Self-induction, inductance. self-induction each conductor through which electric current flows is in its own magnetic field. Physics phenomenon of self-induction lesson phenomenon of self-induction






The manifestation of the phenomenon of self-induction Closing the circuit Opening the circuit When the circuit is closed, the current increases, which causes an increase in the magnetic flux in the coil, a vortex electric field arises, directed against the current, i.e. an EMF of self-induction occurs in the coil, which prevents the current from rising in the circuit (the vortex field slows down the electrons). As a result, L1 lights up later than L2. When the electric circuit is opened, the current decreases, there is a decrease in the m.flow in the coil, a vortex electric field appears, directed like a current (tending to maintain the same current strength), i.e. A self-inductive emf appears in the coil, which maintains the current in the circuit. As a result, L flashes brightly when turned off.




INDUCTIVITY What determines the EMF of self-induction? Electric current creates its own magnetic field. The magnetic flux through the circuit is proportional to the induction magnetic field(Ф ~ B), induction is proportional to the strength of the current in the conductor (B ~ I), therefore the magnetic flux is proportional to the strength of the current (Ф ~ I). The self-induction emf depends on the rate of change in the current strength in the electric circuit, on the properties of the conductor (size and shape) and on the relative magnetic permeability of the medium in which the conductor is located. A physical quantity showing the dependence of the self-induction EMF on the size and shape of the conductor and on the environment in which the conductor is located is called the self-induction coefficient or inductance.












ENERGY OF THE MAGNETIC FIELD OF THE CURRENT There is a magnetic field around a conductor with current, which has energy. Where does it come from? The current source included in the electric circuit has an energy reserve. At the moment of closing the electric circuit, the current source expends part of its energy to overcome the action of the emerging EMF of self-induction. This part of the energy, called the self-energy of the current, goes to the formation of a magnetic field. The energy of the magnetic field is equal to the self-energy of the current. The self-energy of the current is numerically equal to the work that the current source must do to overcome the self-induction EMF in order to create a current in the circuit.


The energy of the magnetic field created by the current is directly proportional to the square of the current strength. Where does the energy of the magnetic field disappear after the current stops? - stands out (when the circuit is opened with enough great strength current, sparks or arcs may occur)

According to Lenz's rule, an inductive current that occurs in a closed circuit always opposes the change in the external magnetic flux that caused its appearance. Today we will consider the case when the appearance electromagnetic induction due to a change in the strength of the current passing through the coil with a large number of turns. If the cause of induction current consists in an increase in current, then the induction current with its magnetic field will counteract this increase. You can verify this in the following experiment. Let's connect two bulbs in parallel, the current gets to the first bulb, passing through the rheostat, and to the second bulb, passing through the inductor, and the number of turns in this coil is quite large, and inside there is a core consisting of interconnected plates of transformer steel (magnetic field , which will arise around such a coil, is large). Lock the chain with the key. Both bulbs lit up, but the second bulb lit up with a visible delay. What is the reason for this phenomenon? When the key is closed, total strength current I, and the current in each branch of I1 and I2 begin to increase. And if there is an increase in the magnetic field around the conductors, then, in accordance with Lenz's rule, induction currents arise in the rheostat and the coil, which will prevent their action from further increasing the current strength in the circuit. Of course, the magnetic field that will develop around the current coil is stronger, so light bulb number two lights up later.
Please note that in the experiments that we considered earlier, the induction current in the circuit arose due to the influence of an external magnetic field. In our example, the induction current in the circuit arose due to a change in the current strength in the circuit. This phenomenon is called the phenomenon of self-induction. The phenomenon of self-induction is a phenomenon due to the occurrence of an inductive current in a conductor or coil, due to a change in the current in it. The resulting current is called the self-induction current. The bulb lit up later, passing through the coil, because. in the coil, the induction current is greater than in the rheostat (the coil has more turns and core). Therefore, they say that it has more inductance than a rheostat.
What is inductance? Inductance is a new physical quantity that can be used to evaluate the ability of a coil to resist a change in the current strength in it. Designate the inductance with the letter L (el). Units of inductance change in international system units (SI) - henry (H). The inductance of different coils will be different. It depends on the size and shape of the coil, the number of turns, the presence of the core and the material from which it is made. And of course, the more inductance the coil has, the more late the bulb will light up.
Let's carry out the second experiment, which will demonstrate the phenomenon of self-induction when the circuit is opened. In the circuit that we collected earlier, we will make some substitutions. We remove the first light bulb, and connect a neon light bulb in parallel to the coil, which we denote in the diagram as Ln (el with the index en). When the circuit is closed, we observe the burning of only one light bulb. The voltage on the current source is less than necessary for the burning of a neon light bulb (the voltage must be at least 80 volts). Let's open the circuit, the incandescent bulb goes out, and the neon bulb lights up with a short flash.
Why it happens? When the current in the circuit decreases, an induction current arises in the coil, with its magnetic field, which prevents the decrease in current in the circuit. Moreover, the resulting inductive current is so large that its voltage is sufficient to burn a neon light bulb, but it weakens very quickly.
Think and answer the question, in what case does the phenomenon of self-induction occur in the circuit?
A) when the current in the circuit decreases,
B) with increasing current in the circuit,
C) in both cases.
The phenomenon of self-induction occurs when passing through an alternating current coil (this can be an increase in current and a decrease).
When the circuit is closed, the inductive current
A) prevents an increase in current in the circuit,
B) contributes to an increase in current in the circuit,
C) does not affect the flow of current in the circuit.
When the key is closed, the resulting inductive current prevents the current from increasing in the circuit. Self-induction occurs in all conductors when the current in the circuit changes, however, it will be noticeable and have a significant effect on other elements in the circuit, only if a coil with a sufficiently large number of turns and a core is used.

1st semester

ELECTRODYNAMICS

3. Electromagnetic field

LESSON 9/36

Theme. Self-induction. Inductance

The purpose of the lesson: to expand students' understanding of the phenomenon of electromagnetic induction; explain the essence of the phenomenon of self-induction.

Type of lesson: lesson learning new material.

LESSON PLAN

Knowledge control

1. The phenomenon of electromagnetic induction.

2. The law of electromagnetic induction.

3. Lenz's rule.

Demonstrations

1. The phenomenon of self-induction during the opening and closing of the circle.

2. Using self-induction to light the fluorescent lamp.

3. Fragments of the video film "The phenomenon of self-induction".

Learning new material

1. Self-induction.

2. EMF of self-induction.

3. Inductance

Consolidation of the studied material

1. Qualitative questions.

2. Learning to solve problems.

STUDY NEW MATERIAL

First level

1. At what moment does the switch spark: in the case of closing or opening the circle?

2. When can one observe the phenomenon of self-induction in a circuit direct current?

3. Why is it impossible to instantly change the current strength in a closed circuit?

Second level

1. How does the value of the modulus of the magnetic induction vector depend on the current strength?

2. Experiments show that the inductance of the coil increases in accordance with the increase in the number of turns in the coil. How can this fact be explained?

CONFIGURATION OF THE STUDYED MATERIAL

) . Qualitative questions

1. Why does sparking occur when the tram arc breaks from the overhead wire?

2. An open-core electromagnet is connected to a DC circuit. When the armature closes the core, there is a short-term decrease in the current strength in the circuit. Why?

3. Why are powerful electric motors disconnected from the mains smoothly and slowly using a rheostat?

) . Learning to solve problems

1. A superconducting coil with an inductance of 5 H is closed to a current source with an EMF of 20 V and a very low internal resistance. Assuming that the current in the coil increases evenly, determine the time it takes for the current to reach 10 A.

Solutions. The current in the coil increases gradually due to the phenomenon of self-induction. Let's use Ohm's law for a complete circuit: where is the total EMF of the circuit, consisting of the EMF of the source and the EMF of self-induction: Then Ohm's law takes the form.

In this lesson, we will learn how and by whom the phenomenon of self-induction was discovered, we will consider an experiment with which we will demonstrate this phenomenon, we will determine that self-induction is a special case of electromagnetic induction. At the end of the lesson, enter physical quantity, showing the dependence of the EMF of self-induction on the size and shape of the conductor and on the environment in which the conductor is located, i.e. inductance.

Henry invented flat copper strip coils, with which he achieved force effects that were more pronounced than with wire solenoids. The scientist noticed that when a powerful coil is in the circuit, the current in this circuit reaches its maximum value much more slowly than without a coil.

Rice. 2. Scheme experimental setup D. Henry

On fig. 2 shows the electrical circuit of the experimental setup, on the basis of which it is possible to demonstrate the phenomenon of self-induction. Electrical circuit consists of two lamps connected in parallel, connected through a key to a direct current source. A coil is connected in series with one of the bulbs. After the circuit is closed, it can be seen that the light bulb, which is connected in series with the coil, lights up more slowly than the second light bulb (Fig. 3).

Rice. 3. Different incandescence of bulbs at the moment the circuit is turned on

When the source is turned off, the light bulb connected in series with the coil goes out more slowly than the second light bulb.

Why do the lights go out at the same time?

When the key is closed (Fig. 4), due to the occurrence of self-induction EMF, the current in the bulb with the coil increases more slowly, so this bulb lights up more slowly.

Rice. 4. Key lock

When the key is opened (Fig. 5), the emerging EMF of self-induction prevents the current from decreasing. Therefore, the current continues to flow for some time. For the existence of current, a closed circuit is needed. There is such a circuit in the circuit, it contains both light bulbs. Therefore, when the circuit is opened, the bulbs should glow the same for some time, and the observed delay may be due to other reasons.

Rice. 5. Opening the key

Consider the processes occurring in this circuit when the key is closed and opened.

1. Closing the key.

There is a conductive loop in the circuit. Let the current in this coil flow counterclockwise. Then the magnetic field will be directed upwards (Fig. 6).

Thus, the coil is in the space of its own magnetic field. With an increase in current, the coil will be in the space of a changing magnetic field of its own current. If the current increases, then the magnetic flux created by this current also increases. As is known, with an increase in the magnetic flux penetrating the plane of the circuit, a electromotive force induction and, as a consequence, the induction current. According to Lenz's rule, this current will be directed in such a way that its magnetic field prevents a change in the magnetic flux penetrating the circuit plane.

That is, for the one considered in Fig. 6 turns, the induction current must be directed clockwise (Fig. 7), thereby preventing the increase in the own current of the turn. Consequently, when the key is closed, the current in the circuit does not increase instantly due to the fact that a braking induction current appears in this circuit, directed in the opposite direction.

2. Opening the key

When the key is opened, the current in the circuit decreases, which leads to a decrease in the magnetic flux through the plane of the coil. A decrease in the magnetic flux leads to the appearance of an EMF of induction and an induction current. In this case, the induction current is directed in the same direction as the loop's own current. This leads to a slower decrease in the intrinsic current.

Conclusion: when the current in the conductor changes, electromagnetic induction occurs in the same conductor, which generates an induction current directed in such a way as to prevent any change in the intrinsic current in the conductor (Fig. 8). This is the essence of the phenomenon of self-induction. Self-induction is a special case of electromagnetic induction.

Rice. 8. Moment of switching on and off the circuit

Formula for finding magnetic induction direct conductor with current:

where - magnetic induction; - magnetic constant; - current strength; - distance from the conductor to the point.

The flux of magnetic induction through the site is equal to:

where is the surface area that is penetrated by the magnetic flux.

Thus, the flux of magnetic induction is proportional to the magnitude of the current in the conductor.

For a coil in which is the number of turns, and is the length, the magnetic field induction is determined by the following relationship:

The magnetic flux created by a coil with the number of turns N, is equal to:

Substituting the formula for the magnetic field induction into this expression, we obtain:

The ratio of the number of turns to the length of the coil is denoted by the number:

We get the final expression for the magnetic flux:

It can be seen from the relation obtained that the value of the flux depends on the magnitude of the current and on the geometry of the coil (radius, length, number of turns). A value equal to is called inductance:

The unit for inductance is the henry:

Therefore, the flux of magnetic induction caused by the current in the coil is:

Taking into account the formula for the EMF of induction, we obtain that the EMF of self-induction is equal to the product of the rate of change of current and inductance, taken with the “-” sign:

self induction- this is the phenomenon of the occurrence of electromagnetic induction in a conductor when the strength of the current flowing through this conductor changes.

Electromotive force of self-induction is directly proportional to the rate of change of the current flowing through the conductor, taken with a minus sign. The proportionality factor is called inductance, which depends on the geometric parameters of the conductor.

A conductor has an inductance equal to 1 H if, at a rate of change of current in the conductor equal to 1 A per second, an electromotive force of self-induction equal to 1 V arises in this conductor.

A person encounters the phenomenon of self-induction every day. Each time we turn on or off the light, we thereby close or open the circuit, while exciting induction currents. Sometimes these currents can reach such high values ​​that a spark jumps inside the switch, which we can see.

Bibliography

  1. Myakishev G.Ya. Physics: Proc. for 11 cells. general education institutions. - M.: Education, 2010.
  2. Kasyanov V.A. Physics. Grade 11: Proc. for general education institutions. - M.: Bustard, 2005.
  3. Gendenstein L.E., Dick Yu.I., Physics 11. - M .: Mnemosyne.
  1. Internet portal Myshared.ru ().
  2. Internet portal Physics.ru ().
  3. Internet portal Festival.1september.ru ().

Homework

  1. Questions at the end of paragraph 15 (p. 45) - Myakishev G.Ya. Physics 11 (see list of recommended reading)
  2. Which conductor has an inductance of 1 henry?

Physics lesson number 47 in grade 9.

Date of:

Topic: "Self-induction"

The purpose of the lesson:

  • The study of the essence of the phenomenon of self-induction; familiarity with the value of inductance, the formula for calculating the energy of a magnetic field, finding out physical sense this formula.
  • Development logical thinking, attention, skills to analyze the results of the experiment, to draw conclusions.
  • Education of a culture of mental work; interest in physics; formation of communicative qualities of a person.

Lesson type: combined.

Lesson form: mixed.

D/W:§ 49, 50.

During the classes

  1. Org. moment.
  2. Checking d / z.
  1. Oral survey.
  • The phenomenon of electromagnetic induction.
  • Methods of current induction.
  1. Individual work on cards.
  1. Explanation of new material.
  1. Additional material.

The direction of the induction current.

Questions to students to update previous knowledge:

  • Name two series of Faraday's experiments on the study of the phenomenon of electromagnetic induction (the appearance of an induction current in a coil when a magnet or coil with current is pushed in and out; the appearance of an induction current in one coil when the current changes in another by closing or opening a circuit or using a rheostat).
  • Does the direction of deviation of the galvanometer needle depend on the direction of movement of the magnet relative to the coil? (it depends: when the magnet approaches the coil, the arrow deviates in one direction, when the magnet is removed, in the other direction).
  • What is the difference (according to the readings of the galvanometer) induction current that occurs in the coil when the magnet approaches, from the current that occurs when the magnet is removed (at the same speed of the magnet)? (current direction is different).

Thus, when the magnet moves relative to the coil, the direction of deviation of the galvanometer needle (and, therefore, the direction of the current) can be different. Using the Lenz experiment, we formulate the rule for finding the direction of the induction current (video clip "Demonstration of the phenomenon of electromagnetic induction").

Explanation of Lenz's experiment: If you bring a magnet closer to a conducting ring, it will start to repel from the magnet. This repulsion can only be explained by the fact that an induction current arises in the ring, due to an increase in the magnetic flux through the ring, and the ring with the current interacts with the magnet.

Lenz's rule and the law of conservation of energy.

increases, then the direction of the induction current in the circuit is such that the magnetic induction vector of the field created by this current is directed opposite to the magnetic induction vector of the external magnetic field.

If the magnetic flux through the circuit decreases, then the direction of the induction current is such that the magnetic induction vector of the field created by this current co-directed the magnetic induction vector of the external field.

The formulation of Lenz's rule: the induction current has such a direction that the magnetic flux created by it always tends to compensate for the change in the magnetic flux that caused this current.

Lenz's rule is a consequence of the law of conservation of energy.

  1. The phenomenon of self-induction.
  • Before considering the phenomenon of self-induction, let us recall what the essence of the phenomenon of electromagnetic induction is - this is the occurrence of an induction current in a closed circuit when the magnetic flux penetrating this circuit changes. Consider one of the variants of Faraday's experiments: If the current strength is changed in a circuit containing a closed circuit (coil), then an induction current will also appear in the circuit itself. This current will also obey Lenz's rule.

Consider an experiment on closing a circuit containing a coil. When the circuit with the coil is closed, a certain value of the current strength is set only after some time.

  • Video fragment "Self-induction"
  • Definition of self-induction: SELF-INDUCTION - the occurrence of a vortex electric field in the conductive circuit when the current strength in it changes; a special case of electromagnetic induction.
    Due to self-induction, a closed circuit has "inertia": the current strength in the circuit containing the coil cannot be changed instantly.

3. Inductance.

Ф=LI

Units of inductance in the SI system: [L] = 1 = 1 H (henry).

  1. Application and accounting of self-induction in technology.

Due to the phenomenon of self-induction, when opening circuits containing coils with steel cores (electromagnets, motors, transformers), a significant self-induction EMF is created and sparking or even an arc discharge may occur. As homework I propose (optionally) to prepare a presentation on the topic “How to eliminate unwanted self-induction when the circuit is opened?”.

  1. Magnetic field energy
  1. Consolidation.
  1. Ex. 41 - orally.
  2. No. 830, 837 - at the board.
  3. No. 834 - in the workplace.
  1. Reflection.
  2. Summary of the lesson.
  3. D / s.

style="&6�#:.��I �E s New Roman""> Faraday experience.

Magnetic and electric fields connected to each other. Email current can generate a magnetic field. Can a magnetic field create an electric current? Many scientists tried to solve this problem in the early 19th century. But the first decisive contribution to the discovery of EM interactions was made by Michael Faraday.

“Turn magnetism into electricity,” Faraday wrote in his diary. 1821 And only 10 years later he was able to solve this problem. You and I will discover what Faraday could not discover for 10 years, in a few minutes. Faraday could not understand one thing: that only a moving magnet causes a current. A magnet at rest causes no current in it. What experiments did Faraday conduct? Let's repeat the experiments of Faraday, with the help of which he discovered the EMP phenomenon.

Demo: induction current generation (coil, milliammeter, permanent magnet)

Definition: Occurrence in a closed conductor electric current, caused by a change in the magnetic field is called the phenomenon of ELECTROMAGNETIC INDUCTION.

The resulting current is called - induction.

CONCLUSION: Induction current occurs only when the coil and magnet move relative. The direction of the induction current depends on the direction of the vector B of the external magnetic field.

  1. Methods for obtaining induction current.

The inductive current in a closed circuit appears only when the magnetic flux that passes through the area covered by the circuit changes.

Group work (using textbook, Internet)

1 group: 1 way (Fig. 127)

  1. Consolidation of new material.
  1. Ex. 39 (1.2) - orally;
  2. Ex. 40 (2) - orally.
  1. Reflection.
  2. Summary of the lesson.
  3. D / s.