If changes magnetic field doesn't happen, then there won't be any electric current. Even if a magnetic field exists. We can say that the induced electric current is directly proportional, firstly, to the number of turns, and secondly, to the speed of the magnetic field with which this magnetic field changes relative to the turns of the coil.
Rice. 3. What does the magnitude of the induction current depend on?
To characterize the magnetic field, a quantity called magnetic flux is used. It characterizes the magnetic field as a whole; we will talk about this in the next lesson. For now, let us just note that it is the change magnetic flux, i.e. the number of magnetic field lines penetrating a current-carrying circuit (a coil, for example), leads to the appearance of an induction current in this circuit.
Physics. 9th grade
Topic: Electromagnetic field
Lesson 44 Magnetic flux
Eryutkin E.S., physics teacher highest category GOU secondary school No. 1360
Introduction. Faraday's experiments
Continuing our study of the topic “Electromagnetic induction”, let’s take a closer look at such a concept as magnetic flux.
You already know how to detect the phenomenon electromagnetic induction- if a closed conductor is crossed magnetic lines, an electric current arises in this conductor. This current is called induction.
Now let's discuss how this electric current is formed and what is important for this current to appear.
First of all, let's turn to Faraday's experiment and look again at its important features.
So, we have an ammeter, a coil with a large number turns, which is short-circuited to this ammeter.
We take a magnet, and just like in the previous lesson, we lower this magnet inside the coil. The arrow deviates, that is, there is an electric current in this circuit.
Rice. 1. Experience in detecting induction current.
But when the magnet is inside the coil, there is no electric current in the circuit. But as soon as you try to remove this magnet from the coil, an electric current appears in the circuit again, but the direction of this current changes to the opposite.
Please also note that the value of the electric current that flows in the circuit also depends on the properties of the magnet itself. If you take another magnet and do the same experiment, the value of the current changes significantly, in in this case the current becomes less.
After conducting experiments, we can conclude that the electric current that arises in a closed conductor (in a coil) is associated with a magnetic field permanent magnet.
In other words, the electric current depends on some characteristic of the magnetic field. And we have already introduced such a characteristic - magnetic induction.
Let us recall that magnetic induction is denoted by the letter, this is - vector quantity. And magnetic induction is measured in Tesla.
⇒ - Tesla - in honor of the European and American scientist Nikola Tesla.
Magnetic induction characterizes the effect of a magnetic field on a current-carrying conductor placed in this field.
But, when we talk about electric current, we must understand that electric current, and you know this from 8th grade, arises under the influence electric field.
Therefore, it can be concluded that the electrical induced current appears due to the electric field, which in turn is formed as a result of the action of the magnetic field. And this relationship is precisely achieved through magnetic flux.
If there is a closed conducting circuit in a magnetic field that does not contain current sources, then when the magnetic field changes, an electric current appears in the circuit. This phenomenon is called electromagnetic induction. The appearance of a current indicates the emergence of an electric field in the circuit, which can provide closed motion electric charges or, in other words, about the occurrence of EMF. The electric field that arises when the magnetic field changes and the work of which when moving charges along a closed circuit is not zero, has closed lines of force and is called a vortex field.
For quantitative description Electromagnetic induction introduces the concept of magnetic flux (or flux of the magnetic induction vector) through a closed loop. For a flat circuit located in a uniform magnetic field (and only such situations can schoolchildren encounter in a single state exam), magnetic flux is defined as
where is the field induction, is the contour area, is the angle between the induction vector and the normal (perpendicular) to the contour plane (see figure; the perpendicular to the contour plane is shown by a dotted line). Unit of magnetic flux in international system The SI unit of measurement is Weber (Wb), which is defined as the magnetic flux through a contour of an area of 1 m 2 of a uniform magnetic field with an induction of 1 T, perpendicular to the plane contour.
The magnitude of the induced emf that occurs in a circuit when the magnetic flux through this circuit changes is equal to the rate of change of the magnetic flux
|
Here is the change in magnetic flux through the circuit over a short time interval. Important property the law of electromagnetic induction (23.2) is its universality in relation to the reasons for changes in magnetic flux: the magnetic flux through the circuit can change due to a change in the magnetic field induction, a change in the area of the circuit or a change in the angle between the induction vector and the normal, which occurs when the circuit rotates in the field . In all these cases, according to law (23.2), an induced emf and an induced current will appear in the circuit.
The minus sign in formula (23.2) is “responsible” for the direction of the current resulting from electromagnetic induction (Lenz’s rule). However, it is not so easy to understand in the language of the law (23.2) to which direction of the induction current this sign will lead with a particular change in the magnetic flux through the circuit. But it’s quite easy to remember the result: the induced current will be directed in such a way that the magnetic field it creates will “tend” to compensate for the change in the external magnetic field that generated this current. For example, when the flux of an external magnetic field through a circuit increases, an induced current will appear in it, the magnetic field of which will be directed opposite to the external magnetic field so as to reduce the external field and thus maintain the original value of the magnetic field. When the field flux through the circuit decreases, the induced current field will be directed in the same way as the external magnetic field.
If the current in a circuit with current changes for some reason, then the magnetic flux through the circuit of the magnetic field that is created by this current itself also changes. Then, according to law (23.2), an induced emf should appear in the circuit. The phenomenon of the occurrence of induced emf in some electrical circuit as a result of a change in current in this circuit itself is called self-induction. To find Self-induced emf in some electrical circuit it is necessary to calculate the flux of the magnetic field created by this circuit through itself. This calculation is complex problem due to the inhomogeneity of the magnetic field. However, one property of this flow is obvious. Since the magnetic field created by the current in the circuit is proportional to the magnitude of the current, the magnetic flux of its own field through the circuit is proportional to the current in this circuit
|
where is the current strength in the circuit, is the proportionality coefficient, which characterizes the “geometry” of the circuit, but does not depend on the current in it and is called the inductance of this circuit. The SI unit of inductance is Henry (H). 1 H is defined as the inductance of such a circuit, the induction flux of its own magnetic field through which is equal to 1 Wb with a current strength of 1 A. Taking into account the definition of inductance (23.3) from the law of electromagnetic induction (23.2), we obtain for the self-induction EMF
|
Due to the phenomenon of self-induction, the current in any electrical circuit has a certain “inertia” and, therefore, energy. Indeed, to create a current in the circuit, it is necessary to do work to overcome the self-induction EMF. The energy of the current circuit is equal to this work. It is necessary to remember the formula for the energy of a current circuit
|
where is the inductance of the circuit, is the current strength in it.
The phenomenon of electromagnetic induction is widely used in technology. The creation of electric current in electric generators and power plants is based on it. Thanks to the law of electromagnetic induction, a transformation occurs mechanical vibrations in electric microphones. Based on the law of electromagnetic induction, it works, in particular, electrical circuit, which is called oscillatory circuit(see next chapter), and which is the basis of any radio transmitting or receiving equipment.
Let's now consider the tasks.
Of those listed in problem 23.1.1 phenomena, there is only one consequence of the law of electromagnetic induction - the appearance of a current in the ring when a permanent magnet is passed through it (answer 3 ). Everything else is the result of the magnetic interaction of currents.
As stated in the introduction to this chapter, the phenomenon of electromagnetic induction underlies the operation of the generator alternating current (problem 23.1.2), i.e. device that creates alternating current at a given frequency (answer 2 ).
The induction of the magnetic field created by a permanent magnet decreases with increasing distance to it. Therefore, when the magnet approaches the ring ( problem 23.1.3) the flux of the magnetic field of the magnet through the ring changes, and an induced current appears in the ring. Obviously, this will happen as the magnet approaches the ring with both the north and south poles. But the direction of the induction current in these cases will be different. This is due to the fact that when a magnet approaches the ring with different poles, the field in the plane of the ring in one case will be directed opposite to the field in the other. Therefore, to compensate for these changes external field the magnetic field of the induction current should be directed differently in these cases. Therefore, the directions of the induction currents in the ring will be opposite (answer 4 ).
For induced emf to occur in the ring, it is necessary that the magnetic flux through the ring changes. And since the magnetic induction of a magnet’s field depends on the distance to it, then in the considered problem 23.1.4 In this case, the flow through the ring will change, and an induced current will arise in the ring (answer 1
).
When rotating the frame 1 ( problem 23.1.5) the angle between the lines of magnetic induction (and, therefore, the induction vector) and the plane of the frame at any time equal to zero. Consequently, the magnetic flux through frame 1 does not change (see formula (23.1)), and the induced current does not arise in it. In frame 2, an induction current will arise: in the position shown in the figure, the magnetic flux through it is zero, when the frame turns a quarter turn it will be equal to , where is the induction and is the area of the frame. After another quarter turn, the flow will again be zero, etc. Therefore, the flux of magnetic induction through frame 2 changes during its rotation, therefore, an induced current appears in it (answer 2 ).
IN problem 23.1.6 induced current occurs only in case 2 (answer 2
). Indeed, in case 1, the frame, when moving, remains at the same distance from the conductor, and, therefore, the magnetic field created by this conductor in the plane of the frame does not change. When the frame moves away from the conductor, the magnetic induction of the conductor's field in the area of the frame changes, the magnetic flux through the frame changes, and an induced current appears
The law of electromagnetic induction states that an induced current will flow in a ring at times when the magnetic flux through the ring changes. Therefore, while the magnet is at rest near the ring ( problem 23.1.7) no induced current will flow in the ring. Therefore, the correct answer in this problem is 2 .
According to the law of electromagnetic induction (23.2), the induced emf in the frame is determined by the rate of change of the magnetic flux through it. And since by condition problems 23.1.8 the magnetic field induction in the frame area changes uniformly, the rate of its change is constant, the value of the induced emf does not change during the experiment (answer 3 ).
IN problem 23.1.9 The induced emf that occurs in the frame in the second case is four times greater than the induced emf that occurs in the first (answer 4 ). This is due to a fourfold increase in the frame area and, accordingly, the magnetic flux through it in the second case.
IN task 23.1.10 in the second case, the rate of change of the magnetic flux doubles (the field induction changes by the same amount, but in half the time). Therefore, the emf of electromagnetic induction that occurs in the frame in the second case is twice as large as in the first (answer 1 ).
When the current in a closed conductor doubles ( problem 23.2.1), the magnitude of the magnetic field induction will double at each point in space without changing in direction. Therefore, the magnetic flux through any small area and, accordingly, the entire conductor will change exactly twice (answer 1
). But the ratio of the magnetic flux through a conductor to the current in this conductor, which represents the inductance of the conductor 
, it will not change ( problem 23.2.2- answer 3
).
Using formula (23.3) we find in problem 32.2.3 Gn (answer 4 ).
The relationship between the units of magnetic flux, magnetic induction and inductance ( problem 23.2.4) follows from the definition of inductance (23.3): a unit of magnetic flux (Wb) is equal to the product of a unit of current (A) by a unit of inductance (H) - answer 3 .
According to formula (23.5), with a twofold increase in the inductance of the coil and a twofold decrease in the current in it ( problem 23.2.5) the energy of the magnetic field of the coil will decrease by 2 times (answer 2 ).
When the frame rotates in a uniform magnetic field, the magnetic flux through the frame changes due to a change in the angle between the perpendicular to the plane of the frame and the magnetic field induction vector. And since in both the first and second cases in problem 23.2.6 this angle changes according to the same law (according to the condition, the frequency of rotation of the frames is the same), then the induced emf changes according to the same law, and, therefore, the ratio of the amplitude values of the induced emf within the frame is equal to one (answer 2 ).
A magnetic field, generated by conductor with current in the frame area ( problem 23.2.7), directed “from us” (see solutions to problems in Chapter 22). The magnitude of the field induction of the wire in the area of the frame will decrease as it moves away from the wire. Therefore, the induced current in the frame should create a magnetic field directed inside the frame “away from us”. Using now the gimlet rule to find the direction of magnetic induction, we conclude that the induced current in the frame will be directed clockwise (answer 1 ).
As the current in the wire increases, the magnetic field it creates will increase and an induced current will appear in the frame ( problem 23.2.8). As a result, there will be an interaction between the induction current in the frame and the current in the conductor. To find the direction of this interaction (attraction or repulsion), you can find the direction of the induction current, and then, using the Ampere formula, the force of interaction between the frame and the wire. But you can do it differently, using Lenz's rule. All inductive phenomena must have such a direction as to compensate for the cause that causes them. And since the reason is an increase in current in the frame, the force of interaction between the induction current and the wire should tend to reduce the magnetic flux of the wire's field through the frame. And since the magnetic induction of the wire’s field decreases with increasing distance to it, this force will push the frame away from the wire (answer 2 ). If the current in the wire decreased, the frame would be attracted to the wire.
Problem 23.2.9 also related to the direction of induction phenomena and Lenz's rule. When a magnet approaches a conducting ring, an induced current will arise in it, and its direction will be such as to compensate for the cause that causes it. And since this reason is the approach of the magnet, the ring will be repelled from it (answer 2 ). If the magnet is moved away from the ring, then for the same reasons an attraction of the ring to the magnet would arise.
Problem 23.2.10 is the only computational problem in this chapter. To find the induced emf you need to find the change in magnetic flux through the circuit 
. It can be done like this. Let at some point in time the jumper be in the position shown in the figure, and let a small time interval pass. During this time interval, the jumper will move by an amount. This will lead to an increase in the contour area 
by the amount 
. Therefore, the change in magnetic flux through the circuit will be equal to , and the magnitude of the induced emf 
(answer 4
).
Physics teacher, Secondary School No. 58, Sevastopol, Safronenko N.I.
Lesson topic: Faraday's experiments. Electromagnetic induction.
Laboratory work “Study of the phenomenon of electromagnetic induction”
Lesson Objectives : Know/understand: definition of the phenomenon of electromagnetic induction. Be able to describe and explain electromagnetic induction,be able to make observations natural phenomena, use simple measuring instruments to study physical phenomena.
- developing: develop logical thinking, cognitive interest, observation.
- educational: To form confidence in the possibility of knowing nature,necessitywise use of scientific achievements for further development human society, respect for the creators of science and technology.
Equipment: Electromagnetic induction: a coil with a galvanometer, a magnet, a coil with a core, a current source, a rheostat, a coil with a core through which alternating current flows, a solid and a ring with a slot, a coil with a light bulb. Film about M. Faraday.
Lesson type: combined lesson
Lesson method: partially search, explanatory and illustrative
§21(pp.90-93), answer questions orally p.90, test 11 p.108
Laboratory work
Study of the phenomenon of electromagnetic induction
Goal of the work: to figure out
1) under what conditions does an induced current appear in a closed circuit (coil);
2) what determines the direction of the induction current;
3) what does the strength of the induction current depend on?
Equipment : milliammeter, coil, magnet
During the classes.
Connect the ends of the coil to the terminals of the milliammeter.
1. Find out what An electric current (induction) in a coil occurs when the magnetic field inside the coil changes. Changes in the magnetic field inside the coil can be caused by moving a magnet into or out of the coil.
A) Insert the magnet with the south pole into the coil and then remove it.
B) Insert the magnet with the north pole into the coil and then remove it.
When the magnet moves, does a current (induction) appear in the coil? (When the magnetic field changes, does an induced current appear inside the coil?)
2. Find out what the direction of the induction current depends on the direction of movement of the magnet relative to the coil (the magnet is added or removed) and on which pole the magnet is inserted or removed.
A) Insert the magnet with the south pole into the coil and then remove it. Observe what happens to the milliammeter needle in both cases.
B) Insert the magnet with the north pole into the coil and then remove it. Observe what happens to the milliammeter needle in both cases. Draw the direction of deflection of the milliammeter needle:
Magnet poles
To reel
From the reel
North Pole
3. Find out what the strength of the induction current depends on the speed of the magnet (the rate of change of the magnetic field in the coil).
Slowly insert the magnet into the coil. Observe the milliammeter reading.
Quickly insert the magnet into the coil. Observe the milliammeter reading.
Conclusion.
During the classes
The road to knowledge? She's easy to understand. You can simply answer: “You make mistakes and make mistakes again, but less, less each time. I hope that today's lesson will be one less on this road of knowledge. Our lesson is devoted to the phenomenon of electromagnetic induction, which was discovered by the English physicist Michael Faraday on August 29, 1831. Rare case, when the date of a new wonderful discovery is known so accurately!
The phenomenon of electromagnetic induction is the phenomenon of the occurrence of electric current in a closed conductor (coil) when the external magnetic field inside the coil changes. The current is called induction. Induction - guidance, receiving.
The purpose of the lesson: study the phenomenon of electromagnetic induction, i.e. under what conditions does an induction current appear in a closed circuit (coil); find out what determines the direction and magnitude of the induction current.
At the same time as studying the material, you will perform laboratory work.
At the beginning of the 19th century (1820), after the experiments of the Danish scientist Oersted, it became clear that electric current creates a magnetic field around itself. Let's remember this experience again. (A student tells Oersted's experiment ). After this, the question arose about whether it was possible to obtain current using a magnetic field, i.e. produce reverse actions. In the first half of the 19th century, scientists turned to just such experiments: they began to look for the possibility of creating an electric current due to a magnetic field. M. Faraday wrote in his diary: “Convert magnetism into electricity.” And I walked towards my goal for almost ten years. He coped with the task brilliantly. As a reminder of what he should always think about, he carried a magnet in his pocket. With this lesson we will pay tribute to the great scientist.
Let's remember Michael Faraday. Who is he? (A student talks about M. Faraday ).
Son of a blacksmith, newspaper delivery man, book binder, self-taught person who independently studied physics and chemistry from books, laboratory assistant outstanding chemist Devi and finally the scientist did great job, showed ingenuity, perseverance, and perseverance until he received an electric current using a magnetic field.
Let's take a trip to those distant times and reproduce Faraday's experiments. Faraday is considered the largest experimentalist in the history of physics.
N S
1) 2)
SN
The magnet was inserted into the coil. When the magnet moved in the coil, a current (induction) was recorded. The first scheme was quite simple. Firstly, M. Faraday used a coil with a large number of turns in his experiments. The coil was connected to a milliammeter device. It must be said that in those distant times there was not enough good tools for measuring electric current. Therefore, we used unusual technical solution: they took a magnetic needle, placed a conductor next to it through which current flowed, and by the deviation of the magnetic needle they judged the flow of current. We will judge the current based on the readings of the milliammeter.
Students reproduce the experience, perform step 1 in laboratory work. We noticed that the milliammeter needle deviates from its zero value, i.e. shows that a current appears in the circuit when the magnet moves. As soon as the magnet stops, the arrow returns to the zero position, i.e. there is no electric current in the circuit. Current appears when the magnetic field inside the coil changes.
We came to what we talked about at the beginning of the lesson: we received an electric current using a changing magnetic field. This is the first merit of M. Faraday.
The second merit of M. Faraday is that he established what the direction of the induction current depends on. We will establish this too.Students perform step 2 in laboratory work. Let's turn to point 3 of the laboratory work. Let's find out that the strength of the induction current depends on the speed of movement of the magnet (the rate of change of the magnetic field in the coil).
What conclusions did M. Faraday make?
Electric current appears in a closed circuit when the magnetic field changes (if the magnetic field exists but does not change, then there is no current).
The direction of the induction current depends on the direction of movement of the magnet and its poles.
The strength of the induction current is proportional to the rate of change of the magnetic field.
M. Faraday's second experiment:
I took two coils on a common core. I connected one to a milliammeter, and the second using a key to a current source. As soon as the circuit was closed, the milliammeter showed the induced current. When it opened, it also showed current. While the circuit is closed, i.e. there is current flowing in the circuit, the milliammeter did not show any current. The magnetic field exists, but does not change.
Let's consider modern version experiments of M. Faraday. We insert and remove an electromagnet and a core into a coil connected to a galvanometer, turn the current on and off, and use a rheostat to change the current strength. A coil with a light bulb is placed on the core of the coil through which alternating current flows.
Found out conditions occurrence of induction current in a closed circuit (coil). And what isreason its occurrence? Let us recall the conditions for the existence of electric current. These are: charged particles and electric field. The fact is that a changing magnetic field generates an electric field (vortex) in space, which acts on free electrons in the coil and sets them in directional motion, thus creating an induction current.
The magnetic field changes, the number of magnetic field lines through a closed loop changes. If you rotate the frame in a magnetic field, an induced current will appear in it.Show generator model.
The discovery of the phenomenon of electromagnetic induction had great value for the development of technology, for the creation of generators with the help of which Electric Energy, which are on energy industrial enterprises(power plants).A film about M. Faraday “From electricity to power generators” is shown from 12.02 minutes.
Transformers operate on the phenomenon of electromagnetic induction, with the help of which they transmit electricity without loss.A power line is on display.
The phenomenon of electromagnetic induction is used in the operation of a flaw detector, with the help of which steel beams and rails are examined (inhomogeneities in the beam distort the magnetic field and an induction current appears in the flaw detector coil).
I would like to remember the words of Helmholtz: “As long as people enjoy the benefits of electricity, they will remember the name of Faraday.”
“Let those be holy who, in creative fervour, exploring the whole world, discovered laws in it.”
I think that on our road of knowledge there are even fewer mistakes.
What new did you learn? (That current can be obtained using a changing magnetic field. We found out what the direction and magnitude of the induction current depends on).
What did you learn? (Receive induced current using a changing magnetic field).
Questions:
A magnet is pushed into the metal ring during the first two seconds, during the next two seconds it is motionless inside the ring, and during the next two seconds it is removed. At what time intervals does current flow in the coil? (From 1-2s; 5-6s).
A ring with or without a slot is put on the magnet. Where does induced current occur? (In a closed ring)
On the core of the coil, which is connected to an alternating current source, there is a ring. The current is turned on and the ring jumps. Why?
Board design:
"Turn magnetism into electricity"
M. Faraday
Portrait of M. Faraday
Drawings of M. Faraday's experiments.
Electromagnetic induction is the phenomenon of the occurrence of electric current in a closed conductor (coil) when the external magnetic field inside the coil changes.
This current is called induction current.
INDUCTION CURRENT is an electric current that occurs when the flux of magnetic induction changes in a closed conductive circuit. This phenomenon is called electromagnetic induction. Do you want to know which direction is the induction current? Rosinductor is a trading informational portal, where you will find information about current.
The rule determining the direction of the induction current sounds in the following way: “The induced current is directed so as to counteract with its magnetic field the change in magnetic flux that causes it.” Right hand palm turned towards magnetic power lines, wherein thumb directed in the direction of movement of the conductor, and four fingers indicate in which direction the induced current will flow. By moving a conductor, we move along with the conductor all the electrons contained in it, and when moving electric charges in a magnetic field, a force will act on them according to the left-hand rule.
The direction of the induction current, as well as its magnitude, is determined by Lenz’s rule, which states that the direction of the induction current always weakens the effect of the factor that excited the current. When the magnetic field flux through the circuit changes, the direction of the induced current will be such as to compensate for these changes. When a magnetic field exciting a current in a circuit is created in another circuit, the direction of the induction current depends on the nature of the changes: when the external current increases, the induction current has the opposite direction; when it decreases, it is directed in the same direction and tends to increase the flow.
An induction current coil has two poles (north and south), which are determined depending on the direction of the current: the induction lines come out of north pole. The approach of a magnet to a coil causes a current to appear in a direction that repels the magnet. When the magnet is removed, the current in the coil has a direction that favors the attraction of the magnet.
Induction current occurs in a closed circuit located in an alternating magnetic field. The circuit can be either stationary (placed in a changing flux of magnetic induction) or moving (the movement of the circuit causes a change in the magnetic flux). The occurrence of an induction current causes a vortex electric field, which is excited under the influence of a magnetic field.
You can learn how to create a short-term induction current from school course physics.
There are several ways to do this:
- - movement of a permanent magnet or electromagnet relative to the coil,
- - movement of the core relative to the electromagnet inserted into the coil,
- - closing and opening the circuit,
- - regulation of current in the circuit.
The basic law of electrodynamics (Faraday's law) states that the strength of the induced current for any circuit is equal to the rate of change of the magnetic flux passing through the circuit, taken with a minus sign. The strength of the induction current is called electromotive force.
As we have already found out, electric current can generate magnetic fields. The question arises: can a magnetic field cause the appearance of an electric current? This problem was solved English physicist Michael Faraday, who discovered the phenomenon of electromagnetic induction in 1831. A conductor wound into a coil is connected to a galvanometer (Fig. 3.19). If you slide a permanent magnet into the coil, the galvanometer will show the presence of current during the entire period of time while the magnet moves relative to the coil. When a magnet is pulled from a coil, the galvanometer indicates the presence of current. opposite direction. Changes in the direction of the current occur when the sliding or retractable pole of the magnet changes.
Similar results were observed when replacing a permanent magnet with an electromagnet (coil with current). If both coils are fixed motionless, but the current value in one of them is changed, then at that moment an induced current is observed in the other coil.
THE PHENOMENON OF ELECTROMAGNETIC INDUCTION consists in the appearance of an electromotive force (emf) of induction in a conductive circuit through which the flux of the magnetic induction vector changes. If the circuit is closed, then an induced current appears in it.
Discovery of the phenomenon of electromagnetic induction:
1) showed relationship between electric and magnetic field;
2) suggested method of producing electric current using a magnetic field.
Basic properties of induction current:
1. Induction current always occurs when there is a change in the magnetic induction flux associated with the circuit.
2. The strength of the induction current does not depend on the method of changing the flux of magnetic induction, but is determined only by the rate of its change.
Faraday's experiments established that the magnitude of the electromotive force of induction is proportional to the rate of change of the magnetic flux penetrating the conductor circuit (Faraday's law of electromagnetic induction)
Or , (3.46)
where (dF) is the change in flow over time (dt). MAGNETIC FLUX or FLUX OF MAGNETIC INDUCTION is a quantity that is determined based on the following relationship: ( magnetic flux through a surface of area S): Ф=ВScosα, (3.45), angle a – the angle between the normal to the surface under consideration and the direction of the magnetic field induction vector
unit of magnetic flux in the SI system it is called weber– [Wb=Tl×m2].
The “–” sign in the formula means that the emf. induction causes an induced current, the magnetic field of which counteracts any change in the magnetic flux, i.e. at >0 e.m.f. induction e AND<0 и наоборот.
e.m.f. induction is measured in volts
To find the direction of the induction current, there is Lenz's rule (the rule was established in 1833): the induction current has such a direction that the magnetic field it creates tends to compensate for the change in the magnetic flux that caused this induction current.
For example, if you move the north pole of a magnet into a coil, i.e., increase the magnetic flux through its turns, an induced current appears in the coil in such a direction that a north pole appears at the end of the coil closest to the magnet (Fig. 3.20). So, the magnetic field of the induced current tends to neutralize the change in magnetic flux that caused it.
Not only does an alternating magnetic field generate an induced current in a closed conductor, but also when a closed conductor of length l moves in a constant magnetic field (B) at a speed v, an emf appears in the conductor:
a (B Ùv) (3.47)
As you already know, electromotive force in a chain is the result of the action of external forces. When the conductor moves in a magnetic field the role of external forces performs Lorentz force(which acts from the magnetic field on a moving electric charge). Under the influence of this force, charges are separated and a potential difference arises at the ends of the conductor. E.m.f. Induction in a conductor is the work of moving unit charges along the conductor.
Direction of induction current can be determined according to the right hand rule:Vector B enters the palm, the abducted thumb coincides with the direction of the conductor's velocity, and 4 fingers indicate the direction of the induction current.
Thus, an alternating magnetic field causes the appearance of an induced electric field. It not potentially(as opposed to electrostatic), because Job by moving a single positive charge equal to e.m.f. induction, not zero.
Such fields are called vortex. Vortex force lines electric field - are closed in on themselves, in contrast to the lines of electrostatic field strength.
E.m.f. induction occurs not only in neighboring conductors, but also in the conductor itself when the magnetic field of the current flowing through the conductor changes. Emergence of e.m.f. in any conductor, when the current strength in it itself changes (hence, the magnetic flux in the conductor) is called self-induction, and the current induced in this conductor is - self-induction current.
The current in a closed circuit creates a magnetic field in the surrounding space, the intensity of which is proportional to the current strength I. Therefore, the magnetic flux Ф penetrating the circuit is proportional to the current strength in the circuit
Ф=L×I, (3.48).
L is the proportionality coefficient, which is called the self-inductance coefficient, or, simply, inductance. Inductance depends on the size and shape of the circuit, as well as on the magnetic permeability of the environment surrounding the circuit.
In this sense, the inductance of the circuit is analogue the electrical capacitance of an isolated conductor, which also depends only on the shape of the conductor, its dimensions and the dielectric constant of the medium.
The unit of inductance is henry (H): 1Gn - the inductance of such a circuit, the self-induction magnetic flux of which at a current of 1A is equal to 1Wb (1Gn=1Wb/A=1V s/A).
If L=const, then emf. self-induction can be represented in the following form:
, or 
, (3.49)
where DI (dI) is the change in current in a circuit containing an inductor (or circuit) L over time Dt (dt). The “–” sign in this expression means that the emf. self-induction prevents a change in current (i.e., if the current in a closed circuit decreases, then the emf of self-induction leads to the appearance of a current in the same direction and vice versa).
One of the manifestations of electromagnetic induction is the occurrence of closed induction currents in continuous conducting media: metal bodies, electrolyte solutions, biological organs, etc. Such currents are called eddy currents or Foucault currents. These currents arise when a conducting body moves in a magnetic field and/or when the induction of the field in which the bodies are placed changes over time. The strength of Foucault currents depends on the electrical resistance of bodies, as well as on the rate of change of the magnetic field.
Foucault's currents also obey Lenz's rule : Their magnetic field is directed to counteract the change in magnetic flux that induces eddy currents.
Therefore, massive conductors are decelerated in a magnetic field. In electrical machines, in order to minimize the influence of Foucault currents, transformer cores and magnetic circuits of electrical machines are assembled from thin plates insulated from each other with a special varnish or scale.
Eddy currents cause the conductors to become very hot. Joule heat generated by Foucault currents, used in induction metallurgical furnaces for melting metals, according to the Joule-Lenz law.