What is the direction of the current in the skein. Laboratory work in physics: "Study of the phenomenon of electromagnetic induction." Directions of deflection of the milliampere meter needle

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Laboratory work № 9

Study of the phenomenon electromagnetic induction

Purpose of the work: study the conditions of occurrence induced current, Induction emf.

Equipment: coil, two strip magnets, milliammeter.

Theory

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

Numerous Faraday experiments show that with the help magnetic field can be obtained electric current in Explorer.

The phenomenon of electromagnetic inductionconsists in the occurrence of electric current in closed loop when the magnetic flux passing through the circuit changes.

The current arising from the phenomenon of electromagnetic induction is called induction.

IN electrical circuit(Figure 1) an induced 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 induced current if there is no relative movement of the coil and magnet.

Figure 1.

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

Figure 2.

Faraday experimentally established that when the magnetic flux changes in the conducting circuit, an induced emf E ind occurs, equal to speed changes in magnetic flux through a surface bounded by a contour taken with a minus sign:

This formula expresses Faraday's law:e. d.s. induction is equal to the rate of change of 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 loop when the magnetic flux changes is always directed in such a way that the magnetic field it creates prevents the change in the magnetic flux causing the induced 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.

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

Lenz's rule has 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 in such a way 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 in such a way that its magnetic field supports this decreasing magnetic field.

The induced emf depends on various reasons. If pushed into the reel 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 moves quickly. In each of the experiments carried out in this work, the direction of the induction current is determined by Lenz’s rule. The procedure for determining the direction of the induction current is shown in Figure 2.

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

According to Lenz's rule, the induced electric current in a conductor, arising 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 force lines of the permanent magnet, because the magnet moves towards the coil. We find the direction of the current using the gimlet rule: if a gimlet (with a right-hand 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 induced current will coincide in direction with the field lines of the permanent magnet, and the current will flow from right to left.

Work progress.

Prepare a table for the report and fill it out as you conduct experiments.

Purpose 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 is either at rest in a time-varying magnetic field 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 over time, since it is created by a moving (freely) magnet. According to Lenz's rule, the induced current arising in a closed loop with its magnetic field counteracts the change in the magnetic flux that causes it. In this case, we can observe this by the deflection of the milliammeter needle.
Equipment: Milliammeter, power supply, coils with cores, arc-shaped 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 induced current occurred if:

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

3. Find out how the magnetic flux F passing through the coil changed in each case. Draw a conclusion about the condition under which an induced current appeared in the coil.
II. Studying the direction of induction current.

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

1. Approach the magnet to the stationary coil slowly and at a faster speed, noting how many divisions (N 1, N 2) the milliammeter needle deflects.

2. Bring the magnet closer to the coil with its north pole. Note how many divisions N 1 The milliammeter needle deflects.

Attach the north pole of a strip magnet to the north pole of the arc-shaped magnet. Find out how many divisions N 2, the milliammeter needle deflects when two magnets approach simultaneously.

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

Answer the questions:

1.To a coil of copper wire first quickly, then slowly push in the magnet. Is it the same electric charge is it transferred through the cross-section of the coil wire?
2. Will an induction current appear in the rubber ring when a magnet is inserted into it?

You already know that there is always a magnetic field around an electric current. Electric current and magnetic field are inseparable from each other.

But if an electric current is said to “create” a magnetic field, isn’t there the opposite phenomenon? Is it possible to “create” an electric current using a magnetic field?

Such a task in early XIX V. Many scientists have tried to solve it. The English scientist Michael Faraday also put it before himself. “Convert magnetism into electricity” - this is how Faraday wrote this problem in his diary in 1822. It took the scientist almost 10 years of hard work to solve it.

Michael Faraday (1791-1867)
English physicist. Discovered the phenomenon of electromagnetic induction, extra currents during closing and opening

To understand how Faraday managed to “transform magnetism into electricity,” let’s perform some of Faraday’s experiments using modern instruments.

Figure 119, a shows that if a magnet is moved into a coil closed to a galvanometer, the galvanometer needle is deflected, indicating the appearance of an inductive (induced) current in the coil circuit. The induced current in a conductor is the same ordered movement of electrons as the current received from a galvanic cell or battery. The name “induction” only indicates the reason for its occurrence.

Rice. 119. The occurrence of induction current when a magnet and coil move relative to each other

When the magnet is removed from the coil, a deflection of the galvanometer needle is again observed, but in the opposite direction, which indicates the occurrence of a current in the coil in the opposite direction.

As soon as the movement of the magnet relative to the coil stops, the current stops. Consequently, current in the coil circuit exists only while the magnet is moving relative to the coil.

Experience can be changed. We will put a coil on a stationary magnet and remove it (Fig. 119, b). And again you can find that as the coil moves relative to the magnet, current appears again in the circuit.

Figure 120 shows coil A connected to the current source circuit. This coil is inserted into another coil C connected to the galvanometer. When the circuit of coil A is closed and opened, an induced current appears in coil C.

Rice. 120. The occurrence of induction current when closing and opening an electrical circuit

You can cause the appearance of an induction current in coil C by changing the current strength in coil A or by moving these coils relative to each other.

Let's do one more experiment. Let's place a flat contour of a conductor in a magnetic field, the ends of which will be connected to a galvanometer (Fig. 121, a). When the circuit is rotated, the galvanometer notes the appearance of an induction current in it. A current will also appear if a magnet is rotated near the circuit or inside it (Fig. 121, b).

Rice. 121. When a circuit rotates in a magnetic field (magnet relative to the circuit), a change in magnetic flux leads to the appearance of an induced current

In all the experiments considered, the induced current arose when the magnetic flux piercing the area covered by the conductor changed.

In the cases shown in Figures 119 and 120, the magnetic flux changed due to a change in the magnetic field induction. Indeed, when the magnet and the coil moved relative to each other (see Fig. 119), the coil fell into field areas with greater or lesser magnetic induction (since the magnet’s field is non-uniform). When the circuit of coil A (see Fig. 120) was closed and opened, the induction of the magnetic field created by this coil changed due to a change in the current strength in it.

When a wire loop was rotated in a magnetic field (see Fig. 121, a) or a magnet relative to the loop (see Fig. 121, b"), the magnetic flux changed due to a change in the orientation of this loop relative to the lines of magnetic induction.

Thus,

• with any change in the magnetic flux penetrating the area limited by a closed conductor, an electric current arises in this conductor, existing throughout the entire process of changing the magnetic flux

This is the phenomenon of electromagnetic induction.

The discovery of electromagnetic induction is one of the most remarkable scientific achievements of the first half of the 19th century V. It caused the emergence and rapid development of electrical engineering and radio engineering.

Powerful generators were created based on the phenomenon of electromagnetic induction electrical energy, in the development of which scientists and technicians took part different countries. Among them were our compatriots: Emilius Khristianovich Lenz, Boris Semenovich Jacobi, Mikhail Iosifovich Dolivo-Dobrovolsky and others, who made a great contribution to the development of electrical engineering.

Questions

1. What was the purpose of the experiments depicted in Figures 119-121? How were they carried out?
2. Under what condition in the experiments (see Fig. 119, 120) did an induced current arise in a coil closed to a galvanometer?
3. What is the phenomenon of electromagnetic induction?
4. What is the importance of the discovery of the phenomenon of electromagnetic induction?

Exercise 36

1. How to create a short-term induction current in the K 2 coil shown in Figure 118?
2. The wire ring is placed in a uniform magnetic field (Fig. 122). The arrows shown next to the ring show that in cases a and b the ring moves rectilinearly along the lines of induction of the magnetic field, and in cases c, d and e it rotates around the axis OO." In which of these cases can an induced current arise in the ring ?

Purpose of the work: To study the phenomenon of electromagnetic induction.
Equipment: Milliammeter, coil-coil, arc-shaped magnet, power source, coil with an iron core from a dismountable electromagnet, rheostat, key, connecting wires, model of electric current generator (one per class).
Directions for work:
1. Connect the coil coil to the clamps of the milliammeter.
2. Observing the readings of the milliammeter, bring one of the poles of the magnet to the coil, then stop the magnet for a few seconds, and then bring it closer to the coil again, pushing it into it (Fig. 196). Record whether an induced current arose in the coil while the magnet was moving relative to the coil; while it is stopped.

Write down whether the magnetic flux F passing through the coil changed while the magnet was moving; while it is stopped.
4. Based on your answers to the previous question, draw and write down a conclusion about the condition under which an induced current appeared in the coil.
5. Why did the magnetic flux passing through this coil change when the magnet approached the coil? (To answer this question, remember, firstly, on what values ​​​​the magnetic flux Ф depends and, secondly, is the same
whether the magnitude of the induction vector B of the magnetic field of a permanent magnet is near this magnet and far from it.)
6. The direction of the current in the coil can be judged by the direction in which the milliammeter needle deviates from the zero division.
Check whether the direction of the induction current in the coil will be the same or different when the same magnet pole approaches and moves away from it.

4. Approach the magnet pole to the coil at such a speed that the milliammeter needle deviates by no more than half the limit value of its scale.
Repeat the same experiment, but at a higher speed of the magnet than in the first case.
At a higher or lower speed of movement of the magnet relative to the coil, did the magnetic flux F passing through this coil change faster?
When the magnetic flux through the coil changed rapidly or slowly, was the current in it greater?
Based on your answer to the last question, draw and write down a conclusion about how the modulus of the strength of the induction current arising in the coil depends on the rate of change of the magnetic flux F passing through this coil.
5. Assemble the setup for the experiment according to Figure 197.
6. Check whether an induced current occurs in coil 1 in the following cases:
a) when closing and opening the circuit in which coil 2 is connected;
b) when direct current flows through coil 2;
c) by increasing and decreasing the current flowing through coil 2 by moving the rheostat slider to the corresponding side.
10. In which of the cases listed in paragraph 9 does the magnetic flux passing through coil 1 change? Why is it changing?
11. Observe the occurrence of electric current in the generator model (Fig. 198). Explain why an induced current appears in a frame rotating in a magnetic field.
Rice. 196

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. By using necessary equipment We will conduct laboratory work, at the end of which we will learn how to correctly study and define this phenomenon.

Purpose - study electromagnetic induction phenomena.

Equipment:

1. Milliammeter.

2. Magnet.

3. Reel-skein.

4. Current source.

5. Rheostat.

6. Key.

7. Coil from an electromagnet.

8. Connecting wires.

Rice. 1. Experimental equipment

Let's start the laboratory work by assembling the setup. To assemble the circuit that we will use in laboratory work, we will connect a skein-coil to a milliammeter and use a magnet, which we will move closer or further from the coil. At the same time, we must remember what will happen when the induced current appears.

Rice. 2. Experiment 1

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

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

Note that the magnetic induction lines leave the north pole and enter the south pole. Moreover, the number of these lines and their density are different in different parts of the magnet. Please 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 that penetrates the area limited by the turns of this coil changes.

The next stage of our study of electromagnetic induction is related to the determination direction of induction current. We can judge the direction of the induction current by the direction in which the milliammeter needle deviates. Let's use an arc-shaped magnet and see that when the magnet approaches, the arrow will deviate in one direction. If the magnet is now moved in the other direction, the arrow will deflect in the other direction. As a result of the experiment, we can say that the direction of the magnet’s movement also determines the direction of the induction current. Let us 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 will be related to another experiment. Let's look at the design 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 induction current. The same milliammeter attached to a coil of 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 explore the presence induced current when closing and opening the circuit. So, the first part of the experiment: we close the key. Please note that the current is increasing in the circuit, the arrow has deviated in one direction, but note that now the key is closed, and the milliammeter does not show any electric current. The fact is that there is no change in the magnetic flux, we have already talked about this. If you now open the key, the milliammeter will show that the direction of the current has changed.

In the second experiment we will trace how induced current when the electric current in the second circuit changes.

The next part of the experiment will be to observe how the induction current will change if the magnitude of the current in the circuit is changed by means of a 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 the sliding contact of the rheostat moves, the magnetic field changes, which leads to the appearance of an induction current.

To conclude the lab, we need to look at how an induced 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 you now rotate the wheel of this generator, an inductive electric current will be induced in the coil winding. The experiment shows that an increase in the number of revolutions leads to the fact that the light bulb begins to burn brighter.

List of additional literature:

Aksenovich L. A. Physics in high school: Theory. Assignments. Tests: Textbook. benefits 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. 347-348. Myakishev G.Ya. Physics: Electrodynamics. 10-11 grades. Textbook for advanced study of physics / G.Ya. Myakishev, A.3. Sinyakov, V.A. Slobodskov. - M.: Bustard, 2005. - 476 p. Purysheva N.S. Physics. 9th grade. Textbook. / Purysheva N.S., Vazheevskaya N.E., Charugin V.M. 2nd ed., stereotype. - M.: Bustard, 2007.