Lenz's rule law of electromagnetic induction abstract. Physics lesson notes_law of electromagnetic induction. Learning new material

Today we will reveal such a physics phenomenon as the “law of electromagnetic induction”. We will tell you why Faraday conducted the experiments, give the formula and explain the importance of the phenomenon for everyday life.

Ancient Gods and Physics

Ancient people worshiped the unknown. And now man is afraid of the abyss of the sea and the depths of space. But science can explain why. Submarines are documenting the incredible life of the oceans at depths of more than a kilometer, and space telescopes are studying objects that existed only a few million years after the big bang.

But then people deified everything that fascinated and worried them:

• Sunrise;
• awakening of plants in spring;
• rain;
• birth and death.

In every object and phenomenon lived unknown forces that ruled the world. Until now, children tend to humanize furniture and toys. Left without adult supervision, they fantasize: a blanket will hug you, a stool will fit, a window will open on its own.

Perhaps the first evolutionary step of humanity was the ability to maintain a fire. Anthropologists suggest that the earliest fires were lit by a tree struck by lightning.

Thus, electricity has played a huge role in the life of mankind. The first lightning gave impetus to the development of culture, the basic law of electromagnetic induction led humanity to the modern state.

From vinegar to a nuclear reactor

Strange ceramic vessels were found in the Cheops pyramid: the neck was sealed with wax, and a metal cylinder was hidden in the depths. Residues of vinegar or sour wine were found on the inside of the walls. Scientists have come to a sensational conclusion: this artifact is a battery, a source of electricity.

But until 1600, no one undertook to study this phenomenon. Before moving electrons, the nature of static electricity was studied. The ancient Greeks knew that amber produces shocks when rubbed against fur. The color of this stone reminded them of the light of the star Electra from the Pleiades. And the name of the mineral became, in turn, a reason to christen a physical phenomenon.

The first primitive direct current source was built in 1800

Naturally, as soon as a sufficiently powerful capacitor appeared, scientists began to study the properties of the conductor connected to it. In 1820, the Danish scientist Hans Christian Oersted discovered that the magnetic needle deviates near a conductor connected to the network. This fact gave impetus to the discovery of the law of electromagnetic induction by Faraday (the formula will be given below), which allowed humanity to produce electricity from water, wind and nuclear fuel.

Primitive but modern

The physical basis for Max Faraday's experiments was laid by Oersted. If a switched-on conductor affects a magnet, then the reverse is also true: the magnetized conductor must cause a current.

The structure of the experiment, which helped to derive the law of electromagnetic induction (we will consider EMF as a concept a little later), was very simple. The wire wound into a spring was connected to a device that records the current. The scientist brought a large magnet to the coils. While the magnet moved next to the circuit, the device recorded the flow of electrons.

Since then, the technology has improved, but the basic principle of creating electricity at huge stations is still the same: a moving magnet excites a current in a conductor wound by a spring.

Development of the idea

The very first experiment convinced Faraday that electric and magnetic fields are interconnected. But it was necessary to find out exactly how. Does a magnetic field also arise around a current-carrying conductor, or are they simply capable of influencing each other? Therefore, the scientist went further. He wound one wire, supplied a current to it, and pushed this coil into another spring. And I also received electricity. This experiment proved that moving electrons create not only an electric, but also a magnetic field. Later, scientists figured out how they are located in space relative to each other. The electromagnetic field is also the reason why light exists.

By experimenting with different options for the interaction of live conductors, Faraday found out that current is transmitted best if both the first and second coils are wound on one common metal core. The formula expressing the law of electromagnetic induction was derived precisely on this device.

Formula and its components

Now that the history of the study of electricity has been brought to Faraday’s experiment, it’s time to write the formula:

Let's decipher:

ε is electromotive force (abbreviated emf). Depending on the value of ε, electrons move more intensely or weakly in the conductor. The EMF is affected by the power of the source, and it is influenced by the strength of the electromagnetic field.

Φ is the magnitude of the magnetic flux that is currently passing through a given area. Faraday rolled the wire into a spring because he needed a certain space through which the conductor would pass. Of course, it would be possible to make a very thick conductor, but this would be expensive. The scientist chose the shape of a circle because this flat figure has the greatest ratio of surface area to surface length. This is the most energy efficient form. Therefore, water droplets on a flat surface become round. In addition, a spring with a round cross-section is much easier to obtain: you just need to wind the wire around some round object.

t is the time during which the flow passed through the circuit.

The prefix d in the formula for the law of electromagnetic induction means that the quantity is differential. That is, a small magnetic flux must be differentiated over short periods of time in order to obtain the final result. This mathematical operation requires some preparation from people. To better understand the formula, we strongly encourage the reader to review differentiation and integration.

Consequences from the law

Immediately after the discovery, they began to study the phenomenon of electromagnetic induction. Lenz's law, for example, was derived experimentally by a Russian scientist. It was this rule that added a minus to the final formula.

It looks like this: the direction of the induction current is not random; the flow of electrons in the second winding tends to reduce the effect of the current in the first winding. That is, the occurrence of electromagnetic induction is actually the resistance of the second spring to interference in “personal life”.

Lenz's rule has another consequence.

• if the current in the first coil increases, then the current of the second spring will also tend to increase;
• if the current in the inducing winding drops, then the current in the second winding will also decrease.

According to this rule, the conductor in which the induced current occurs actually tends to compensate for the effect of the changing magnetic flux.

Grain and donkey

People have long sought to use the simplest mechanisms for their own benefit. Grinding flour is a complicated matter. Some tribes grind the grain by hand: placing the wheat on one stone, covering it with another flat and round stone, and turning the millstone. But if you need to grind flour for an entire village, then you can’t do it with muscular labor alone. At first, people thought of tying a draft animal to a millstone. The donkey pulled the rope - the stone rotated. Then people probably thought: “The river flows all the time, it pushes all sorts of things downstream. Why don’t we use it for good?” This is how water mills appeared.

Wheel, water, wind

Of course, the first engineers who built these structures knew nothing about the force of gravity, due to which water always tends downward, or about the force of friction or surface tension. But they saw: if you put a wheel with blades in diameter in a stream or river, it will not only rotate, but will also be able to do useful work.

But this mechanism was also limited: not everywhere there is running water with sufficient current strength. So people moved on. They built mills that were powered by the wind.

Coal, fuel oil, gasoline

When scientists understood the principle of exciting electricity, a technical task was set: to produce it on an industrial scale. At that time (mid-nineteenth century) the world was gripped by machine fever. They tried to entrust all the difficult work to the expanding steam.

But then they could only heat large volumes of water with fossil fuels - coal and fuel oil. Therefore, those that were rich in ancient carbons immediately attracted the attention of investors and workers. And the redistribution of people led to the industrial revolution.

Holland and Texas

However, this state of affairs has a bad impact on the environment. And scientists wondered: how to get energy without destroying nature? The well-forgotten old thing came to the rescue. The mill used torque to directly perform rough mechanical work. Hydroelectric turbines rotate magnets.

Currently, the cleanest electricity comes from wind energy. The engineers who built the first generators in Texas relied on experience from the windmills of Holland.

Sections: Physics

Lesson objectives:

• educational: study the phenomenon of electromagnetic induction and the conditions for its occurrence; show cause-and-effect relationships when observing the phenomenon of electromagnetic induction; reveal the essence of the phenomenon when setting up experiments, study Lenz’s rule (rules for determining the direction of the induction current), explain the law of electromagnetic induction.
• developing: develop logical thinking and attention, the ability to analyze, compare the results obtained, draw appropriate conclusions, present the results of the work done, develop a general culture of speech, group work skills.
• educational: arouse interest in the topic being studied from the point of view of the profession being acquired, and promote independent acquisition of knowledge.

Lesson type: learning new material

Teaching methods: Method of problem presentation, partially search.

Forms of organization of cognitive activity:

• Group
• Frontal

Equipment: electronic board, presentation, multimedia course Physics: complete course. 7-11 grades (edited by V. Akopyan), strip magnet, connecting wires, galvanometer, milliammeter, coils, current source, key, wire coils, arc-shaped magnet, device for demonstration of Lenz's rule.

Lesson Plan

During the classes

1. Creation of a problem situation (long-term perspective)

Hello guys! The slide (Slide 1) of the presentation shows power line supports in different countries: in Finland, for example, in the form of deer. But the supports do not change the content: all power lines are designed to transmit electric current over long distances, and all power lines are high-voltage.

Why are all power lines high voltage?

(Students’ answers, as a rule, are “High voltage current is flowing”).

Why increase the voltage? (Slide 2). Look at the power transmission diagram: the transformer increases the already high voltage, but in everyday life, in the lighting network, only 220V is needed! So why increase the voltage? ( Students' answers)

While we were having a conversation with you, an electric current flowed through a coil of wire.

Demo 1: A coil of wire is fixed in the tripod leg, and an electric current is passed through it.

(Students’ answers, as a rule, are “The conductor through which current flows heats up. This is the thermal effect of current.”).

Well done, that's right! The current flowing through the power line heats the line (wire), and energy is lost: part of the electrical energy is converted into thermal energy. Thermal energy losses must be minimized. (Slide 3) Let's remember the Joule-Lenz law: you can reduce heat losses by reducing, for example, the current strength. A device that reduces the current and at the same time increases the voltage by the same amount (and vice versa), with virtually no loss of power, was invented in 1878 by the Russian scientist P.N. Yablochkov and was called a transformer.

Let's summarize briefly: in order to reduce heat losses when transmitting electricity over long distances, it is necessary to reduce the current strength, and this role will be performed by a step-up transformer, but at the same time it will increase the voltage by the same amount. This is why all power lines are high voltage.

2. Creation of a problematic situation (short term)

But on what principle is the operation of a transformer based?

(Students find it difficult to answer)

His work is based on the phenomenon of electromagnetic induction, which was discovered by Michael Faraday in 1831 and is the greatest discovery of the 19th century. (Slide 4)

The principle of operation of induction furnaces (OMD, steelmaking) and logs, induction hobs (Technologist), metal detectors, transformers (Welder) and alternating current generators (Maintenance of electrical and electromechanical equipment) is based on this phenomenon. Your future profession (specialty) is inextricably linked with this phenomenon: without the electric current generated by generators on the ES, the operation of machine tools (Machine Operator), electromagnets (Crane Operator), electric furnaces and stoves (Technologist), etc. is impossible.

Demonstration 2. The skein is fixed in the tripod leg, an electric current is passed through it, and a magnet is brought in.

What effect of electric current can be seen?

(Students’ answers, as a rule, are “Magnetic. If current flows through a conductor, a magnetic field appears around the conductor.”). Well done!

Right. If an electric current generates a magnetic field, then couldn’t the magnetic field, in turn, generate an electric current?

In 1821, Michael Faraday was puzzled by this question. “Convert magnetism into electricity” was written in his diary. Ten years later, on August 29, 1831, this problem was solved.

Write down the topic of the lesson. PHENOMENON OF ELECTROMAGNETIC INDUCTION. LAZY RULE. LAW OF ELECTROMAGNETIC INDUCTION.

Let's experimentally establish under what conditions a magnetic field can generate an electric current in a conductor (circuit).

(Students perform experimental tasks in groups).

• Group 1: Appendix 1
• Group 2: Appendix 2
• Group 3: Appendix 3

Let's summarize the work of our groups:

1 group (Students' answers). (Slide 5) (answers from students in group 1 are supplemented by responses from students from other groups)

Conclusion: In a conductive closed circuit arises electricity , if the contour is in alternating magnetic field or moves in a time-constant field so that the number of magnetic induction lines penetrating the circuit changes.

From the history of the issue: Almost simultaneously with Faraday, the Swiss physicist Colladon tried to obtain an electric current in a coil using a magnet. When working, he used a galvanometer, the light magnetic needle of which was placed inside the coil of the device. To prevent the magnet from directly influencing the needle, the ends of the coil into which the magnet was inserted were brought into the next room and there connected to a galvanometer. Having inserted the magnet into the coil, Colladon went into the next room and was disappointed to see that the galvanometer did not show any current. If only he had been near the galvanometer all the time and asked someone to work on the magnet, a remarkable discovery would have been made. But this did not happen. A magnet at rest relative to the coil does not generate current in it.

Let us introduce the concept of magnetic flux. (Slide 6)

Magnetic flux is a physical quantity equal to the product of the magnitude of the magnetic induction vector B by the area S cosine of the angle? between vectors and

1 Wb = 1 T*1m 2

A magnetic flux of 1 Weber is created by a magnetic field with an induction of 1 T through a surface with an area of ​​1 m 2 located perpendicular to the magnetic induction vector.

The current that arises in a closed circuit when the magnetic flux passing through the circuit changes is called induced current.

2nd group (Students' answers).

Conclusion: The magnitude of the induction current depends (Slide 7)

• the strength of the induction current does not depend on the rate of change of magnetic induction, but on the rate of change flow magnetic induction (from the rate of change of magnetic flux)
• on the number of turns in the circuit

General conclusion of the work of groups 1 and 2:

The phenomenon of the appearance of an induced current in a closed circuit when the magnetic flux penetrating the circuit changes is called the phenomenon of electromagnetic induction.

3 group (Students' answers). (Slide 8). Lenz's rule.

Investigating the phenomenon of electromagnetic induction, E. H. Lenz in 1833 established a general rule for determining the direction of the induction current:

The induced current arising in a closed circuit with its magnetic field counteracts the change in the magnetic flux that caused it.

Direction of induction current.

Right hand rule

If the right hand is positioned so that vector B enters the palm, and the thumb bent by 90° is directed along the movement of the conductor, then the four fingers of the hand will indicate the direction of the induction current to the conductor.

When explaining the material, you can use the multimedia course Physics: complete course. Grades 7-11 (edited by V. Akopyan) (lesson “The phenomenon of electromagnetic induction”)

Law of Electromagnetic Induction

It is known that an electric current appears in a circuit when external forces act on the free charges of a conductor. The work of these external forces when moving a single positive charge along a closed loop is called electromotive force. Consequently, when the magnetic flux changes, through the surface limited by the contour, extraneous forces appear in the latter, the action of which is characterized by an emf, called induced emf.

~ and =, then = - for 1 turn = * N- for N turns

According to Lenz's rule:

= - *N - for N turns

The induced emf in a closed loop is equal in magnitude to the rate of change of the magnetic flux through the surface bounded by the loop.

Guys, today we got acquainted with the phenomenon of electromagnetic induction (EMI). The operation of many devices is based on this phenomenon; a special role should be given to alternating current generators, in which mechanical energy is converted into electrical energy. Without electric current, it is almost impossible to imagine the life of a modern person, as well as your future work: induction hobs - Technologist, induction furnaces - OMD, transformer - Welder, etc.

Let's summarize the lesson and answer the questions:

Questions:

1. What is the phenomenon of electromagnetic induction?

2. What is magnetic flux called?

3. How is the work of a machine operator (crane operator, locomotive driver, etc.) related to the EMP phenomenon?

4. Why is the law of electromagnetic induction formulated for emf, and not for current? Formulate the law of EMR.

5. Why is there a minus sign in the law of electromagnetic induction?

6. How to determine the direction of the induction current?

Today we worked fruitfully, conducted experiments, guys, evaluate the work of each group: the work of your group and the work of students in other groups.

(Discussion, student dialogue)

3. Homework:

8-11, abstract, p. 27 (give examples of the occurrence of induction current using two coils on a common core), prepare messages (Metal detectors, magnetic levitation train, induction furnaces, induction hobs).

Chain:

As usual, we leave the class along the “chain” (it is necessary to name the physical quantity and the units of measurement of the physical quantity).

Annex 1

Appendix 2

Appendix 3

In 1831, the English physicist M. Faraday discovered the phenomenon in his experiments electromagnetic induction. Then the Russian scientist E.Kh. studied this phenomenon. Lenz and B. S. Jacobi.

Currently, many devices are based on the phenomenon of electromagnetic induction, for example in a motor or electric current generator, in transformers, radio receivers, and many other devices.

Electromagnetic induction- this is the phenomenon of the occurrence of current in a closed conductor when a magnetic flux passes through it. That is, thanks to this phenomenon, we can convert mechanical energy into electrical energy - and this is wonderful. After all, before the discovery of this phenomenon, people did not know about methods of producing electric current, except for galvanization.

When a conductor is exposed to a magnetic field, an emf arises in it, which can be quantitatively expressed through the law of electromagnetic induction.

Law of Electromagnetic Induction

The electromotive force induced in a conducting circuit is equal to the rate of change of the magnetic flux coupling to that circuit.

In a coil that has several turns, the total emf depends on the number of turns n:

But in the general case, the EMF formula with general flux linkage is used:

The EMF excited in the circuit creates a current. The simplest example of the appearance of current in a conductor is a coil through which a permanent magnet passes. The direction of the induced current can be determined using Lenz's rules.

Lenz's rule

The current induced when the magnetic field passing through the circuit changes, its magnetic field prevents this change.

In the case when we introduce a magnet into the coil, the magnetic flux in the circuit increases, which means that the magnetic field created by the induced current, according to Lenz’s rule, is directed against the increase in the magnet’s field. To determine the direction of the current, you need to look at the magnet from the north pole. From this position we will screw the gimlet in the direction of the magnetic field of the current, that is, towards the north pole. The current will move in the direction of rotation of the gimlet, that is, clockwise.

In the case when we remove the magnet from the coil, the magnetic flux in the circuit decreases, which means the magnetic field created by the induced current is directed against the decrease in the magnet's field. To determine the direction of the current, you need to unscrew the gimlet; the direction of rotation of the gimlet will indicate the direction of the current in the conductor - counterclockwise.