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.
Henry invented flat coils of strip copper, 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. Schematic of the experimental setup by 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. The electrical circuit consists of two parallel-connected light bulbs connected through a key to a DC 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 you know, 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 result, 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 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 arises 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
The formula for finding the magnetic induction of a 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
- Myakishev G.Ya. Physics: Proc. for 11 cells. general education institutions. - M.: Education, 2010.
- Kasyanov V.A. Physics. Grade 11: Proc. 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 reading)
- Which conductor has an inductance of 1 henry?
The purpose of the lesson: to form the idea that a change in the current strength in a conductor creates a vortex wave, which can either accelerate or decelerate moving electrons.
During the classes
Checking homework by the method of individual survey
1. Get 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 arranged?
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?
Decision. ξi=ΔF/Δt; ξi= Iiʹ·R; Ii =Δq/Δt; ΔF/Δt = Δq R/Δt; Δq = ΔФΔt/ RΔt; Δq= ΔФ/R;
Learning new material
1. Self-induction.
If an alternating current flows through the conductor, then it creates an induction EMF in the same conductor - this is a phenomenon
Self-induction. The conducting circuit plays a dual role: a current flows through it, and an EMF of induction is created in it by this current.
Based on Lenz's rule; when the current increases, the intensity of the eddy electric field is directed against the current, i.e. prevents it from growing.
During the current decrease, the vortex field maintains it.
Consider a circuit that shows that the current strength reaches a certain
values gradually over time.
Demonstration of experiments with schemes. Using the first circuit, we will show how the induction 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 that the core coil creates.
With the help of the second circuit, we will demonstrate the appearance of the induction EMF when the circuit is opened.
At the moment of opening, a directed current will flow through the ammeter, against the initial current.
When opened, the current may exceed the original current value. This means that the EMF of self-induction can be greater than the EMF of the current source.
Draw an analogy between inertia and self-induction
Inductance.
The magnetic flux is proportional to the magnitude of the magnetic induction and the strength of the current. F~B~I.
F= L I; where L is the proportionality factor between current and magnetic flux.
This ratio is often called loop inductance or the coefficient of self-induction.
Using the value 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 EMF of self-induction that occurs in the circuit when the current changes by 1 A in 1 s.
Measure the inductance in Henry (H) 1 H = 1 V s / A
On the importance of self-induction in electrical engineering and radio engineering.
Conclusion: when a changing current flows through a conductor, a vortex electric field appears.
The vortex field slows down the free electrons as the current increases and maintains it as the current decreases.
Consolidation of the studied material.
– How to explain the phenomenon of self-induction?
– Draw an analogy between inertia and self-induction.
- What is the inductance of the circuit, in what units is the inductance measured?
- Task. At a current strength of 5 A, a magnetic flux of 0.5 mWb appears in the circuit. What will be the inductance of the circuit?
Decision. ΔF/Δt = – L ΔI/Δt; L = ΔF/ΔI; L \u003d 1 10-4H
Summing up the lesson
Homework: §15, rep. §13, ex. 2 No. 10
- The purpose of the lesson: to formulate the quantitative law of electromagnetic induction; students should learn what is the EMF of magnetic induction and what is magnetic flux. Lesson progress Checking homework...
- The purpose of the lesson: to form in students the idea of the existence of resistance only in an alternating current circuit - these are capacitive and inductive resistances. Lesson progress Checking homework...
- The purpose of the lesson: to form an idea of the energy that an electric current has 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; to create in students an idea of the difference between EMF, voltage and potential difference. Move...
- The purpose of the lesson: to form students' understanding of the active resistance in an alternating current circuit, and the effective value of current and voltage. Lesson progress Checking home...
- The purpose of the lesson: to form the concept that the EMF of induction can occur either in a stationary conductor placed in a changing magnetic field, or in a moving conductor in a constant ...
- The purpose of the lesson: to find out how the discovery of electromagnetic induction took place; to form the concept of electromagnetic induction, the significance of Faraday's discovery for modern electrical engineering. Course of the lesson 1. Analysis of the control work ...
- The purpose of the lesson: to consider the device and the principle of operation of transformers; give evidence that electric current would never have had such a wide application, if at one time ...
- The purpose of the lesson: to find out what causes the induction EMF in moving conductors placed in a constant magnetic field; bring students to the conclusion that a force acts on charges ...
- The purpose of the lesson: control of assimilation by students of the studied topic, development of logical thinking, improvement of computational skills. Course of the lesson Organization of students to perform the test Option 1 No. 1. Phenomenon...
- The purpose of the lesson: to form students' understanding of the electric and magnetic fields, as a single whole - the electromagnetic field. Lesson progress Checking homework by testing ...
- The purpose of the lesson: to test students' knowledge on the topic studied, to improve the skills of solving problems of various types. Lesson progress Checking homework Students' answers according to 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 computational problems. Lesson progress Checking homework 1....
- The purpose of the lesson: to prove to students that free electromagnetic oscillations in the circuit have no practical application; undamped forced oscillations are used, which are of great use in practice. Move...
- The purpose of the lesson: to form the concept of the modulus of magnetic induction and the Ampere force; be able to solve problems to determine these quantities. Course of the lesson Checking homework by the method of individual ...
slide 2
SELF-INDUCTION
Each conductor through which electric current flows is in its own magnetic field.
slide 3
When the current strength changes in the conductor, the m.field changes, i.e. the magnetic flux created by this current changes. A change in the magnetic flux leads to the emergence of a vortex electric field and an induction EMF appears in the circuit.
slide 4
Self-induction - the phenomenon of the occurrence of induction EMF in an electric circuit as a result of a change in current strength. The resulting emf is called the self-induction emf.
slide 5
Manifestation of the phenomenon of self-induction
slide 6
Conclusion in electrical engineering, the phenomenon of self-induction manifests itself when the circuit is closed (the electric current increases gradually) and when the circuit is opened (the electric current does not disappear immediately).
Slide 7
INDUCTANCE
What does the EMF of self-induction depend on? Electric current creates its own magnetic field. The magnetic flux through the circuit is proportional to the magnetic field induction (Ф ~ B), the induction is proportional to the current strength in the conductor (B ~ I), therefore the magnetic flux is proportional to the current strength (Ф ~ I). EMF of self-induction depends on the rate of change of 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.
Slide 8
Inductance - physical. a value numerically equal to the EMF of self-induction that occurs in the circuit when the current strength changes by 1 ampere in 1 second.
Slide 9
Also, the inductance can be calculated by the formula:
where F is the magnetic flux through the circuit, I is the current strength in the circuit.
Slide 10
SI units for inductance:
slide 11
The inductance of a coil depends on:
the number of turns, the size and shape of the coil, and the relative magnetic permeability of the medium (a core is possible).
slide 12
SELF-INDUCTION EMF
EMF of self-induction prevents the increase in current strength when the circuit is turned on and the decrease in current strength when the circuit is opened.
slide 13
ENERGY OF THE MAGNETIC FIELD OF THE CURRENT
Around a conductor with current there is a magnetic field that 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.
Slide 14
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 a circuit with a sufficiently large current is opened, a spark or arc may occur)
View all slides
1st semester
ELECTRODYNAMICS
3. Electromagnetic field
LESSON 9/36
Subject. 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 DC circuit?
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.
Outline of the lesson in physics “Self-induction. Inductance. Energy of the magnetic field of the current "(Grade 8)
Lesson topic: Self-induction. Inductance. The energy of the magnetic field.
Target : Formation of the concept of the phenomenon of self-induction, its manifestation in electric current circuits. The use of self-induction in electrical devices.
Tasks:
Educational: Repeat students' knowledge about the phenomenon of electromagnetic induction, deepen them; on this basis to study the phenomenon of self-induction.
Educational: To cultivate interest in the subject, diligence and the ability to carefully evaluate the answers of comrades. Show the importance of cause-and-effect relationships in the cognizability of phenomena.
Developing: The development of the physical thinking of students, the expansion of the conceptual apparatus of students, the formation of skills to analyze information, draw conclusions from observations and experiments.
Lesson type: lesson learning new material.
Equipment: Core inductor - demo, power supply, key, two 3.5 V bulbs, 100 Ohm rheostat, 200V neon bulb.
Experiences: 1) experience in observing the phenomenon of self-induction when the circuit is closed; 2) experience in observing the phenomenon of self-induction when the circuit is opened;
Lesson plan:
Organizing time.
Updating of basic knowledge.
Motivation.
Learning new material.
Consolidation.
Homework.
During the classes
Organizing time.(1 min)
Updating of basic knowledge.
What is the phenomenon of electromagnetic induction?
What hypothesis of Faraday led to the discovery of electromagnetic induction?
How did Faraday discover the phenomenon of electromagnetic induction?
Under what conditions does an induction current occur in a coil?
What determines the direction of the induced current?
What explains the repulsion of the aluminum ring when a magnet is inserted into it and the attraction to the magnet when it is removed from the ring?
Why does a cut aluminum ring not interact with a moving magnet?
Formulate Lenz's rule.
How to use the Lenz rule to determine the direction of the inductive current in the conductor?
3 . Motivation.
The foundations of electrodynamics were laid by Ampère 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. Creating various electromagnets, in 1832 the scientist discovered a new phenomenon in electromagnetism - the phenomenon of self-induction. We will talk about this in this lesson.
4. Learning new material.
Consider a special case of electromagnetic induction: the occurrence of an inductive current in a coil when the current strength in it changes.
To do this, we will carry out the experiment shown in the figure. We close the circuit with the key Kl. Lamp L1 will light up immediately, and L2 - with a delay of approximately 1 s. The reason for the delay is as follows. According to the phenomenon of electromagnetic induction, inductive currents arise in the rheostat and in the coil. They prevent an increase in the strength of the current I 1 and I 2 (this follows from the Lenz rule and the right hand rule). But in the coil K, the induction current will be much greater than in the rheostat P, since the coil has a much larger number of turns and a core, i.e., it has a greater inductance than the rheostat.
In our experiment, we observe the phenomenon of self-induction.
The phenomenon of self-induction is the occurrence of an induction current in the coil when the current strength in it changes. In this case, the resulting induction current is called the self-induction current. This phenomenon was discovered by Joseph Henry, almost simultaneously with the discovery of the phenomenon of electromagnetic induction by Faraday.
Self-induction when opening an electric circuit and the energy of a magnetic field. The appearance of a powerful induction current when the circuit is opened indicates that the magnetic field of the current in the coil has energy. It is by reducing the energy of the magnetic field that work is done to create an induction current. At this moment, the Ln lamp flashes, which, under normal conditions, lights up at a voltage of 200V. And this energy accumulated earlier, when the circuit was closed, when due to the energy of the current source, work was done to overcome the self-induction current, which prevents the increase in current in the circuit, and its magnetic field.
Inductance- this is a value equal to the EMF of self-induction when the current strength in the conductor changes by 1 A in 1 s. The unit of inductance is henry (H). 1 H = 1 V s/A. 1 henry is the inductance of such a conductor in which an EMF of self-induction of 1 volt occurs at a rate of change in current strength of 1 A / s. L is called inductance. Demonstration of various inductors used in radio engineering and electrical engineering. We use handouts for students to view. (inductors) 
Fluorescent Lamp are gas-discharge light sources. Their luminous flux is formed due to the glow of phosphors, which are affected by the ultraviolet radiation of the discharge. Its visible glow usually does not exceed 1-2%. Fluorescent lamps (LL) are widely used in the lighting of various types of premises. Their luminous efficiency is many times greater than that of conventional incandescent lamps. A starter device is used as a switch. The starter is a small gas-discharge glow lamp. The glass flask is filled with an inert gas (neon or helium-hydrogen mixture) and placed in a metal or plastic case. When the circuit is turned on for mains voltage, it will be completely applied to the starter. The starter electrodes are open, and a glow discharge occurs in it. A small current (20-50 mA) will flow in the circuit. This current heats the bimetallic electrodes, and they, bending, close the circuit, and the glow discharge in the starter will stop. After the lamp is ignited, a current equal to the rated operating current of the lamp will be established in the circuit. This current will cause such a voltage drop across the inductor that the lamp voltage will become approximately equal to half the nominal mains voltage. Since the starter is connected in parallel with the lamp, the voltage on it will be equal to the voltage on the lamp, and due to the fact that it is not enough to ignite the glow discharge in the starter, its electrodes will remain open when the lamp burns.
5. Consolidation.
1. What phenomenon was studied in the experiment.
2. What is the phenomenon of self-induction?
3. Can a self-induction current occur in a direct current-carrying conductor? If not, please explain why; if so, under what condition.
4. By reducing what energy was the work done to create an inductive current when the circuit was opened?
5. What facts prove that the magnetic field has energy?
6. What is inductance?
7. What is the SI unit of inductance and what is it called?
8. What is a choke and why is it needed when operating a fluorescent lamp?
Task1. What is the inductance of the coil if, with a gradual change in the current strength from 5 to 10A in 0.1 s, an EMF of self-induction occurs equal to 20V?
