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ELECTROMAGNETIC FIELD

Lesson objectives: Give the concept of an electromagnetic field, explain the properties of an electromagnetic field

Course of the lesson Frontal survey Problem solving 3. History reference 4. The concept of an electromagnetic field 5. Consolidation of the material 6. Homework

1. Frontal poll 1) What current is called variable? Answer: alternating current is called electricity, which periodically changes with time in absolute value and direction.

2) What is the standard AC frequency in Russia? ν = 50 Hz

3) Where is alternating current used? Answer: alternating electric current is mainly used by electromechanical induction generators, i.e. in which mechanical energy is converted into electrical energy.

2. Problem Solving 1) The rotor of a two-pole AC machine makes 120 revolutions per minute. Determine the period of current oscillation.

Given: Solution: N= 120 T= t/N t=1min T=60s/120 rpm=0.5s T-? Answer: T \u003d 0.5 s.

2) According to the graph, determine the amplitude, period and frequency of oscillations

Answer: X m \u003d 0.1m T \u003d 1 with ν \u003d 1 Hz

3. Historical background The history of the magnet has more than two and a half thousand years. In the VI century BC. Ancient Chinese scientists discovered a mineral that can attract iron objects to itself.

In ancient times, they tried to explain the properties of a magnet by attributing a "living soul" to it. The magnet, according to the ancient people, “rushed to the iron for the same reason as a dog to a piece of meat”

Now we know: Every magnet has a magnetic field around it.

In 1808, a ship half-destroyed by lightning barely reached the berth of one of the French ports under its own power. A commission boarded it, which included Francois Argo, a brilliant scientist who became an academician at the age of 23. Argo noticed that the arrows of all the compasses were remagnetized as a result of a lightning strike. But Argo failed to draw a conclusion about the connection between electricity and magnetism.

Hans Christian Oersted on February 15, 1820 established: a magnetic needle located near a conductor rotates through a certain angle when current is passed. When the circuit is opened, the arrow returns to its original position.

ERSTED Hans Christian

Oersted's experience made it possible to conclude that there is a magnetic field in the space surrounding a conductor with an electric current.

1820 Ampere suggested that the magnetic properties of permanent magnets are due to the many circular currents circulating inside the molecules of these bodies.

The experiments of Oersted and Ampère, which proved the connection between electricity and magnetism, aroused in the young Faraday a deep interest in electromagnetism. Not surprisingly, already in 1821. Faraday writes in his diary as a task: "Turn magnetism into electricity"

1831 Michael Faraday Discovers the phenomenon of electromagnetic induction. What is the phenomenon of electromagnetic induction?

With any change in the magnetic flux penetrating the circuit of a closed conductor, an induction current arises in this conductor.

Induction current is the current that occurs in an alternating magnetic field penetrating closed loop conductor, creating an electric field in it, under the action of which a current arises.

Faraday proved that an alternating magnetic field penetrating a closed loop of a conductor created an electric field in it, under the action of which an induction current arose.

In 1831, James Clark Maxwell was born in England, who in 1865 introduced the concept of the electromagnetic field into physics.

Theoretically, he proved. Any change in the magnetic field with time leads to the appearance of an alternating electric field, and any change in the electric field with time generates an alternating magnetic field

These alternating electric and magnetic fields that generate each other form a single electromagnetic field. The sources of the electromagnetic field are rapidly moving electric charges

Indeed, electric and magnetic fields arise around electric charges, and the electric field exists in any frame of reference, and the magnetic field exists in the one relative to which the charges move.

Around charges moving at a constant speed, a constant magnetic field is created (for example, around a conductor with a direct current flowing through it).

But if electric charges move with acceleration or oscillate, then the electric field created by them changes periodically. An alternating electric field creates an alternating magnetic field in space, which, in turn, generates an alternating electric field, and so on.

It is impossible to create an alternating magnetic field without simultaneously generating an electric field in space. Conversely, an alternating electric field cannot exist without a magnetic one.

An alternating electric field is called a vortex field, since its lines of force are closed like the lines of induction of a magnetic field.

An electrostatic field (i.e. a constant field that does not change with time) that exists around stationary charged bodies. Electrostatic field lines start at positive charges and end at negative charges.

Which figure shows the vortex and electrostatic fields?

Electrostatic field Vortex electric field

The theory created by Maxwell, which made it possible to predict the existence of an electromagnetic field 22 years before it was discovered experimentally, is considered the greatest of scientific discoveries, the role of which in the development of science and technology can hardly be overestimated.

5. Fixation of material Electromagnetic field properties

Properties of the electromagnetic field The magnetic field is generated only by moving charges, in particular by electric current; The sources of the electromagnetic field are rapidly moving electric charges; The magnetic field is detected by the action on the magnetic needle.

Homework §51. answer questions 1-4

Properties of lines of force: Properties of lines of force: Start on positive charges, Start on positive charges, end on negative ones; end in negative The denser the lines of force are, the denser the lines of force are, the greater the tension. the more tension.

B N E Electromagnetic induction An alternating magnetic field creates an alternating electric field. The faster the magnetic field induction changes, the greater the electric field strength. An alternating electric field is called a vortex field, since its lines of force are closed like the lines of induction of a magnetic field. 0

E B E E V V V The electromagnetic field is a combination of two inseparably connected with each other, mutually generating each other changing fields: an alternating electric and an alternating magnetic field. The source of the electromagnetic field are charges moving with acceleration.

electromagnetic fields cell phones The most harmful is the high-frequency radiation of the centimeter range. Facilities mobile communications work so far at the very beginning of this range, but gradually the operating frequency increases. The effect of electromagnetic fields on the human body is manifested in a functional disorder of the central nervous system. Subjective sensations in this case are increased fatigue, drowsiness or, conversely, sleep disturbance, headaches, etc. With systematic exposure, persistent neuropsychiatric diseases, changes in blood pressure, and slowing of the pulse are observed.

Safety Tips: Don't talk on mobile phone for a long time, and not at all for reasons tariff plan; do not bring the phone to your head immediately after pressing the start dialing button, because. at this moment, electromagnetic radiation is several times greater than during the conversation itself; beware of being near the provider's repeater antenna for a long time, since it constantly, and in all directions, emits a sufficiently powerful signal; when choosing a phone model, give preference to devices with external antennas and good sensitivity declared in certificates.

Is electromagnetic radiation harmful to health? At the moment, science has not quantitatively proven a direct relationship between the level of electromagnetic fields and oncological and other types of morbidity. However, such a relationship can be traced qualitatively: in places where people are exposed to electromagnetic radiation, cancers and disorders of the cardiovascular and autonomic nervous systems are more often detected. The most sensitive nervous system and organs of vision.

Artificial electromagnetic fields are harmful to everyone, but especially to high-risk groups: children, pregnant women, people with diseases of the central nervous, hormonal, cardiovascular systems, and allergy sufferers. It is not bad to put an air ionizer in the apartment - it reduces the effects of electrostatic fields. Home flowers - begonias and violets - also saturate the air with very useful ions. A low-frequency electromagnetic field is formed around operating microwave ovens, which decreases to a safe level within a radius of at least 0.5 m. Televisions emit an electromagnetic field in all directions, even in standby mode. Therefore, at night it is better to turn them off from the network.

Receiver elements: 1.Power supply. Supplies energy to the circuit. 2. Antenna. Receives electromagnetic waves. 3. Grounding. Increases the range of receiving waves. 4. Coherer. Controls the current in the receiver circuit. 5. Call. Registers received electromagnetic waves. Provides automatic reception of waves.

Improvement of the receiver On May 7, 1895, at a meeting of the Russian Physical and Chemical Society, A.S. Popov demonstrated the world's first radio receiver. 10 months later, on March 24, 1896, A. S. Popov transmitted the world's first radiogram of their two words "Heinrich Hertz" at a distance of 250 m. next year the wireless range has been increased to 5 km. In 1899, he designed a receiver for receiving signals by ear using a telephone receiver. In 1897 A.S. Popov conducted the reception of radio waves from thunderclouds. The reception range was 30 km. Thunderstorm indicator A.S. Popova

In 1900, A. S. Popov made contact in the Baltic Sea at a distance of over 45 km between the islands of Hogland and Kutsalo, not far from the city of Kotka. This world's first practical wireless communication line served a rescue expedition to remove the battleship "General-Admiral Apraksin" from the stones, which had landed on the stones off the southern coast of Gogland. The first radiogram, transmitted by A. S. Popov to the island of Gogland on February 6, 1900, contained an order to the icebreaker "Ermak" to go to the aid of fishermen carried away on an ice floe into the sea. The icebreaker complied with the order and 27 fishermen were rescued. The world's first practical line, which began its work by rescuing people carried away at sea, clearly proved the advantages of this type of communication. Battleship General-Admiral Apraksin. Behind the icebreaker "Ermak".

In order to perpetuate the memory of A. S. Popov, a gold medal named after A. S. Popov was established, awarded annually for outstanding works and inventions in the field of radio. The fatherland appreciated the merits of the brilliant inventor and patriotic scientist before the motherland. In 1945, the 50th anniversary of the invention of radio was widely celebrated in our country. The anniversary was celebrated on May 7 on the day when A. S. Popov publicly demonstrated his invention for the first time. In this regard, the government has established May 7 - Radio Day.

Propagation of radio waves Radar questions: 1. Definition of radio waves. 2. Types of radio waves. Wavelength range. 3. Due to what phenomena do radio waves propagate? Explain with a drawing. 4. Definition of radar. 5. What phenomenon is radar based on?

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Presentation on the topic: Electromagnetic field

slide number 1

Description of the slide:

The electromagnetic field is alternating electric and magnetic fields that generate each other. The electromagnetic field theory was created by James Maxwell in 1865. He theoretically proved that: Any change in the magnetic field over time leads to the emergence of a changing electric field, and any change in the electric field over time generates a changing magnetic field. If electric charges move with acceleration, then the electric field created by them periodically changes and itself creates an alternating magnetic field in space, etc.

slide number 2

Description of the slide:

The sources of the electromagnetic field can be a moving magnet; - an electric charge moving with acceleration or oscillating (unlike a charge moving at a constant speed, for example, in the case of a direct current in a conductor, a constant magnetic field is created here).

slide number 3

Description of the slide:

Conditions for the existence of fields An electric field always exists around an electric charge, in any reference frame, a magnetic field exists in the one relative to which electric charges move, an electromagnetic field exists in a reference frame relative to which electric charges move with acceleration.

slide number 4

Description of the slide:

TRY SOLUTION! A piece of amber was rubbed against a cloth and charged with static electricity. What field can be found around immobile amber? Around a moving? A charged body is at rest relative to the surface of the earth. The car moves uniformly and rectilinearly relative to the surface of the earth. Is it possible to detect a constant magnetic field in the reference frame associated with the car? What field arises around an electron if it: is at rest; moving at a constant speed; moving with acceleration?

slide number 5

Description of the slide:

slide number 6

Description of the slide:

Properties of electromagnetic waves: - propagate not only in matter, but also in vacuum; - propagate in vacuum at the speed of light (C \u003d 300,000 km / s); - these are transverse waves; - these are traveling waves (transfer energy). The source of electromagnetic waves are rapidly moving electric charges. Oscillations of electric charges are accompanied by electromagnetic radiation having a frequency equal to the frequency of charge oscillations. Abstracts 1. Radio waves2. Infrared radiation3. Visible light 4. Ultraviolet radiation5. X-ray radiation6. Gamma radiation

Description of the slide:

INTERESTING WHAT... Reinforced concrete houses shield external "street" electromagnetic fields, so inside such a house the influence of external fields is not felt. There are many electrical appliances in our homes today. All of them create electromagnetic fields during operation. Even a turned-on iron is surrounded by an electromagnetic field within a radius of about 25 cm, an electric kettle has an electromagnetic field twice as wide. The electromagnetic field of an ordinary electric razor is strong enough, so an electric razor is good only for short-term use. field (moreover, color - to a greater extent than black and white), but at a distance of 1.5 meters from it, the electromagnetic background becomes already safe. When using a working microwave oven, it is safe to stay at a distance of 1-1.5 meters from it, although turning on the oven should also be quite short. It is safe to sit at arm's length in front of the screen.

Electromagnetic field Electromagnetic waves

Grade 9

Michael Faraday 1791-1867 In 1831, he discovered the phenomenon of electromagnetic induction - the occurrence of an electric current in a conductor when the magnetic flux changes through the conductor circuit.

What forces cause the charges in the loop to move? The magnetic field itself, penetrating the coil, cannot do this, because. the magnetic field acts exclusively on moving charges, and the conductor with the electrons in it is motionless.

James Clerk Maxwell 1831-1879 The greatest scientific achievement in 1865 is the theory of the electromagnetic field he created, which he formulated as a system of several equations expressing all the basic laws of electromagnetic phenomena.

Fundamental field property: Any change in the magnetic field over time results in an alternating electric field, and any change over time in the electric field generates an alternating magnetic field.

The source of a single electromagnetic field is fast moving electric charges

The mechanism of occurrence of induction current

The resulting vortex electric field, under the influence of which free charges, always present in the conductor, come into directional motion. The galvanometer plays the role of an indicator, detecting an electric field in space (electric current).

From Maxwell's theory follows the conclusion: A rapidly changing electromagnetic field propagates in space in the form of transverse waves.

James Maxwell based on theory:

• Waves propagate not only in matter, but also in vacuum. Wave propagation speed in vacuum is 300,000 km/s.
• Waves propagate not only in matter, but also in vacuum.
• Wave propagation speed in vacuum is 300,000 km/s.

• An electromagnetic wave is a system of generating each other and propagating in space electric and magnetic fields

Characteristic of the electric field - strength ()

The strength of the electric field at any point is equal to the ratio of the force , with which the field acts on a point positive charge placed at this point, to the value of this charge q.

The characteristic of the magnetic field is the vector of magnetic induction (

For electromagnetic waves, the same relationships between the wavelength and its speed are valid

with = 3 10 8 m / s, period T and frequency ν, as for mechanical waves. λ= = with T

Heinrich Rudolf Hertz 1857-1894 In 1888 he experimentally proved the existence of electromagnetic waves predicted by Maxwell. Established that the speed of propagation of electromagnetic waves is equal to the speed of light

slide 2

Electromagnetic field theory

According to Maxwell's theory, alternating electric and magnetic fields cannot exist separately: a changing magnetic field generates an electric field, and a changing electric field generates a magnetic one.

slide 3

Is it true that at a given point in space there is only an electric or only a magnetic field?

A charge at rest creates an electric field. But the charge is at rest only with respect to a certain frame of reference. Relative to others, it can move and, therefore, create a magnetic field. A magnet lying on a table creates only a magnetic field. But an observer moving relative to it will also detect an electric field

slide 4

The statement that at a given point in space there is only an electric or only a magnetic field is meaningless, if you do not specify in relation to which frame of reference these fields are considered.

Conclusion: electric and magnetic fields are a manifestation of a single whole: the electromagnetic field. The source of the electromagnetic field are rapidly moving electric charges.

slide 5

What is an electromagnetic wave?

What is the nature of an electromagnetic wave?

slide 6

Electromagnetic waves are the propagation in space over time of perturbations of the electromagnetic field.

The existence of electromagnetic waves was predicted by J. Maxwell, and only Heinrich Hertz managed to prove their existence in 1888.

Slide 7

Causes of electromagnetic waves

Imagine a conductor carrying an electric current. If the current is constant, then the magnetic field around the conductor will also be constant. When the current strength changes, the magnetic field will change: with an increase in current, this field will become stronger, with a decrease, weaker. An electromagnetic field will be disturbed. What will happen next?

Slide 8

An alternating magnetic field will create a changing electric field. This electric field will generate an alternating magnetic field. That, in turn, is again electrical, and so on. The perturbation of the electromagnetic field will begin to spread from its source (a conductor with alternating current), capturing more and more areas of space. This means that electromagnetic waves will appear in the space around the conductor.

Slide 9

Properties of electromagnetic waves:

electromagnetic waves are transverse; Electromagnetic waves can propagate not only in various media, but also in vacuum. The speed of electromagnetic waves in vacuum is denoted by the Latin letter c: c ≈ 300,000 km/s. The speed of electromagnetic waves in matter v is always less than in vacuum: v‹с

Slide 10

Electromagnetic waves are divided by wavelength (and, accordingly, by frequency) into six ranges:

Radio waves Infrared radiation (thermal) Visible radiation (light) Ultraviolet radiation X-rays γ - radiation

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