Topic 11. PHENOMENON OF ELECTROMAGNETIC INDUCTION.

11.1. Faraday's experiments. Induction current. Lenz's rule. 11.2. Magnitude induced emf.

11.3. The nature of induced emf.

11.4. Circulation of the vortex intensity vector electric field.

11.5. Betatron.

11.6. Toki Fuko.

11.7. Skin effect.

11.1. Faraday's experiments. Induction current. Lenz's rule.

WITH Since the discovery of the connection between the magnetic field and the current (which confirms the symmetry of the laws of nature), numerous attempts have been made to obtain current using a magnetic field. The problem was solved by Michael Faraday in 1831. (The American Joseph Henry also discovered, but did not have time to publish his results. Ampere also claimed the discovery, but was not able to present his results).

Michael Faraday (1791 - 1867) - famous English physicist. Research in the field of electricity, magnetism, magnetooptics, electrochemistry. Created a laboratory model of an electric motor. He opened the extra currents when closing and opening the circuit and established their direction. Discovered the laws of electrolysis, was the first to introduce the concepts of field and dielectric constant, in 1845 used the term “magnetic field”.

Among other things, M. Faraday discovered the phenomena of dia and paramagnetism. He found that all materials in a magnetic field behave differently: they are oriented along the field (steam and ferromagnets) or across

fields are diamagnetic.

From school course physicists Faraday's experiments are well known: a coil and a permanent magnet (Fig. 11.1)

Rice. 11.1 Fig. 11.2

If you bring a magnet close to the coil or vice versa, an electric current will arise in the coil. The same thing with two closely spaced coils: if you connect a source to one of the coils AC, then alternating current will also appear in the other

(Fig. 11.2), but this effect is best manifested if two coils are connected with a core (Fig. 11.3).

According to Faraday's definition, what these experiments have in common is that: if the flow

As the induction vector penetrating the closed, conducting circuit changes, an electric current arises in the circuit.

This phenomenon is called the phenomenon of electromagnetic induction, and the current is induction . Moreover, the phenomenon is completely independent of the method of changing the flux of the magnetic induction vector.

So, it turns out that moving charges (current) create a magnetic field, and a moving magnetic field creates a (vortex) electric field and, in fact, induced current.

For each specific case, Faraday indicated the direction of the induction current. In 1833 Lenz established a general rule for finding the direction of current:

induced current is always directed in such a way that the magnetic field of this current prevents a change magnetic flux, causing an induced current. This statement is called Lenz's rule.

Filling the entire space with a homogeneous magnet leads, under other conditions, equal conditions to an increase in induction by µ times. This fact confirms that

the induced current is caused by a change in the flux of the magnetic induction vector B, and not the flux of the intensity vector H.

11.2. The magnitude of the induced emf.

To create current in a circuit, an electromotive force must be present. Therefore, the phenomenon of electromagnetic induction indicates that when the magnetic flux changes in the circuit, an electromotive force of induction E i arises. Our

task, using the laws of conservation of energy, find the value E i and find out it

Let's consider the movement of the moving section 1 - 2 of the circuit with current in a magnetic field

B (Fig. 11.4).

Let us first assume that there is no magnetic field B. A battery with an emf equal to E 0 creates

current I 0 . During time dt, the battery does work

dA = E I0 dt(11.2.1)

– this work will turn into heat, which can be found according to the Joule-Lenz law:

Q = dA = E 0 I0 dt = I0 2 Rdt,

here I 0 = E R 0, R is the total resistance of the entire circuit.

Let's place the circuit in a uniform magnetic field with induction B. LinesB ||n and are related to the direction of the current by the gimlet rule. FluxF associated with the circuit is positive.r

Each circuit element experiences mechanical force d F . The moving side of the frame will experience a force F 0 . Under the influence of this force, section 1 – 2

will move with speed υ = dx dt. In this case, the magnetic flux will also change.

induction.

Then, as a result of electromagnetic induction, the current in the circuit will change and become

resulting). This force will produce work dA in time dt: dA = Fdx = IdФ.

As in the case when all elements of the frame are stationary, the source of work is E 0 .

With a stationary circuit, this work was reduced only to the release of heat. In our case, heat will also be released, but in a different amount, since the current has changed. In addition, it is done mechanical work. General work for time dt, is equal to:

We have the right to consider the resulting expression as Ohm’s law for a circuit in which, in addition to the source E 0, E i acts, which is equal to:

induction running through this circuit.

This expression for the induced emf of a circuit is completely universal, independent of the method of changing the flux of magnetic induction and is called

Faraday's law.

Sign (-) – mathematical expression Lenz's rules on the direction of induction current: the induced current is always directed so that its field

counteract the change in the initial magnetic field.

The direction of the induction current and the direction d dt Ф are related gimlet rule(Fig. 11.5).

Dimension of induced emf: [ E i ] =[ Ф ] =B c =B .t c

If the circuit consists of several turns, then we must use the concept

flux linkage (total magnetic flux):

Ψ = Ф·N,

where N is the number of turns. So if

11.3. The nature of induced emf.

Let's answer the question: what is the reason for the movement of charges, the reason for the occurrence of induction current? Consider Figure 11.6.

1) If you move a conductor in a uniform magnetic field B, then under the influence of the Lorentz force, the electrons will be deflected down, and positive charges up - a potential difference arises. This will be the E i -sided force, under the influence

which current flows. As we know, for positive charges

F l = q + ; for electrons F l = –e - .

2) If the conductor is stationary and the magnetic field changes, what force excites the induced current in this case? Let's take an ordinary transformer (Fig. 11.7).

As soon as we close the circuit of the primary winding, a current immediately arises in the secondary winding. But the Lorentz force has nothing to do with it, because it acts on moving charges, and at the beginning they were at rest (they were in thermal motion - chaotic, but here we need directed motion).

The answer was given by J. Maxwell in 1860: Any alternating magnetic field excites an electric field (E") in the surrounding space. This is the reason for the occurrence of induction current in the conductor. That is, E" occurs only in the presence of an alternating magnetic field (at DC transformer does not work).

The essence of the phenomenon of electromagnetic induction not at all in the appearance of induction current (current appears when there are charges and the circuit is closed), and in the emergence of a vortex electric field (not only in the conductor, but also in the surrounding space, in vacuum).

This field has a completely different structure than the field created by charges. Since it is not created by charges, the lines of force cannot begin and end on charges, as we did in electrostatics. This field is a vortex, its lines of force are closed.

Since this field moves charges, it therefore has force. Let's introduce

vector of the vortex electric field strength E ". The force with which this field acts on the charge

F "= q E ".

But when a charge moves in a magnetic field, it is acted upon by the Lorentz force

MAGNETIC FIELD

The magnetic interaction of moving electric charges, according to the concepts of field theory, is explained as follows: every moving electric charge creates a magnetic field in the surrounding space that can act on other moving electric charges.

IN - physical quantity, which is power characteristic magnetic field. It is called magnetic induction (or magnetic field induction).

Magnetic induction - vector quantity. Magnetic induction vector module equal to the ratio maximum value Ampere force acting on a straight conductor with current, to the current strength in the conductor and its length:

Unit of magnetic induction. IN International system units per unit of magnetic induction is the induction of such a magnetic field in which for each meter of length of the conductor at a current strength of 1 A acts maximum strength Ampere 1 N. This unit is called tesla (abbreviated: T), in honor of the outstanding Yugoslav physicist N. Tesla:

LORENTZ FORCE

The movement of a current-carrying conductor in a magnetic field shows that the magnetic field acts on moving electric charges. Ampere force acts on the conductor F A = ​​IBlsin a, and the Lorentz force acts on a moving charge:

Where a- angle between vectors B and v.

Movement of charged particles in a magnetic field. In a uniform magnetic field, a charged particle moving at a speed perpendicular to the magnetic field induction lines is acted upon by a force m, constant in magnitude and directed perpendicular to the velocity vector. Under the influence of a magnetic force, the particle acquires acceleration, the modulus of which is equal to:

In a uniform magnetic field, this particle moves in a circle. The radius of curvature of the trajectory along which the particle moves is determined from the condition from which it follows,

The radius of curvature of the trajectory is a constant value, since the force perpendicular to the vector speed, only its direction changes, but not its magnitude. And this means that this trajectory is a circle.

The period of revolution of a particle in a uniform magnetic field is equal to:

The last expression shows that the period of revolution of a particle in a uniform magnetic field does not depend on the speed and radius of its trajectory.

If the electric field strength is zero, then the Lorentz force l is equal to the magnetic force m:

ELECTROMAGNETIC INDUCTION

The phenomenon of electromagnetic induction was discovered by Faraday, who established that an electric current arises in a closed conducting circuit with any change in the magnetic field penetrating the circuit.

MAGNETIC FLUX

Magnetic flux F(magnetic induction flux) through a surface area S- size, equal to the product module of the magnetic induction vector per area S and cosine of the angle A between the vector and the normal to the surface:

Ф=BScos

In SI, the unit of magnetic flux is 1 Weber (Wb) - magnetic flux through a surface of 1 m2 located perpendicular to the direction of a uniform magnetic field, the induction of which is 1 T:

Electromagnetic induction -occurrence phenomenon electric current in a closed conducting circuit with any change in the magnetic flux passing through the circuit.

Arising in a closed circuit, the induced current has such a direction that its magnetic field counteracts the change in the magnetic flux that causes it (Lenz's rule).

LAW OF ELECTROMAGNETIC INDUCTION

Faraday's experiments showed that the strength of the induced current I i in a conducting circuit is directly proportional to the rate of change in the number of magnetic induction lines penetrating the surface bounded by this circuit.

Therefore, the strength of the induction current is proportional to the rate of change of the magnetic flux through the surface bounded by the contour:

It is known that if a current appears in the circuit, this means that free charges outside forces act on the conductor. The work done by these forces to move a unit charge along a closed loop is called electromotive force (EMF). Let's find the induced emf ε i.

According to Ohm's law for a closed circuit

Since R does not depend on , then

The induced emf coincides in direction with the induced current, and this current, in accordance with Lenz’s rule, is directed so that the magnetic flux it creates counteracts the change in the external magnetic flux.

Law of Electromagnetic Induction

The induced emf in a closed loop is equal to that taken from opposite sign rate of change of magnetic flux penetrating the circuit:

SELF-INDUCTION. INDUCTANCE

Experience shows that magnetic flux F associated with a circuit is directly proportional to the current in that circuit:

Ф = L*I .

Loop inductance L- proportionality coefficient between the current passing through the circuit and the magnetic flux created by it.

The inductance of a conductor depends on its shape, size and properties of the environment.

Self-induction- the phenomenon of the occurrence of induced emf in a circuit when the magnetic flux changes caused by a change in the current passing through the circuit itself.

Self-induction - special case electromagnetic induction.

Inductance - value, numerically equal to emf self-induction that occurs in a circuit when the current in it changes by one per unit of time. In SI, the unit of inductance is taken to be the inductance of a conductor in which, when the current strength changes by 1 A in 1 s, a self-inductive emf of 1 V occurs. This unit is called henry (H):

MAGNETIC FIELD ENERGY

The phenomenon of self-induction is similar to the phenomenon of inertia. Inductance plays the same role when changing current as mass does when changing the speed of a body. The analogue of speed is current.

This means that the energy of the magnetic field of the current can be considered a value similar to kinetic energy body:

Let us assume that after disconnecting the coil from the source, the current in the circuit decreases with time according to a linear law.

The self-induced emf in this case has a constant value:

where I - initial value current, t is the period of time during which the current decreases from I to 0.

During time t, an electric charge passes through the circuit q = I cp t. Because I cp = (I + 0)/2 = I/2, then q=It/2. Therefore, the work of electric current:

This work is done due to the energy of the magnetic field of the coil. Thus we again get:

Example. Determine the energy of the magnetic field of the coil in which, at a current of 7.5 A, the magnetic flux is 2.3 * 10 -3 Wb. How will the field energy change if the current strength is halved?

The energy of the magnetic field of the coil is W 1 = LI 1 2 /2. By definition, the inductance of the coil is L = Ф/I 1. Hence,

Occurrence in EMF conductor induction

If you place it in a conductor and move it so that during its movement it intersects the field lines, then something called induced emf will arise in the conductor.

An induced emf will occur in a conductor even if the conductor itself remains stationary, and the magnetic field moves, crossing the conductor with its lines of force.

If the conductor in which the induced emf is induced is closed to any external circuit, then under the influence of this emf a current called induction current.

The phenomenon of EMF induction in a conductor when it is crossed by magnetic field lines is called electromagnetic induction.

Electromagnetic induction is a reverse process, i.e. transformation mechanical energy to electric.

The phenomenon of electromagnetic induction has found wide application in. The design of various electrical machines is based on its use.

Magnitude and direction of induced emf

Let us now consider what the magnitude and direction of the EMF induced in the conductor will be.

The magnitude of the induced emf depends on the number of field lines crossing the conductor per unit time, i.e., on the speed of movement of the conductor in the field.

The magnitude of the induced emf is directly dependent on the speed of movement of the conductor in the magnetic field.

The magnitude of the induced emf also depends on the length of that part of the conductor that is intersected by the field lines. How most the conductor is crossed by the field lines, the greater the emf is induced in the conductor. And finally, the stronger the magnetic field, i.e., the greater its induction, the greater the emf that appears in the conductor crossing this field.

So, the magnitude of the induced emf that occurs in a conductor when it moves in a magnetic field is directly proportional to the induction of the magnetic field, the length of the conductor and the speed of its movement.

This dependence is expressed by the formula E = Blv,

where E is the induced emf; B - magnetic induction; I is the length of the conductor; v is the speed of movement of the conductor.

It should be firmly remembered that In a conductor moving in a magnetic field, an induced emf occurs only if this conductor is crossed by magnetic field lines. If the conductor moves along the field lines, that is, does not cross, but seems to slide along them, then no EMF is induced in it. Therefore, the above formula is valid only in the case when the conductor moves perpendicular to the magnetic power lines fields.

The direction of the induced EMF (as well as the current in the conductor) depends on which direction the conductor is moving. To determine the direction of the induced EMF there is a rule right hand.

If you hold the palm of your right hand so that the magnetic field lines enter it, and the bent thumb would indicate the direction of movement of the conductor, then the extended four fingers will indicate the direction of action of the induced emf and the direction of the current in the conductor.

Right hand rule

Induction emf in a coil

We have already said that in order to create an inductive emf in a conductor, it is necessary to move either the conductor itself or the magnetic field in a magnetic field. In both cases, the conductor must be crossed by magnetic field lines, otherwise the EMF will not be induced. The induced EMF, and therefore the induced current, can be obtained not only in a straight conductor, but also in a conductor twisted into a coil.

When moving inside permanent magnet an EMF is induced in it due to the fact that the magnetic flux of the magnet crosses the turns of the coil, i.e., exactly the same as it was when moving straight conductor in the field of a magnet.

If the magnet is lowered into the coil slowly, then the EMF arising in it will be so small that the needle of the device may not even deviate. If, on the contrary, the magnet is quickly inserted into the coil, then the deflection of the needle will be large. This means that the magnitude of the induced emf, and therefore the current strength in the coil, depends on the speed of movement of the magnet, i.e., on how quickly the field lines intersect the turns of the coil. If you now alternately introduce a strong magnet and then a weak one into the coil at the same speed, you will notice that when strong magnet the instrument needle will deviate by larger angle. Means, the magnitude of the induced emf, and therefore the current strength in the coil, depends on the magnitude of the magnetic flux of the magnet.

And finally, if you introduce the same magnet at the same speed first into a coil with a large number turns, and then with significantly less, then in the first case the instrument needle will deviate at a larger angle than in the second. This means that the magnitude of the induced emf, and therefore the current strength in the coil, depends on the number of its turns. The same results can be obtained if an electromagnet is used instead of a permanent magnet.

The direction of the induced emf in the coil depends on the direction of movement of the magnet. The law established by E. H. Lenz tells how to determine the direction of the induced emf.

Lenz's law for electromagnetic induction

Any change in the magnetic flux inside the coil is accompanied by the appearance of an induced emf in it, and the faster the magnetic flux passing through the coil changes, the greater the emf is induced in it.

If the coil in which the induced emf is created is closed to an external circuit, then an induced current flows through its turns, creating a magnetic field around the conductor, due to which the coil turns into a solenoid. It turns out that a changing external magnetic field causes an induced current in the coil, which, in turn, creates its own magnetic field around the coil - the current field.

Studying this phenomenon, E. H. Lenz established a law that determines the direction of the induced current in the coil, and therefore the direction of the induced emf. The induced emf that occurs in a coil when the magnetic flux changes in it creates a current in the coil in such a direction that the magnetic flux of the coil created by this current prevents a change in the extraneous magnetic flux.

Lenz's law is valid for all cases of current induction in conductors, regardless of the shape of the conductors and the way in which a change in the external magnetic field is achieved.

When a permanent magnet moves relative to a wire coil connected to the terminals of a galvanometer, or when a coil moves relative to a magnet, an induced current occurs.

Induction currents in massive conductors

A changing magnetic flux is capable of inducing an emf not only in the turns of the coil, but also in massive metal conductors. Penetrating the thickness of a massive conductor, the magnetic flux induces an EMF in it, creating induced currents. These so-called ones spread along a massive conductor and short-circuit in it.

The cores of transformers, magnetic circuits of various electrical machines and devices are precisely those massive conductors that are heated by the induction currents arising in them. This phenomenon is undesirable, therefore, to reduce the magnitude of induced currents, parts of electrical machines and transformer cores are not made massive, but consist of thin sheets, isolated from one another with paper or a layer of insulating varnish. This prevents the spread of eddy currents by the mass of the conductor.

But sometimes in practice eddy currents They are also used as useful currents. For example, the work of so-called magnetic dampers of moving parts of electrical measuring instruments is based on the use of these currents.

In our world all kinds existing forces, with the exception of gravitational forces, are represented electromagnetic interactions. In the Universe, despite the amazing variety of influences of bodies on each other, in any substances or living organisms there is always a manifestation electromagnetic forces. We will describe below how the discovery of electromagnetic induction (EI) occurred.

VKontakte

Opening EI

The rotation of a magnetic needle near a current-carrying conductor in Oersted's experiments first indicated the connection between electrical and magnetic phenomena. Obviously: An electric current “surrounds” itself with a magnetic field.

So is it possible to achieve its occurrence through a magnetic field? similar task directed by Michael Faraday. In 1821, he noted this property in his diary on the transformation of magnetism into .

Success did not come to the scientist immediately. Only deep confidence in unity natural forces and hard work led him ten years later to a new great discovery.

The solution to the problem was not given to Faraday and his other colleagues for a long time, because they tried to generate electricity in a stationary coil using the action of a constant magnetic field. Meanwhile, it later became clear: the number of power lines piercing the wires changes, and electricity arises.

EI phenomenon

The process of the appearance of electricity in a coil as a result of a change in the magnetic field is characteristic of electromagnetic induction and defines this concept. It is quite natural that the variety that arises during this process, is called induction. The effect will continue if the coil itself is left motionless, but the magnet is moved. By using a second coil, you can do without a magnet altogether.

If you pass electricity through one of the coils, then when they move mutually in the second there will be an induced current. You can put one coil on another and change the voltage of one of them by closing and opening the switch. In this case, the magnetic field penetrating the coil, which is acted upon by the key, changes, and this causes the appearance of an induction current in the second.

Law

During experiments, it is easy to discover that the number of lines of force piercing the coil increases - the needle of the device used (galvanometer) shifts in one direction, and decreases in the other. A more thorough study shows that the strength of the induction current is directly proportional to the rate of change in the number of power lines. This is the basic law of electromagnetic induction.

This law expresses the formula:

It is applied if over a period of time t the magnetic flux changes by the same amount, when the rate of change of the magnetic flux Ф/t is constant.

Important! For induced currents, Ohm's law is valid: I=/R, where is the induced emf, which is found according to the EI law.

The remarkable experiments once carried out by the famous English physicist and which became the basis of the law he discovered, today any schoolchild can do without much difficulty. For these purposes the following are used:

• magnet,
• two wire spools,
• source of electricity,
• galvanometer.

Let's fix the magnet on the stand and bring the coil with the ends attached to the galvanometer to it.

By turning, tilting and moving it up and down, we change the number of magnetic field lines that penetrate its turns.

Galvanometer registers the emergence of electricity with the magnitude and direction constantly changing during the experiment.

If the coil and magnet are at rest relative to each other, they will not create conditions for the generation of electricity.

Other Faraday laws

Based on the research conducted, two more laws of the same name were formed:

1. The essence of the first is the following pattern: mass of substance m, allocated electrical voltage on the electrode, is proportional to the amount of electricity Q passing through the electrolyte.
2. The definition of Faraday's second law, or the dependence of the electrochemical equivalent on the atomic weight of an element and its valence, is formulated as follows: the electrochemical equivalent of a substance is proportional to its atomic weight, and also inversely proportional to valence.

Of all existing species induction great importance has isolated view this phenomenon– self-induction. If we take a coil that has large number turns, then when the circuit is closed, the light bulb does not light up immediately.

This process may take a few seconds. A very surprising fact at first glance. To understand what's going on here, you need to understand what's going on in circuit closing moment. A closed circuit seems to “awaken” an electric current, which begins its movement along the turns of the wire. At the same time, an increasing magnetic field is instantly created in the space around it.

The coil turns are penetrated by a changing electromagnetic field, concentrated by the core. The induction current excited in the turns of the coil when the magnetic field increases (at the moment the circuit is closed) counteracts the main one. It is impossible for it to instantly reach its maximum value at the moment the circuit is closed; it “grows” gradually. Here is the explanation why the light bulb does not light up immediately. When the circuit is opened, the main current is enhanced by induction as a result of the phenomenon of self-induction, and the light bulb flashes brightly.

Important! The essence of the phenomenon, called self-induction, is characterized by the dependence of the change exciting the induction current electromagnetic field from changes in the strength of the electric current flowing through the circuit.

The direction of the self-induction current is determined by Lenz's rule. Self-induction is easily comparable to inertia in the field of mechanics, since both phenomena have similar characteristics. And indeed, in as a result of inertia under the influence of force, the body acquires a certain speed gradually, and not instantly. Not immediately - under the influence of self-induction - when the battery is connected to the circuit, electricity appears. Continuing the comparison with speed, we note that it is also not capable of instantly disappearing.

Eddy currents

The presence of eddy currents in massive conductors can serve as another example of electromagnetic induction.

Experts know that metal transformer cores, generator and electric motor armatures are never solid. During their manufacture, a layer of varnish is applied to the individual thin sheets of which they are composed, isolating one sheet from the other.

It's not hard to understand what force forces a person to create such a device. Under the influence of electromagnetic induction in an alternating magnetic field, the core is penetrated by the lines of force of the vortex electric field.

Let's imagine that the core is made of solid metal. Since it's electrical resistance small, the occurrence of inductive voltage large size would be quite understandable. The core would eventually heat up, and a considerable part of the electrical energy would be lost uselessly. In addition, it would be necessary to take special measures for cooling. And the insulating layers do not allow achieve great values.

Induction currents inherent in massive conductors are called eddy currents for a reason - their lines are closed like the electric field lines where they arise. Most often, eddy currents are used in the operation of induction metallurgical furnaces for smelting metals. Interacting with the magnetic field that gave birth to them, they sometimes become the cause of interesting phenomena.

Let's take a powerful electromagnet and place, for example, a five-kopeck coin between its vertically located poles. Contrary to expectations, it will not fall, but will slowly descend. It will take seconds for her to travel a few centimeters.

Let us place, for example, a five-kopeck coin between vertically located poles powerful electromagnet and let her go.

Contrary to expectation, it will not fall, but will slowly descend. It will take seconds for her to travel a few centimeters. The movement of a coin resembles the movement of a body in a viscous medium. Why does this happen?

According to Lenz's rule, the directions of eddy currents arising when a coin moves in a non-uniform magnetic field are such that the magnet field pushes the coin upward. This feature is used to “calm” the needle in measuring instruments. Aluminum plate located between magnetic poles, is attached to the arrow, and the eddy currents arising in it contribute to the rapid attenuation of the oscillations.

Demonstration of the phenomenon of electromagnetic induction of amazing beauty suggested by Moscow University professor V.K. Arkadyev. Let's take a lead bowl that has superconducting properties and try to drop a magnet over it. It will not fall, but will seem to “hover” above the bowl. The explanation here is simple: equal to zero the electrical resistance of a superconductor contributes to the generation of large amounts of electricity in it, which can persist for a long time and “hold” the magnet above the bowl. According to Lenz's rule, the direction of their magnetic field is such that it repels the magnet and prevents it from falling.

We study physics - the law of electromagnetic induction

Correct formulation of Faraday's law

Conclusion

Electromagnetic forces are the forces that allow people to see the world around us and are found more often than others in nature, for example, light is also an example electromagnetic phenomena. It is impossible to imagine the life of mankind without this phenomenon.

magnet. Mark which ones the following statements correct and some are incorrect.
A. The magnet and the coil attract each other.
B. Inside the coil, the magnetic field of the induction current is directed upward.
B. Inside the coil, the magnetic induction lines of the magnet's fields are directed upward.
D. The magnet is removed from the coil.

2. Which reference systems are inertial and non-inertial? Give examples.
3. What is the property of bodies called inertia? What value characterizes inertia?
4. What is the relationship between the masses of bodies and the acceleration modules that they receive during interaction?
5. What is strength and how is it characterized?
6. Formulation of Newton's 2nd law? What is it mathematical notation?
7. How is Newton’s 2nd law formulated in impulse form? Its mathematical notation?
8. What is 1 Newton?
9. How does a body move if a force is applied to it that is constant in magnitude and direction? What is the direction of the acceleration caused by the force acting on it?
10. How is the resultant of forces determined?
11. How is Newton’s 3rd law formulated and written?
12. How are the accelerations of interacting bodies directed?
13. Give examples of the manifestation of Newton’s 3rd law.
14. What are the limits of applicability of all Newton’s laws?
15. Why we can count the Earth inertial system counting if it moves with centripetal acceleration?
16. What is deformation, what types of deformation do you know?
17. What force is called the elastic force? What is the nature of this force?
18. What are the features of elastic force?
19. How is the elastic force directed (support reaction force, thread tension force?)
20. How is Hooke’s law formulated and written? What are its limits of applicability? Construct a graph illustrating Hooke's law.
21. How the law is formulated and written Universal gravity when is it applicable?
22. Describe the experiments to determine the value of the gravitational constant?
23. What is the gravitational constant, what is it physical meaning?
24. Does the work done by the gravitational force depend on the shape of the trajectory? What is the work done by gravity in a closed loop?
25. Does the work of the elastic force depend on the shape of the trajectory?
26. What do you know about gravity?
27. How is acceleration calculated? free fall on Earth and other planets?
28. What is the first escape velocity? How is it calculated?
29. What is called free fall? Does the acceleration of gravity depend on the mass of the body?
30. Describe the experience Galileo Galilei, proving that all bodies in a vacuum fall with the same acceleration.
31. What force is called the friction force? Types of friction forces?
32. How are the forces of sliding and rolling friction calculated?
33. When does the static friction force occur? What is it equal to?
34. Does the force of sliding friction depend on the area of ​​contacting surfaces?
35. On what parameters does the sliding friction force depend?
36. What does the force of resistance to body motion in liquids and gases depend on?
37. What is body weight called? What is the difference between the weight of a body and the force of gravity acting on the body?
38. In what case is body weight numerically equal to modulus gravity?
39. What is weightlessness? What is overload?
40. How to calculate the weight of a body during its accelerated movement? Does the weight of a body change if it moves along a stationary horizontal plane with acceleration?
41. How does the weight of a body change when it moves along a convex and concave part of a circle?
42. What is the algorithm for solving problems when a body moves under the influence of several forces?
43. What force is called the Archimedes Force or buoyant force? What parameters does this force depend on?
44. What formulas can be used to calculate the Archimedes force?
45. Under what conditions does a body in a liquid float, sink, or float?
46. ​​How does the depth of immersion of a floating body in liquid depend on its density?
47. Why balloons filled with hydrogen, helium or hot air?
48. Explain the influence of the rotation of the Earth around its axis on the value of the acceleration of gravity.
49. How does the value of gravity change when: a) the body moves away from the surface of the Earth, B) when the body moves along the meridian, parallel

electrical circuit?

3. What is the physical meaning of EMF? Define volt.

4. Connect to short time voltmeter source electrical energy, observing polarity. Compare its readings with the calculation based on the experimental results.

5. What does the voltage at the terminals of current sources depend on?

6. Using the measurement results, determine the voltage on the external circuit (if the work is performed using method I), the resistance of the external circuit (if the work is performed using method II).

Question 6 in attachment: calculation

1. Under what conditions do friction forces appear?
2. What determine the modulus and direction of the static friction force?
3. Within what limits can the static friction force vary?
4. What force imparts acceleration to a car or diesel locomotive?
5. Can sliding friction force increase the speed of a body?
6. What is the main difference between the resistance force in liquids and gases and the friction force between two solids?
7. Give examples of useful and harmful action friction forces of all types