at the same time is transferred through the section of the wire of the coil?
2. Will there be an induction current in the rubber ring when a magnet is introduced into it?

The student must:

be able to: handle physical instruments and use them in laboratory work; to investigate the phenomenon of electromagnetic induction - to determine what the magnitude and direction of the induction current depend on; enjoy the necessary reference literature;

know: methods for measuring the power consumed by an electrical appliance; the dependence of the power consumed by the light bulb on the voltage at its terminals; investigate the dependence of the conductor resistance on temperature.

Security of the lesson

Equipment and tools: milliammeter, coil-coil, arcuate magnet, strip magnet, DC power supply, two coils with cores, rheostat, key, long wire, connecting wires.

Handouts:

Brief theoretical materials on the topic of laboratory work

Induction current in a closed loop occurs when the magnetic flux changes through the area bounded by the loop. Changing the magnetic flux through the circuit can be done in two different ways:

1) change in time of the magnetic field in which the fixed circuit is located when the magnet is pushed into the coil or when it is pulled out;

2) the movement of this circuit (or its parts) in a constant magnetic field (for example, when putting a coil on a magnet).

Instructions for performing laboratory work

Connect the coil-coil to the clamps of the milliammeter, and then put it on and take it off the north pole of the arcuate magnet at different speeds (see figure), and for each case note the maximum and minimum strength of the induction current and the direction of deviation of the device arrow.

Figure 9.1

1. Turn the magnet over and slowly push the south pole of the magnet into the coil and then pull it out. Repeat the experiment at a faster rate. Pay attention to where the needle of the milliammeter deviated this time.

2. Fold two magnets (stripe and arcuate) with the same poles and repeat the experiment with different speed movement of the magnets in the coil.

3. Connect to the clamps of the milliammeter instead of the coil a long wire, folded into several turns. Putting on and taking off turns of wire from the pole of the arcuate magnet, note the maximum strength of the induction current. Compare it with the maximum strength of the induction current obtained in experiments with the same magnet and coil, and find the dependence of the induction emf on the length (number of turns) of the conductor.

4. Analyze your observations and draw conclusions regarding the reasons on which the magnitude of the induction current and its direction depend.

5. Assemble the circuit shown in Figure 1. The coils with the cores inserted into them should be located close to one another and so that their axes coincide.

6. Carry out the following experiments:

a) set the rheostat slider to the position corresponding to the minimum resistance of the rheostat. Close the circuit with a key, watching the milliammeter needle;

b) open the circuit with the key. What changed?

c) put the rheostat slider in the middle position. Repeat the experience;

d) set the slider of the rheostat to the position corresponding to the maximum resistance of the rheostat. Close and open the circuit with the key.

7. Analyze your observations and draw conclusions.

Lab #10

DEVICE AND OPERATION OF THE TRANSFORMER

The student must:

be able to: determine the transformation ratio; use the necessary reference literature;

know: device and principle of operation of the transformer.

Security of the lesson

Equipment and tools: regulated alternating voltage source, laboratory collapsible transformer, voltmeters alternating current(or avometer), key, connecting wires;

Handouts: data guidelines for laboratory work.