Oscillatory circuit receiving electromagnetic oscillations presentation. Oscillatory circuit. III. Learning new material

Oscillations are

mechanical, electromagnetic, chemical, thermodynamic

and various others. Despite such a variety, they all have a lot in common.

A magnetic field

generated by electric current

the main physical characteristic is magnetic induction

Electric field

generates with i charge

main physical characteristic-

field strength

these are periodic or almost periodic changes in charge q, current strength I and voltage U .

Oscillatory types

systems

Mathematical

pendulum

Spring loaded

pendulum

Oscillatory types

systems

Mathematical

pendulum

Spring loaded

pendulum

Oscillatory

Circuit

Shock absorber operation diagram

Schematic representation of the types of oscillatory systems

Mathematical pendulum

Spring pendulum

this is simplest system, in which electromagnetic oscillations can occur, consisting of a capacitor and a coil attached to its plates.

By the nature of the processes that cause oscillatory movements

Oscillatory types

movement

Free

Forced

The oscillatory system is left to itself, damped oscillations occur due to the initial supply of energy.

Oscillations occur due to external, periodically changing forces.

Free vibrations are the vibrations in a system that arise after it is taken out of equilibrium.

Forced oscillations are called oscillations in a circuit under the influence of an external periodic EMF.

To bring the system out of equilibrium, it is necessary to impart additional charge to the capacitor.

The origin of the EMF: electrons moving along with the conductors of the frame are affected by a force from the side of the magnetic field, which causes a change in the magnetic flux and, accordingly, the EMF of induction.

for observation and research, the most suitable device is electronic oscilloscope

OSCILLOSCOPE

(from lat. oscillo - swing and "graph"), measuring

device for observing the relationship between two

or several rapidly changing quantities

(electrical or converted to electrical)

The most common cathode-ray oscilloscopes

in which electrical signals,

proportional to the change in the studied quantities,

go to the deflector plates

oscilloscope tube;

on the screen of the tube they observe or

take pictures of graphic

dependency image.

L - INDUCTANCE COILS, Mr.

C - ELECTRIC CAPACITY CONDENSER, F

CHARGER

CONDENSER

W is the energy of the electric field, J

Discharging the capacitor: the energy of the electric field decreases, but at the same time the energy of the magnetic field of the current increases.

W = Li ² / 2 -

magnetic field energy, J

i- strength alternating current, A

The total energy of the electromagnetic field of the circuit is equal to the sum of the energies of the magnetic and electric fields.

W = L i 2 / 2 + q 2 / 2C

W el W m W el

Energy conversion in an oscillatory circuit

q 2/2 С = q 2/2 С + Li 2/2 = Li 2/2

In real oscillatory circuits

there is always active resistance,

which determines

damping of oscillations.

Mechanical and electromagnetic vibrations and vibrational systems

mechanical and electromagnetic vibrations obey exactly the same quantitative laws

In addition to mechanical vibrations in nature, there are also

electromagnetic vibrations.

They are committed in

oscillatory circuit.

It consists of

coil and capacitor.

What transformations take place in the circuit

transformation of energies

§27-28,
synopsis in a notebook.,
repeat mechanical vibrations: definitions and physical quantities characterizing vibrations.

Back forward

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Lesson objectives:

teaching: to introduce the concepts: “electromagnetic oscillations”, “oscillatory circuit”; to show the universality of the basic laws of oscillatory processes for oscillations of any physical nature; show that oscillations in an ideal circuit are harmonic; to reveal the physical meaning of the characteristics of vibrations;
developing: development of cognitive interests, intellectual and creative abilities in the process of acquiring knowledge and skills in physics using various sources of information, including modern means information technologies; the formation of skills to assess the reliability of natural science information;
educational: education of conviction in the possibility of knowing the laws of nature; the use of the achievements of physics for the benefit of the development of human civilization; the need for cooperation in the process of joint implementation of tasks, readiness for a moral and ethical assessment of the use of scientific achievements, a sense of responsibility for environmental protection.

During the classes

I. Organizational moment.

In today's lesson we start to study the new chapter of the textbook and the topic of today's lesson “Electromagnetic vibrations. Oscillatory circuit ”.

II. Homework check.

Let's start our lesson by checking our homework.

Slide 2. Test for repetition of the passed material and the course of the 10th grade.

You were asked to answer the questions for the diagram shown in the figure.

1. At what position of the SA2 key will the neon lamp flash when the SA1 key is opened?

2. Why does the neon lamp not flash when the SA1 key is closed, whatever position the SA2 switch is in?

The test is performed on a computer. In the meantime, one of the students assembles the diagram.

Answer... The neon lamp flashes at the second position of the SA2 switch: after opening the SA1 key due to the phenomenon of self-induction, a current decreasing to zero flows in the coil, an alternating magnetic field is excited around the coil, generating a vortex electric field, which for a short time supports the movement of electrons in the coil. A short-term current will flow along the upper part of the circuit through the second diode (it is switched on in the throughput direction). As a result of self-induction in the coil when the circuit is opened, a potential difference will appear at its ends (EMF of self-induction), sufficient to maintain the gas discharge in the lamp.

When the SA1 key is closed (the SA2 key is in position 1), the DC voltage source is not enough to maintain the gas discharge in the lamp, so it does not light up.

Let's check the correctness of your assumptions. The proposed scheme is assembled. Let's see what happens to the neon lamp when the SA1 key is closed and opened at different positions of the SA2 switch.

(The test is compiled in the MyTest program. The score is set by the program).

File for running the MyTest program (located in the folder with the presentation)

Test. (Run the MyTest program, open the "Test" file, press the F5 key to start the test)

III. Learning new material.

Slide 3. Problem statement: Let's remember what we know about mechanical vibrations? (The concept of free and forced oscillations, self-oscillations, resonance, etc.) In electrical circuits, as well as in mechanical systems, such as a load on a spring or a pendulum, free oscillations can occur. In today's lesson, we begin to study such systems. Topic of today's lesson: “Electromagnetic vibrations. Oscillatory circuit ”.

Lesson objectives

we will introduce the concepts: “electromagnetic oscillations”, “oscillatory circuit”;
show the universality of the basic laws of oscillatory processes for oscillations of any physical nature;
show that the oscillations in an ideal circuit are harmonic;
we will reveal the physical meaning of the characteristics of the oscillations.

Let us first recall what properties the system must possess in order for free oscillations to arise in it.

(A restoring force should arise in the oscillatory system and a transformation of energy from one type to another should occur, the friction in the system should be small enough.)

In electrical circuits, as well as in mechanical systems such as a spring load or a pendulum, free vibrations can occur.

What vibrations are called free vibrations? (Vibrations that arise in the system after removing it from the equilibrium position) What vibrations are called forced vibrations? (oscillations occurring under the influence of an external periodically changing EMF)

Periodic or almost periodic changes in charge, current and voltage are called electromagnetic oscillations.

Slide 4. After they invented the Leyden jar and learned how to impart a large charge to it using an electrostatic machine, they began to study the electric discharge of the jar. Closing the covers of the Leyden jar with a wire coil, they found that the steel spokes inside the coil were magnetized, but it was impossible to predict which end of the coil core would turn out to be the North Pole, and which South Pole was impossible. An important role in the theory of electromagnetic oscillations was played by the German scientist of the 19th century HELMHOLZ Hermann Ludwig Ferdinand. He is called the first physician among scientists and the first scientist among physicians. He studied physics, mathematics, physiology, anatomy and psychology, achieving worldwide recognition in each of these areas. Paying attention to the oscillatory nature of the discharge of the Leyden jar, in 1869 Helmholtz showed that similar oscillations occur in an induction coil connected to a capacitor (i.e., in essence, he created an oscillatory circuit consisting of inductance and capacitance). These experiments played an important role in the development of the theory of electromagnetism.

Slide 4. Typically, electromagnetic vibrations occur at a very high frequency, much higher than the frequency of mechanical vibrations. Therefore, an electronic oscilloscope is very convenient for observing and researching them. (Demonstration of the device. The principle of its operation in the animation.)

Slide 4. Currently, digital oscilloscopes have replaced electronic oscilloscopes. He will tell us about the principles of their action ...

Slide 5. Oscilloscope animation

Slide 6. But back to electromagnetic oscillations. The simplest electrical system that can oscillate freely is a serial RLC circuit. An oscillating circuit is an electrical circuit consisting of a series-connected capacitor with an electrical capacity C, a coil with an inductance L and an electrical resistance R. We will call it a series RLC circuit.

Physical experiment. We have a circuit, the diagram of which is shown in Figure 1. Let's attach a galvanometer to the coil. Let's observe the behavior of the galvanometer arrow after the switch is moved from position 1 to position 2. You have noticed that the needle begins to oscillate, but these oscillations will soon fade away. All real circuits contain electrical resistance R. For each period of oscillation, part of the electromagnetic energy stored in the circuit is converted into Joule heat, and the oscillations become damped. A graph of damped oscillations is considered.

How do free vibrations occur in an oscillatory circuit?

Consider the case when the resistance R = 0 (model of an ideal oscillatory circuit). What processes take place in the oscillatory circuit?

Slide 7. Oscillating contour animation.

Slide 8. Let's move on to the quantitative theory of processes in an oscillatory circuit.

Consider a serial RLC circuit. When the K key is in position 1, the capacitor is charged to voltage. After switching the key to position 2, the process of discharging the capacitor begins through the resistor R and the inductor L. Under certain conditions, this process can be oscillatory.

Ohm's law for a closed RLC circuit that does not contain an external current source is written as

where is the voltage across the capacitor, q is the charge of the capacitor, - current in the circuit. On the right side of this ratio is the EMF of the self-induction of the coil. If we choose the capacitor charge q (t) as a variable, then the equation describing free oscillations in the RLC circuit can be reduced to the following form:

Let us consider the case when there is no loss of electromagnetic energy in the circuit (R = 0). Let's introduce the notation: ... Then

(*)

Equation (*) is the basic equation that describes free oscillations in an LC-circuit (ideal oscillatory circuit) in the absence of damping. In appearance, it exactly coincides with the equation of free vibrations of a load on a spring or thread in the absence of friction forces.

We wrote down this equation while studying the topic “Mechanical vibrations”.

In the absence of damping, free oscillations in the electrical circuit are harmonic, that is, they occur according to the law

q (t) = q m cos (0 t + 0).

Why? (Since this is the only function, the second derivative of which is equal to the function itself. In addition, cos0 = 1, which means q (0) = q m)

The amplitude of oscillations of the charge q m and the initial phase 0 are determined by the initial conditions, that is, by the way by which the system was brought out of equilibrium. In particular, for the oscillation process, which will begin in the circuit shown in Figure 1, after switching the key K to position 2, q m = C, 0 = 0.

Then the equation of harmonic oscillations of the charge for our circuit will take the form

q (t) = q m cos 0 t.

The strength of the current also performs harmonic oscillations:

Slide 9. Where is the amplitude of current fluctuations. Current fluctuations are phase ahead of charge fluctuations.

With free oscillations, there is a periodic transformation of the electrical energy W e stored in the capacitor into the magnetic energy W m of the coil and vice versa. If there is no energy loss in the oscillatory circuit, then the total electromagnetic energy of the system remains unchanged:

Slide 9. The parameters L and C of the oscillating circuit determine only the natural frequency of free oscillations

Given that, we get.

Slide 9. Formula called the Thomson formula, the English physicist William Thomson (Lord Kelvin), who derived it in 1853.

Obviously, the period of electromagnetic oscillations depends on the inductance of the coil L and the capacitance of the capacitor C. We have a coil, the inductance of which can be increased with the help of an iron core, and a capacitor of variable capacitance. Let's first remember how you can change the capacity of such a capacitor. Let me remind you that this is the material of the 10th grade course.

A variable capacitor consists of two sets of metal plates. When the handle is rotated, the plates of one set enter the gaps between the plates of the other set. In this case, the capacitance of the capacitor changes in proportion to the change in the area of the overlapping part of the plates. If the plates are connected in parallel, then by increasing the area of the plates, we will increase the capacity of each of the capacitors, which means that the capacity of the entire capacitor bank will increase. When capacitors are connected in series into a bank, an increase in the capacity of each capacitor entails a decrease in the capacity of the capacitor bank.

Let's see how the period of electromagnetic oscillations depends on the capacitance of the capacitor C and the inductance of the coil L.

Slide 9. Animation "Dependence of the period of electromagnetic oscillations on L and C"

Slide 10. Let us now compare the electrical vibrations and the vibrations of the weight on the spring. Open page 85 of the tutorial, Figure 4.5.

The figure shows the graphs of the change in the charge q (t) of the capacitor and the displacement x (t) of the load from the equilibrium position, as well as the graphs of the current I (t) and the speed of the load v(t) for one period T of oscillations.

You have a table on your tables that we filled out when studying the topic "Mechanical vibrations". Appendix 2.

One line of this table is filled in. Using Figure 2, paragraph 29 of the textbook and Figure 4.5 on page 85 of the textbook, fill in the remaining lines of the table.

What are the similarities between the processes of free electrical and mechanical vibrations? Let's see the following animation.

Slide 11. Animation "Analogy between electrical and mechanical vibrations"

The obtained comparisons of free oscillations of a load on a spring and processes in an electric oscillatory circuit make it possible to draw a conclusion about the analogy between electrical and mechanical quantities.

Slide 12. These analogies are presented in the table. Appendix 3.

You have the same table on your tables and in the tutorial on page 86.

So, we have considered the theoretical part. Did you understand everything? Maybe someone has questions?

Now let's move on to solving problems.

IV. Physical education.

V. Consolidation of the studied material.

Solving problems:

tasks 1, 2, tasks of part A No. 1, 6, 8 (orally);
tasks No. 957 (answer 5.1 μH), No. 958 (the answer will decrease by 1, 25 times) (at the board);
task of part B (orally);
task number 1 of part C (at the board).

Problems are taken from the collection of problems for grades 10-11 A.P. Rymkevich and Appendices 10. Appendix 4.

Vi. Reflection.

Students fill out a reflective map.

Vii. Summing up the lesson.

Have you met the objectives of the lesson? Summing up the lesson. Assessment of learners.

VIII. Home assignment.

Paragraphs 27 - 30, No. 959, 960, the remaining tasks from Appendix 10.

Literature:

Multimedia physics course “Open Physics” version 2.6 edited by Professor S.М. Goat.
Problem book 10-11 grade. A.P. Rymkevich, Moscow "Education", 2012.
Physics. Textbook for grade 11 educational institutions. G.Ya. Myakishev, B.B. Bukhovtsev, V.M. Charugin. Moscow "Education", 2011.
Electronic supplement to the textbook by G.Ya. Myakishev, B.B. Bukhovtseva, V.M. Charugin. Moscow "Education", 2011.
Electromagnetic induction. Qualitative (logical) tasks. Grade 11, physics and mathematics profile. CM. Novikov. Moscow "Chistye Prudy", 2007. Library "First of September". Series "Physics". Issue 1 (13).
http://pitf.ftf.nstu.ru/resources/walter-fendt/osccirc

P.S. If it is not possible to provide each student with a computer, then the test can be carried out in writing.

"Free vibrations"- Continuous oscillations. Free electromagnetic oscillations. Where i and q are the current strength and electric charge at any given time. According to the law of electromagnetic induction: The total electromagnetic energy of the oscillatory circuit. The number of vibrations per unit of time is called the vibration frequency: Total energy.

"Mechanical resonance"- 1. Chain of the Egyptian bridge in St. Petersburg. Resonance in technology. 3. Mexico City 1985 Takoma suspension bridge. Positive resonance value Frequency meter. 2. State educational institution Gymnasium No. 363 of the Frunzensky District. A mechanical reed frequency meter is a device for measuring vibration frequency.

"Oscillation frequency"- Sound waves. Let's think ???? Infrasound is used in military affairs, fishing, etc. Can sound propagate in gases, liquids, solids? What does the sound volume depend on? What does the pitch of the sound depend on? Sound speed. Ultrasound. In this case, the vibrations of the sound source are obvious.

"Mechanical vibrations"- Transverse. Spring pendulum graph. Oscillatory motion. Free. Longitudinal. "Oscillations and Waves". Harmonic. Free vibrations. Waves are the propagation of vibrations in space over time. Completed: student of grade 11 "A" Oleinikova Yulia. Forced vibrations. Waves. Mathematical pendulum.

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Slide captions:

Oscillatory circuit. Electromagnetic vibrations. The principle of radio communication and television Lesson number 51

Electromagnetic vibrations are periodic changes over time in electrical and magnetic quantities (charge, current, voltage, intensity, magnetic induction, etc.) in an electrical circuit. As you know, to create a powerful electromagnetic wave, which could be recorded by devices at large distances from the emitting antenna, it is necessary that the frequency of the wave is not less than 0.1 MHz.

One of the main parts of the generator is an oscillatory circuit - this is an oscillatory system consisting of a series-connected coil with inductance L, a capacitor with capacitance C and a resistor with resistance R.

After they invented the Leyden jar (the first capacitor) and learned how to impart a large charge to it using an electrostatic machine, they began to study the electric discharge of the jar. By closing the Leyden jar plates with a coil, we found that the steel spokes inside the coil were magnetized. The strange thing was that it was impossible to predict which end of the coil core would be the north pole and which would be the south. They did not immediately understand that when a capacitor is discharged through a coil, oscillations arise in an electrical circuit.

The period of free oscillations is equal to the natural period of the oscillating system, in this case the period of the circuit. The formula for determining the period of free electromagnetic oscillations was obtained by the English physicist William Thomson in 1853.

The Popov transmitter circuit is quite simple - it is an oscillatory circuit, which consists of an inductance (secondary winding of the coil), a powered battery and a capacitance (spark gap). If you press the key, then a spark jumps in the spark gap of the coil, causing electromagnetic oscillations in the antenna. The antenna is an open vibrator and emits electromagnetic waves, which, upon reaching the antenna of the receiving station, excite electrical oscillations in it.

To register the received waves, Alexander Stepanovich Popov used a special device - a coherer (from the Latin word "coherence" - cohesion), consisting of a glass tube containing metal filings. On March 24, 1896, the first words were transferred using Morse code - "Heinrich Hertz".

Although modern radios bear little resemblance to Popov's, the basic principles of their operation are the same.

Main conclusions: - An oscillatory circuit is an oscillatory system consisting of a coil, a capacitor and an active resistance connected in series. - Free electromagnetic oscillations are oscillations that occur in an ideal oscillatory circuit due to the expenditure of the energy imparted to this circuit, which is not replenished in the future. - The period of free electromagnetic oscillations can be calculated using the Thomson formula. - It follows from this formula that the period of the oscillating circuit is determined by the parameters of its constituent elements: the inductance of the coil and the capacitance of the capacitor. - Radio communication is the process of transmitting and receiving information using electromagnetic waves. - Amplitude modulation is the process of changing the amplitude of high-frequency oscillations with a frequency equal to the frequency of an audio signal. - The reverse process of modulation is called detection.

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