Lesson topic: “The phenomenon of self-induction. Inductance

Plan - lesson summary

« Self-induction . AND inductance . Magnetic field energy current"

Completed by a 5th year student

FM-112 group

full-time education

physics and mathematics education

Kezhutina Olga Vladislavovna

Date: 09/23/16

Vladimir 2016

Lesson topic: Self-induction . AND inductance .

Class: "11b"

Lesson type : lesson in learning new knowledge.

Lesson type: lesson-lecture.

Target : form the idea that a change in current strength in a conductor creates a vortex wave, which can either accelerate or slow down moving electrons; form an idea of the energy possessed by an electric current in a conductor and the energy of the magnetic field created by the current.

Tasks:

Educational: Repeat students’ knowledge about the phenomenon of electromagnetic induction, deepen it; on this basis, study the phenomenon of self-induction. Teach to use the law of electromagnetic induction to explain phenomena.Introduce a formula for calculating the energy of the magnetic field of a current and the concept of an electromagnetic field.

Educational: To cultivate interest in the subject, hard work and the ability to carefully evaluate the answers of comrades, the ability to work collectively and in pairs.

Educational: Development of students’ physical thinking, expansion of students’ conceptual apparatus, formation of skills to analyze information, draw conclusions from observations and experiments.

Equipment:

During the classes:

Organizational stage.

11.20 – 11.21

Hello guys, sit down.

The students are getting ready for the lesson.

Updating knowledge.

11.22-11.28

Checking homework, if students have questions, we sort them out.

Frontal survey:

What field is called the vortex electric field?

What is the source of the vortex field?

What are Foucault currents? Give examples of their use.

What determines the induced emf that occurs in a conductor moving in a time-varying magnetic field?

Students check their homework and answer questions:

The field that generatestime-varying magnetic field.

Time-varying magnetic field.

Induction currents reaching large numerical values in massive conductors due to the fact that their resistance is low.

On the speed of movement of a conductor in a uniform magnetic field.

Sample guiding questions:

4.Remember the formula by which you can find the induced emf in moving conductors.

Motivational stage.

11.29-11.31

The foundations of electrodynamics were laid by Ampere in 1820. Ampere's work inspired many engineers to design various technical devices, such as an electric motor (designer B.S. Jacobi), a telegraph (S. Morse), an electromagnet, which was designed by the famous American scientist Henry.

Joseph Henry became famous thanks to the creation of a series of unique powerful electromagnets with a lifting force of 30 to 1500 kg with a dead weight of 10 kg of the magnet. Creating various electromagnets, in 1832 the scientist discovered a new phenomenon in electromagnetism - the phenomenon of self-induction. This lesson is dedicated to this phenomenon.

Write the topic on the board: “ Self-induction . AND inductance . Current magnetic field energy ».

Learning new material.

11.32-11.45

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.

Experience: The figure shows an electrical diagram of the experimental setup, on the basis of which the phenomenon of self-induction can be demonstrated. An electrical circuit consists of two parallel-connected light bulbs connected through a switch to a direct current source. A coil is connected in series with one of the light bulbs. After closing the circuit, 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.

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

Let us consider the processes occurring in this circuit when the key is closed and opened.

1. Key closure.

There is a current-carrying coil in the circuit. Let the current in this turn flow counterclockwise. Then the magnetic field will be directed upward.

Thus, the coil ends up in the space of its own magnetic field. As the current increases, the coil will find itself 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, an electromotive force of induction arises in this circuit and, as a consequence, an 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 plane of the circuit.

That is, for the turn considered in Figure 4, the induction current should be directed clockwise, thereby preventing the increase in the turn’s own current. 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 switch 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 magnetic flux leads to the appearance of induced emf and induced current. In this case, the induced current is directed in the same direction as the coil’s own current. This leads to a slower decrease in the intrinsic current.

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

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.

Inductance. The magnitude of the induction vector B of the magnetic field created by the current is proportional to the current strength. Since the magnetic flux Ф is proportional to B, then Ф ~ В~ I.

It can therefore be argued that

Ф = LI,

where L is the proportionality coefficient between the current in the conductive circuit and the magnetic flux.

The value of L is called the inductance of the circuit, or its self-inductance coefficient.

Using the law of electromagnetic induction and the resulting expression, we obtain the equality

From the formula it follows thatinductance is a physical quantity numerically equal to the self-inductive emf that occurs in a circuit when the current in it changes by 1 A in 1 s.

Inductance, like electrical capacitance, depends on geometric factors: the size of the conductor and its shape, but does not depend directly on the current strength in the conductor. In addition to the geometry of the conductor, inductance depends on the magnetic properties of the environment in which the conductor is located.

Obviously, the inductance of one wire turn is less than that of a coil (solenoid) consisting of N similar turns, since the magnetic flux of the coil increases N times.

The SI unit of inductance is called the henry (denoted by Gn). The inductance of a conductor is equal to 1 H if, with a uniform change in current strength by 1 A in 1 s, a self-inductive emf of 1 V arises in it:

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

Analogy between self-induction and inertia. The phenomenon of self-induction is similar to the phenomenon of inertia in mechanics. Thus, inertia leads to the fact that under the influence of force a body does not instantly acquire a certain speed, but gradually. The body cannot be instantly slowed down, no matter how great the braking force. In the same way, due to self-induction, when the circuit is closed, the current strength does not immediately acquire a certain value, but increases gradually. Turning off the source, we do not stop the current immediately. Self-induction maintains it for some time, despite the resistance of the circuit.

To create an electric current and therefore its magnetic field, work must be done against the forces of the eddy electric field. This work (according to the law of conservation of energy) is equal to the energy of the electric current or the energy of the magnetic field of the current.

Write down the expression for the energy of the currentI, flowing through a circuit with inductanceL, i.e. for the energy of the magnetic field of the current, is possible based on the analogy between inertia and self-induction.

If self-induction is analogous to inertia, then inductance plays the same role in the process of creating current as mass does in mechanics when speed increases. The role of body speed in electrodynamics is played by current strength as a quantity characterizing the movement of electric charges.

Then the current energy can be considered a value similar to kinetic energy in mechanics:

The energy of the magnetic field of the current.

They answer questions, enter into discussions, draw conclusions, and make notes in notebooks.

Reinforcing the material learned

11.46-11.56

Offers to solve the problem:

Solve problems at the board and in the field.

Summarizing. Homework.

11.57-11.58

Marking and substantiation. Writing and discussing homework.

D/Z: §14-16, No. 932, 934, 938.

Write down homework

Reflection

11.59-12.00

A conversation is organized in order for the lesson participants to understand their own actions during the lesson.

Questions:

1. What new did you learn for yourself in the lesson?

2. Was the lesson material clear?

3. Did you like the lesson?

Take part in the conversation

№931. What is the inductance of the circuit if, at a current strength of 5 A, a magnetic flux of 0.5 mWb appears in it?

№933. Find the inductance of a conductor in which, with a uniform change in current strength by 2 A for 0.25 s, a self-inductive emf of 20 mV is excited.

№937. In a coil with an inductance of 0.6 H, the current is 20 A. What is the energy of the magnetic field of this coil? How will the field energy change if the current strength is halved?

№939. Find the energy of the magnetic field of a solenoid in which a magnetic flux of 0.5 Wb occurs at a current of 10 A.

№932. What magnetic flux occurs in a circuit with an inductance of 0.2 mH at a current of 10 A?

№934. What self-inductive emf is excited in the winding of an electromagnet with an inductance of 0.4 H when the current in it uniformly changes by 5 A in 0.02 s?

№938. What should be the current strength in the winding of a choke with an inductance of 0.5 H so that the field energy is equal to 1 J?

The purpose of the lesson: form the idea that a change in current in a conductor creates a vortex that can either accelerate or slow down moving electrons.

During the classes

Checking homework using individual questioning

1. Obtain a formula for calculating the electromotive force of induction for a conductor moving in a magnetic field.

2. Derive a formula for calculating the electromotive force of induction using the law of electromagnetic induction.

3. Where is an electrodynamic microphone used and how is it designed?

4. Task. The resistance of the wire coil is 0.03 Ohm. The magnetic flux decreases inside the coil by 12 mWb. What electric charge passes through the cross section of the coil?

Solution. ξi=ΔФ/Δt; ξi= Iiʹ·R; Ii =Δq/Δt; ΔФ/Δt = Δq R/Δt; Δq = ΔФΔt/ RΔt; Δq= ΔФ/R;

Learning new material

1. Self-induction.

If an alternating current flows through a conductor, then it creates an induced emf in the same conductor - this is a phenomenon

Self-induction. The conductive circuit plays a dual role: current flows through it, and an induced emf is created in it by this current.

Based on Lenz's rule; when the current increases, the strength of the vortex electric field is directed against the current, i.e. prevents its increase.

As the current decreases, the vortex field maintains it.

Let's look at a diagram that shows that the current strength reaches a certain

values gradually, over time.

Demonstration of experiments with circuits. Using the first circuit, we will show how the induced emf appears when the circuit is closed.

When the key is closed, the first lamp lights up instantly, the second with a delay, due to the large self-induction in the circuit created by the coil with the core.

Using the second circuit, we will demonstrate the appearance of induced emf when the circuit is opened.

At the moment of opening, a current will flow through the ammeter, directed against the initial current.

When opening, the current may exceed the original current value. This means that the self-induction emf can be greater than the emf of the current source.

Draw an analogy between inertia and self-induction

Inductance.

Magnetic flux is proportional to the magnitude of magnetic induction and current strength. F~B~I.

Ф= L I; where L is the proportionality coefficient between current and magnetic flux.

This coefficient is often called circuit inductance or self-induction coefficient.

Using the magnitude of inductance, the law of electromagnetic induction can be written as follows:

ξis= – ΔФ/Δt = – L ΔI/Δt

Inductance is a physical quantity numerically equal to the self-inductive emf that occurs in the circuit when the current changes by 1 A in 1 s.

Inductance is measured in henry (H) 1 H = 1 V s/A

On the importance of self-induction in electrical and radio engineering.

Conclusion: when a changing current flows through a conductor, an eddy electric field appears.

The vortex field slows down free electrons when the current increases and maintains it when it decreases.

Consolidation of the studied material.

– How to explain the phenomenon of self-induction?

– Draw an analogy between inertia and self-induction.

– What is circuit inductance, in what units is inductance measured?

- Task. At a current of 5 A, a magnetic flux of 0.5 mWb appears in the circuit. What will be the inductance of the circuit?

Solution. ΔФ/Δt = – L ΔI/Δt; L = ΔФ/ΔI; L \u003d 1 10-4H

Let's summarize the lesson

Homework: §15, rep. §13, ex. 2 No. 10

The purpose of the lesson: to formulate the quantitative law of electromagnetic induction; Students must understand what magnetic induction emf is and what magnetic flux is. Lesson progress Checking homework...
The purpose of the lesson: to form in students an idea of the existence of resistance only in an alternating current circuit - these are capacitive and inductive reactances. Lesson progress Checking homework...
The purpose of the lesson: to form an idea of the energy possessed by an electric current in a conductor and the energy of the magnetic field created by the current. Lesson progress Checking homework by testing ...
The purpose of the lesson: to introduce the concept of electromotive force; get Ohm's law for a closed circuit; create in students an idea of the difference between emf, voltage and potential difference. Progress...
The purpose of the lesson: to form in students an idea of the active resistance in an alternating current circuit, and the effective value of current and voltage. Lesson progress Checking home...
Objective of the lesson: to form the concept that induced emf can occur either in a stationary conductor placed in a changing magnetic field, or in a moving conductor located in a constant...
Purpose of the lesson: to find out how the discovery of electromagnetic induction occurred; form the concept of electromagnetic induction, the significance of Faraday’s discovery for modern electrical engineering. Lesson progress 1. Analysis of the test...
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Purpose of the lesson: to find out what causes the induced emf in moving conductors placed in a constant magnetic field; lead students to the conclusion that a force acts on charges...
The purpose of the lesson: control of students’ assimilation of the topic studied, development of logical thinking, improvement of computational skills. Progress of the lesson Organizing students to complete the test Option 1 No. 1. Phenomenon...
The purpose of the lesson: to form in students an idea of the electric and magnetic field as a single whole - the electromagnetic field. Lesson progress Checking homework using testing...
The purpose of the lesson: to test students’ knowledge on the topic studied, to improve their skills in solving problems of various types. Progress of the lesson Checking homework Students' answers based on what they prepared at home...
The purpose of the lesson: to repeat and summarize knowledge on the topic covered; improve the ability to think logically, generalize, solve qualitative and calculation problems. Progress of the lesson Checking homework 1....
Purpose of the lesson: to prove to students that free electromagnetic oscillations in a circuit have no practical application; continuous forced oscillations are used, which have wide application in practice. Progress...
Purpose of the lesson: to form the concept of the magnetic induction module and Ampere force; be able to solve problems to determine these quantities. Course of the lesson Checking homework by the method of individual ...

Lesson No. 46-169

Self-induction- the phenomenon of the occurrence of induced emf in a conducting circuit when the current strength in it changes. The resulting emf is called Self-induced emf.

Manifestation of the phenomenon of self-induction.

Closing the circuit. When a short circuit occurs in the electrical circuit, the current increases, which causes an increase in the magnetic flux in the coil, and a vortex electric field appears, directed against the current, i.e. A self-induction emf arises in the coil, preventing the increase in current in the circuit (the vortex field inhibits the electrons).

As a result L1 lights up later than L2.

Open circuit.

When the electrical circuit is opened, the current decreases, a decrease in the magnetic flux in the coil occurs, and a vortex electric field appears, directed like a current (trying to maintain the same current strength), i.e. A self-induced emf arises in the coil, maintaining the current in the circuit. As a result, L flashes brightly when turned off.

Inductance, or self-induction coefficient - a parameter of an electrical circuit that determines the self-induction emf induced in the circuit when the current flowing through it changes and/or its deformation. The term “inductance” also refers to a self-induction coil, which determines the inductive properties of the circuit.

Self-induction - the occurrence of induced emf in a conductive circuit when the current strength in it changes. An induced emf occurs when the magnetic flux changes. If this change is caused by its own current, then they speak of self-induced emf:

ε is =–
= –L ,

Where L - circuit inductance, or its coefficientself-induction cit.

Inductance- a physical quantity numerically equal to the self-induction emf that occurs in the circuit when the current changes by 1 A in 1 s.

F - magnetic flux through the circuit, I - current strength in the circuit.SI unit of inductanceHenry(Gn): [ L] = [

] = [

]= Gn; 1 Gn = 1

Inductance, like electrical capacitance, depends on the geometry of the conductor - its size and shape, but does not depend on the current strength in the conductor. In addition, inductance depends on the magnetic properties of the environment in which the conductor is located.

Coil inductance depends on:

− number of turns,

− coil size and shape;

− on the relative magnetic permeability of the medium (possibly a core).

Closing and opening currents Whenever the current is turned on and off in the circuit, so-called extra currents of self-induction (extra currents of closure and timesmooing), arising in a circuit due to the phenomenon of self-induction and preventing (according to Lenz’s rule) an increase or decrease in current in the circuit. Inductance characterizes inertiacircuit in relation to the change in current in it, and itscan be considered as electrodynamicanalogue of body mass in mechanics, which is a measurebody inertia. In this case, the current strength I plays the role of body speed. The energy of the magnetic field of the current. Let's find the energy possessed by the electric current in the conductor. According to the law of conservation of energy, the energy of the magnetic field created by the current is equal to the energy that the current source (galvanic cell, generator at a power plant, etc.) must expend to create the current. When the current stops, this energy is released in one form or another. Let's find out why to create a current it is necessary to expend energy, i.e., it is necessary to do work. This is explained by the fact that when the circuit is closed, when the current begins to increase, a vortex electric field appears in the conductor, acting against the electric field that is created in the conductor due to the current source. In order for the current to become equal I, the current source must do work against the forces of the vortex field. This work goes to increase the energy of the magnetic field of the current.

When the circuit is opened, the current disappears and the vortex field does positive work. The energy stored in the current is released. This is detected by a powerful spark that occurs when a circuit with high inductance is opened.

I flowing through a circuit with inductance L, (i.e. for the energy of the magnetic field of the current), can be based on the analogy between inertia and self-induction discussed above. W m can be considered a value similar to the kinetic energy of the body

in mechanics, and write as W m =

(**) L, and the current strength in it is I. But this same energy can also be expressed through the characteristics of the field. Calculations show that the energy density of the magnetic field (i.e., the energy per unit volume) is proportional to the square of the magnetic induction, just as the energy density of the electric field is proportional to the square of the electric field strength.

The magnetic field created by an electric current has an energy directly proportional to the square of the current.

5. A current of 3 A flows into a coil with a resistance of 2 ohms. The inductance of the coil is 50 mH. What will be the voltage at the coil terminals if the current in it increases uniformly at a speed of 200 ?

Lesson No. 46-169 Self-induction. Inductance. The energy of the magnetic field of the current. D/z:§15; § 161. Self-induction– the phenomenon of the occurrence of EMF in a conducting circuit when the current strength in it changes. The resulting emf is called self-induction emf.According to Lenz's rule, at the moment the current increases, the intensity of the vortex electric field is directed against the current, i.e. the vortex field prevents the current from rising. And at the moment the current decreases, the vortex field supports it.

The phenomenon of self-induction can be observed in simple experiments.

WITH Diagram of parallel connection of two identical lamps. One of them is connected to the source through a resistorR , A the other - in series with the coil L, equipped with an iron core.

P
When the key is closed, the first lamp flashes almost immediately, and the second - with a noticeable delay. The self-inductive emf in the circuit of this lamp is large, and the current strength does not immediately reach its maximum value (Fig.).

The appearance of self-induction EMF during opening:

When the key in the coil is openedL fiddling around shows the self-induced emf that maintains the initialny current. As a result, at the moment of opening, a current flows through the galvanometer (from R to A ), directed againstinitial current before opening ( I to the ammeter). Forcecurrent when opening the circuit may exceed the current strength,

passing through the galvanometer with the switch closed.This means that the EMF of self-inductionε IS . more emf ε ba containers of elements.

2. Inductance. Induction vector module The magnetic field created by the current is proportional to the strength of the current. Since the magnetic flux Ф is proportional , then F ~ B~ I. It can be argued that Ф=LI, (1)

where L - coefficient of proportionality between the current in the conductive circuit and the magnetic flux. The value of L called circuit inductance, or him coefficientvolume of self-induction.

Using the law of electromagnetic induction and expression (1), we obtain the equality

ε IS = -= - L (2), if we assume that the shape of the contour remains unchanged throughoutThe current changes only due to changes in current strength.From formula (2) it follows thatinductance - this is fi ical quantity, numerically equal to the self-induction emf, arising in the circuit when the current strength in it changes by 1 A for 1 s.

Inductance depends on geometric factors: the size of the conductor and its shape, but does not depend directly on the current strength in the conductor. In addition to the geometry of the conductor, inductance depends on the magnetic properties of the environment in which the conductor is located.

The inductance of one wire turn is less than that of a coil (solenoid) consisting of N similar turns, since the magnetic flux of the coil increases by N times.

The SI unit of inductance is called Henry(denoted by Gn). The conductor inductance is equal to 1 Gn, Ifin it with a uniform change in current strength by 1 A behind 1 s self-induced emf occurs 1 V: 1 Gn = = 1

3. Current magnetic field energy According to the law of conservation of energy, the energy of the magnetic field created by the current is equal to the energy that the current source (galvanic cell, generator at a power plant, etc.) must expend to create the current. When the circuit is opened, the current disappears and the vortex field does positive work. The energy stored in the current is released. This is detected, for example, by a powerful spark that occurs when a circuit with high inductance is opened. Write down the expression for current energy I flowing through a circuit with inductance L, (i.e. for the energy of the magnetic field of the current), can be based on the analogy between inertia and self-induction. If self-induction is similar to inertia, then inductance in the process of creating current should play the same role as mass when increasing the speed of a body in mechanics. The role of body speed in electrodynamics is played by current strength I as a quantity characterizing the movement of electric charges. If this is so, then the current energy W m can be considered a quantity similar to the kinetic energy of a body in mechanics, and written in the form W m = (**) It is precisely this expression for the current energy that is obtained as a result of calculations. Current energy (**) is expressed through the geometric characteristics of the conductor L, and the current strength in it is I. But this same energy can also be expressed through the characteristics of the field. Calculations show that the magnetic field energy density (i.e., the energy per unit volume) is proportional to the square of the magnetic induction w M ~ V 2, just as the energy density of the electric field is proportional to the square of the electric field strength w E ~ E 2

Remember: The magnetic field created by an electric current has an energy directly proportional to the square of the current.

Basic formulas: Faraday's law (law of electromagnetic induction): ε = –,where ΔФ is the change in magnetic flux, Δt is the time period during which this change occurred.

The phenomenon of self-induction is that when the current changes in the circuit, an emf appears that counteracts this change. Magnetic flux Ф through a surface bounded by a contour is directly proportional to the current strength I in the circuit: Ф = LI,

where L - proportionality coefficient, called inductance.

Self-induction emf is expressed through the change in current strength in the circuit Δ I by the following formula:

ε = - = -L where Δt is the time during which this change occurred.

Magnetic field energy W is expressed by the formula: W=

Tasks. Self-induction. Inductance.

1. What self-inductive emf occurs in a coil with an inductance of 86 mH if a current of 3.8 A disappears in it in 0.012 s?

2. Determine the self-induction EMF if the current in a coil with an inductance of 0.016 mH decreases at a rate of 0.5 kA / s.

3. What is the inductance of the coil if, with a uniform change in the current in it from 2 to 12 A in 0.1 s, a self-inductive emf equal to 10 V occurs?

4. The magnetic flux penetrating the circuit of a conductor with a resistance of 0.2 Ohm changes uniformly from 1.2∙10 -3 Wb to 0.4∙10 -3 Wb in 2 ms. Determine the current strength in the circuit.

6. What is the rate of change of current in a relay winding with an inductance of 3.5 H if a self-inductive emf of 105 V is excited in it?

7. A coil with negligible resistance and inductance of 3 H is connected to a current source with an emf of 15 V and negligible internal resistance. After what period of time does the current in the coil reach 50A? 8. A coil with an inductance of 0.2 H is connected to a current source with an EMF = 10 V and an internal resistance of 0.4 Ohm. Determine the total EMF at the moment of opening the circuit, if the current in it disappears in 0.04 s, and the resistance of the coil wire is 1.6 ohms. 9. A coil with a resistance of 10 ohms and an inductance of 0.01 H is in an alternating magnetic field. When the magnetic flux created by this field increased by 0.01 Wb, the current in the coil increased by 0.5 A. What charge passed through the coil during this time?

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, we introduce 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, i.e. inductance.

Rice. 2. Diagram of the experimental setup by D. Henry

In 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. An electrical circuit consists of two parallel-connected light bulbs connected through a switch to a direct current source. A coil is connected in series with one of the light 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 light 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 closure

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. Key opening

Let us consider the processes occurring in this circuit when the key is closed and opened.

1. Key closure.

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

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

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. The moment of switching on and off the circuit

Formula for finding the magnetic induction of a straight conductor with current:

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

The flux of magnetic induction through the area 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:

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

Substituting the formula for 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 obtain the final expression for the magnetic flux:

From the resulting relationship it is clear that the flux value depends on the current value and on the geometry of the coil (radius, length, number of turns). A value equal to is called inductance:

The unit of inductance is henry:

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

Taking into account the formula for induced emf, we find that self-induction emf 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 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, a self-inductive electromotive force equal to 1 V arises in this conductor.

Bibliography

Myakishev G.Ya. Physics: Textbook. for 11th grade general education institutions. - M.: Education, 2010.
Kasyanov V.A. Physics. 11th grade: Educational. for general education institutions. - M.: Bustard, 2005.
Gendenstein L.E., Dick Yu.I., Physics 11. - M.: Mnemosyne.

Internet portal Myshared.ru ().
Internet portal Physics.ru ().
Internet portal Festival.1september.ru ().

Homework

Questions at the end of paragraph 15 (p. 45) - Myakishev G.Ya. Physics 11 (see list of recommended readings)
The inductance of which conductor is 1 Henry?

Physics lesson using Internet resources.

11th grade, topic: “Self-induction, inductance” - 2 hours.

Goals:

formation of educational competencies - independently organize the process of studying physical concepts and laws, solve educational problems.
Formation of research competencies - development of independent knowledge acquisition, using Internet resources, analyzing and selecting the necessary information.
Formation of social and personal competencies - the ability to determine the significance of knowledge for oneself and society.

Resources for lesson implementation: a computer lab with Internet connection is required.

Statement of the problem: independently, using Internet resources, study the phenomenon of self-induction, consider the concept of inductance, find out how the EMF of self-induction is determined. Consider the practical application of self-induction. Determine the significance of the phenomenon for yourself and for science.

2. Independent work of students, which involves

research activities to obtain information, select and classify it
graphical representation of one’s own knowledge system based on the information received in the form of a diagram, graph, description. Reflection of the practical orientation of the studied phenomena and laws in the form of drawings and photographs.
reasoning about the significance of the acquired knowledge for oneself and society in graphic form or in the form of a short essay, essay.

All student activities are reflected in the electronic workbook.

3. Self-testing: students are offered a test on the material studied (link to the test in Appendix 2). Students choose their own level. Only answer options are placed in the workbook.

Assessing student work:

Exchange of workbooks over a local network, analysis of acquired knowledge, self-test (answers in Appendix 3). Students evaluate their classmates' workbooks themselves.

Lesson summary: reflection, discussion of difficulties, wishes, results achieved.

Homework: understanding the acquired knowledge, preparing problem material for discussion on the topic “Self-induction, inductivity.” Doing homework involves working independently with a textbook and additional information.

www.physics.nad.ru- Physics in animations

www.physics.ru- Physics at the Open College

http://www.spin.nw.ru/ Physics for schools

http://physicomp.lipetsk.ru/- Electronic magazine "Physikomp"

http://www.omsknet.ru/acad/fr_elect.htm- Electronic textbook on physics.

www.alsak.ru-School physics for teachers and students.

www.physics-regelman.com

Appendix 3

Answers to the test “Self-induction. Inductance"

Level A	Level B	Level C
question number	answer	question number	answer	question number	answer