Presentation “Oscillating circuit. Electromagnetic vibrations. The principle of radio communications and television” presentation for a physics lesson (9th grade) on the topic. Oscillatory circuit Presentations on physics 9th grade oscillatory circuit

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Oscillatory circuit. Electromagnetic vibrations. The principle of radio communications and television Lesson No. 51

Electromagnetic oscillations are periodic changes over time in electrical and magnetic quantities (charge, current, voltage, tension, magnetic induction, etc.) in an electrical circuit. As is known, in order to create a powerful electromagnetic wave that could be recorded by instruments at large distances from the emitting antenna, it is necessary that the wave frequency be at least 0.1 MHz.

One of the main parts of the generator is the oscillatory circuit - this is an oscillatory system consisting of a coil of inductance L connected in series, a capacitor with a 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 linings of a Leyden jar with a coil, they discovered 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 the south. It was not immediately understood that when a capacitor is discharged through a coil, oscillations occur in the electrical circuit.

The period of free oscillations is equal to the natural period of the oscillatory 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 circuit of Popov's transmitter is quite simple - it is an oscillatory circuit, which consists of inductance (the secondary winding of the coil), a powered battery and a capacitance (spark gap). If you press the key, 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 transmitted using Morse code - “Heinrich Hertz”.

Although modern radio receivers bear very little resemblance to Popov's receiver, 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 energy imparted to this circuit, which is not subsequently replenished. – The period of free electromagnetic oscillations can be calculated using Thomson’s formula. – From this formula it follows that the period of the oscillatory 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 the sound signal. – The reverse process of modulation is called detection.

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

“Mechanical resonance” - 1. Chain of the Egyptian Bridge in St. Petersburg. Resonance in technology. 3. Mexico City 1985 Tacoma Suspension Bridge. Positive resonance value Frequency meter. 2. State educational institution Gymnasium No. 363 of the Frunzensky district. Mechanical reed frequency meter is a device for measuring vibration frequency.

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

“Mechanical vibrations” - Transverse. Graph of a spring pendulum. Oscillatory movement. Free. Longitudinal. "Vibrations and Waves." Harmonic. Free vibrations. Waves are the propagation of vibrations in space over time. Completed by: 11th grade student “A” Yulia Oleynikova. Forced vibrations. Waves. Mathematical pendulum.

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Lesson objectives:

educational: 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 oscillations in an ideal circuit are harmonic; 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 information technologies; developing skills to assess the reliability of natural science information;
educational: fostering confidence in the possibility of knowing the laws of nature; using the achievements of physics for the benefit of the development of human civilization; the need for cooperation in the process of jointly performing tasks, readiness for a moral and ethical assessment of the use of scientific achievements, and a sense of responsibility for protecting the environment.

During the classes

I. Organizational moment.

In today's lesson we begin to study a new chapter of the textbook and the topic of today's lesson is “Electromagnetic oscillations. Oscillatory circuit.”

II. Checking homework.

Let's start our lesson by checking your homework.

Slide 2. Test for reviewing the material and the 10th grade course.

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

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

2. Why does the neon lamp not flash when the SA1 key is closed, no matter what position the SA2 switch is in?

The test is performed on a computer. One of the students, meanwhile, is assembling a diagram.

Answer. The neon lamp flashes in the second position of switch SA2: after switch SA1 is opened, 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 maintains 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 connected 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 (self-induction emf), sufficient to maintain a gas discharge in the lamp.

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

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

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

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

Lesson Objectives

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

Let us first remember what properties a system must have in order for free oscillations to occur in it.

(In the oscillatory system, a restoring force should arise and energy should be converted from one type to another; the friction in the system should be quite small.)

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

What oscillations are called free oscillations? (oscillations that occur in a system after it is removed from an equilibrium position) What oscillations are called forced oscillations? (oscillations occurring under the influence of external periodically changing EMF)

Periodic or nearly 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 jars. By closing the linings of a Leyden jar using a wire coil, they discovered that the steel spokes inside the coil were magnetized, but it was impossible to predict which end of the coil core would be the north pole and which end would be the south pole. A significant role in the theory of electromagnetic oscillations was played by the 19th century German scientist HELMHOLTZ Hermann Ludwig Ferdinand. He is called the first doctor among scientists and the first scientist among doctors. He studied physics, mathematics, physiology, anatomy and psychology, achieving worldwide recognition in each of these areas. Drawing attention to the oscillatory nature of the Leyden jar discharge, in 1869 Helmholtz showed that similar oscillations occur in an induction coil connected to a capacitor (i.e., essentially, he created an oscillatory circuit consisting of inductance and capacitance). These experiments played a major role in the development of the theory of electromagnetism.

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

Slide 4. Currently, electronic oscilloscopes have been replaced by digital ones. He will tell us about the principles of their operation...

Slide 5. Animation “Oscilloscope”

Slide 6. But let's return to electromagnetic oscillations. The simplest electrical system capable of free oscillations is a series RLC circuit. An oscillatory circuit is an electrical circuit consisting of a series-connected capacitor with electrical capacity C, a coil with inductance L and 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 connect a galvanometer to the coil. Let's observe the behavior of the galvanometer needle after moving the switch from position 1 to position 2. You notice that the needle begins to oscillate, but these oscillations soon die out. All real circuits contain electrical resistance R. During 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 oscillations occur in an oscillatory circuit?

Let's consider the case when resistance R=0 (model of an ideal oscillatory circuit). What processes occur in the oscillatory circuit?

Slide 7. Animation “Oscillating circuit”.

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

Consider a serial RLC circuit. When switch K 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 resistor R and inductor L. Under certain conditions, this process can have an oscillatory nature.

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

where is the voltage on the capacitor, q is the charge of the capacitor, – current in the circuit. On the right side of this relationship is the self-induction emf 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's consider the case when there are no losses of electromagnetic energy in the circuit (R = 0). Let us 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 oscillations of a load on a spring or thread in the absence of friction forces.

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

In the absence of damping, free oscillations in an 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 charge oscillations q m and the initial phase 0 are determined by the initial conditions, that is, by the way in which the system was brought out of equilibrium. In particular, for the oscillation process that 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 current also performs harmonic oscillations:

Slide 9. Where is the amplitude of current fluctuations. The current oscillations are ahead of the charge oscillations in phase.

With free oscillations, there is a periodic conversion 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. Parameters L and C of the oscillatory circuit determine only the natural frequency of free oscillations

Considering that , we get .

Slide 9. Formula called Thomson's 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 using an iron core, and a variable capacitor. Let's first remember how you can change the capacitance of such a capacitor. Let me remind you that this is 10th grade course material.

A variable capacitor consists of two sets of metal plates. When the handle is rotated, the plates of one set fit into the spaces 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 capacitor, which means the capacity of the entire capacitor bank will increase. When capacitors are connected in series in a battery, 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 electrical oscillations and oscillations of a load on a spring. Open page 85 of the textbook, Figure 4.5.

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

On your desks there is a table that we filled out when studying the topic “Mechanical vibrations”. Appendix 2.

You have completed one row of this table. Using Figure 2, paragraph 29 of the textbook and Figure 4.5 on page 85 of the textbook, fill in the remaining rows of the table.

How are the processes of free electrical and mechanical vibrations similar? Let's watch 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 allow us to draw a conclusion about the analogy between electrical and mechanical quantities.

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

The same table is available on your desks and in your textbook on page 86.

So, we have considered the theoretical part. Was everything clear to you? Maybe someone has questions?

Now let's move on to solving problems.

IV. Physical education minute.

V. Consolidation of the studied material.

Problem solving:

problems 1, 2, problems of part A No. 1, 6, 8 (orally);
problems No. 957 (answer 5.1 μH), No. 958 (answer will decrease by 1.25 times) (at the board);
task part B (orally);
task No. 1 of part C (at the board).

The problems are taken from the collection of problems for grades 10-11 by A.P. Rymkevich and appendices 10. Appendix 4.

VI. Reflection.

Students fill out a reflective card.

VII. Summing up the lesson.

Were the lesson objectives achieved? Summing up the lesson. Student assessment.

VIII. Homework assignment.

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

Literature:

Multimedia physics course “Open Physics” version 2.6 edited by MIPT professor S.M. Goat.
Problem book for grades 10-11. A.P. Rymkevich, Moscow “Enlightenment”, 2012.
Physics. Textbook for 11th grade of general education institutions. G.Ya.Myakishev, B.B. Bukhovtsev, V.M. Charugin. Moscow “Enlightenment”, 2011.
Electronic supplement to the textbook by G.Ya. Myakishev, B.B. Bukhovtseva, V.M. Charugina. Moscow “Enlightenment”, 2011.
Electromagnetic induction. Qualitative (logical) tasks. 11th grade, 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 administered in writing.

There are fluctuations

mechanical, electromagnetic, chemical, thermodynamic

and various others. Despite such diversity, they all have much 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 .

Types of oscillatory

systems

Mathematical

pendulum

Spring

pendulum

Types of oscillatory

systems

Mathematical

pendulum

Spring

pendulum

Oscillatory

Circuit

Shock absorber operating diagram

Schematic representation of types of oscillatory systems

Math pendulum

Spring pendulum

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

According to the nature of the processes causing oscillatory movements

Types of oscillatory

movement

Available

Forced

The oscillatory system is left to its own devices, damped oscillations occur due to the initial energy reserve.

Oscillations occur due to external, periodically changing forces.

Free oscillations are oscillations in a system that occur after it is removed from a state 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 an additional charge to the capacitor.

Origin of EMF: electrons moving along with the conductors of the frame are acted upon by a force from the magnetic field, causing a change in the magnetic flux and, accordingly, the induced emf.