Vortex electric field. Vortex electric field – Knowledge Hypermarket Checking homework
ELECTRIC FIELD The reason for the occurrence of electric current in a stationary conductor is the electric field. Any change in the magnetic field generates an inductive electric field, regardless of the presence or absence of a closed circuit, and if the conductor is open, then a potential difference arises at its ends; If the conductor is closed, then an induced current is observed in it.
Vortex field. The inductive electric field is vortex. The direction of the vortex electric field lines coincides with the direction of the induction current. An inductive electric field has completely different properties compared to an electrostatic field.
The electric field is a vortex field. electrostatic field 1. created by stationary electric charges 2. field lines are open - - potential field 3. sources of the field are electric charges 4. work of field forces to move a test charge along a closed path = 0. induction electric field (vortex electric field) 1. caused by changes in the magnetic field 2. lines of force are closed - - vortex field 3. field sources cannot be specified 4. work of field forces to move a test charge along a closed path = induced emf
Subject. Law of Electromagnetic Induction
Purpose of the lesson: to familiarize students with the law of electromagnetic induction.
Lesson type: lesson on learning new material.
LESSON PLAN
Knowledge control |
1. Magnetic induction flux. 2. The phenomenon of electromagnetic induction. 3. Lenz's rule. |
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Demonstrations |
1. Dependence of induced emf on the rate of change of magnetic flux. 2. Fragments of the video film “The phenomenon of electromagnetic induction.” |
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Learning new material |
1. The law of electromagnetic induction. 2. Vortex electric field. 3. Induction emf in moving conductors. |
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Reinforcing the material learned |
1. Qualitative questions. 2. Learning to solve problems. |
LEARNING NEW MATERIAL
Where do the extraneous forces that act on the charges in the circuit come from? In the case of a conductor stationary relative to the observer, the cause of the appearance of extraneous forces is an alternating magnetic field. The fact is that an alternating magnetic field generates an electric field in the surrounding space - it is this field that acts on free charged particles in the conductor. But the generation of an electric field by a magnetic field occurs even where there is no leading circuit and no electric current arises. As we see, a magnetic field can not only transmit magnetic interactions, but also cause the appearance of another form of matter - an electric field.
However, the electric field generated by an alternating magnetic field is significantly different from the field created by charged particles.
The electric field created by an alternating magnetic field is vortex, that is, its lines of force are closed.
The vortex electric field has some features:
1) the field manifests itself through a force effect on charged particles, therefore the main characteristic of the vortex electric field is intensity;
2) unlike the electrostatic field, the intensity lines of the vortex electric field are closed. The direction of these lines can be determined using, for example, the left hand, as shown in the figure:
3) unlike the electrostatic field, the work of the vortex electric field along a closed trajectory is not zero (the vortex electric field is non-potential).
Let us consider a conductor of length l moving translationally in a uniform magnetic field with induction at a speed directed at an angle to the lines of magnetic induction of the field.
Electrons moving along with a conductor in a magnetic field are acted upon by the Lorentz force directed along the conductor. Its module
where q 0 is the charge of a free charged particle. Under the influence of this force, a separation of charges occurs - free charged particles will move to one end of the conductor, and at the other end there will be a shortage of them, that is, they will exceed the charge of the opposite sign. Therefore, in this case the external force is the Lorentz force. The separation of charges will lead to the appearance of an electric field, which will prevent further separation of charges. This process will stop when the Lorentz force and the force = q 0 balance each other. Consequently, inside the conductor the electric field strength is E = B sin, and the potential difference at the ends of the conductor is U = El = B lsin. Since we are considering an open circle, the potential difference at the ends of the conductor is equal to the induced emf in this conductor. Thus,
If such a conductor is short-circuited, an electric current will flow in a circle. Thus, a conductor moving in a magnetic field can be considered as a kind of current source characterized by induced emf.
QUESTIONS TO STUDENTS DURING PRESENTATION OF NEW MATERIAL
First level
1. Why does an induced current arise in stationary conductors located in an alternating magnetic field?
2. What is the reason for the occurrence of induced current when a conductor moves in a constant magnetic field?
3. What are the features of the vortex electric field?
Second level
1. What is the nature of the external forces that cause the appearance of induced current in a stationary conductor?
2. Why is the law of electromagnetic induction formulated for EMF, and not for current?
3. What is the nature of the induced emf in a conductor moving in a magnetic field?
CONSTRUCTION OF LEARNED MATERIAL
) . Qualitative questions
1. Why do fuses sometimes blow from a lightning strike even when the appliance is unplugged?
2. Why is it better to take a closed conductor in the form of a coil, and not in the form of a straight wire, to detect an induction current?
) . Learning to solve problems
1. Using flexible wires, a straight conductor 60 cm long is connected to a direct current source with an emf of 12 V and an internal resistance of 0.5 Ohm. The conductor moves in a uniform magnetic field with an induction of 1.6 Tesla at a speed of 12.5 m/s perpendicular to the lines of magnetic induction. Determine the current strength in the conductor if the resistance of the external circuit is 2.5 Ohms.
Slide 2
Checking homework
Report about E.H. Lenze (student prepared)
Slide 3
Physical dictation:
1. What is the phenomenon of electromagnetic induction? 2. Under what condition does current occur in a closed conducting circuit? 3.-4 Continue the phrases: 3. The magnetic flux through a surface of area S is the quantity... 4. According to Lenz’s rule, the induced current arising in a closed circuit...
Slide 4
5. Formulate the Law of Electromagnetic Induction. 6. 7. 8. S N V The conductor moves across the magnetic field lines from right to left. Determine the direction of the induction current. V Determine the direction of the magnetic induction vector and the polarity of the permanent magnet. S Determine the polarity of the induction voltage.
Slide 5
Vortex electric field.
When does induced emf occur? An induced emf occurs either in a stationary conductor placed in a field that changes with time, or in a conductor moving in a magnetic field, which may not change with time.
Slide 6
Slide 7
MAXWELL James Clerk (1831-79), English physicist, creator of classical electrodynamics, one of the founders of statistical physics, organizer and first director (from 1871) of the Cavendish Laboratory. Developing the ideas of M. Faraday, he created the theory of the electromagnetic field (Maxwell's equations); introduced the concept of displacement current, predicted the existence of electromagnetic waves, and put forward the idea of the electromagnetic nature of light. Established a statistical distribution named after him. He studied the viscosity, diffusion and thermal conductivity of gases. Showed that the rings of Saturn consist of individual bodies. Works on color vision and colorimetry (Maxwell disk), optics (Maxwell effect), elasticity theory (Maxwell's theorem, Maxwell-Cremona diagram), thermodynamics, history of physics, etc.
Slide 8
Changing over time, the magnetic field generates an electric field
Slide 9
Slide 10
The work of a vortex electric field when moving a single positive charge along a closed stationary conductor is numerically equal to the induced emf in this conductor.
Slide 11
Slide 12
What is the difference between a vortex electric field and a potential one?
Slide 13
Jean Bernard Leon Foucault September 18, 1819, Paris - February 11, 1868, - French physicist and astronomer, member of the Paris Academy of Sciences Foucault Currents Application of induction furnaces In many cases, Foucault currents are undesirable, so special measures have to be taken to reduce them. In particular, these currents cause heating of the ferromagnetic cores of transformers and metal parts of electrical machines. To reduce losses of electrical energy due to the occurrence of eddy currents, transformer cores are made not from a solid piece of ferromagnet, but from individual metal plates isolated from each other by a dielectric layer.
Lesson 15. Vortex electric field. EMF induction in moving conductors
Purpose: to find out the conditions for the occurrence of EMF in moving conductors.
During the classes
I. Organizational moment
II. Repetition
What is the phenomenon of electromagnetic induction?
What conditions are necessary for the existence of the phenomenon of electromagnetic induction?
How is the direction of the induced current determined by Lenz's rule?
What formula is used to determine the induced emf and what is the physical meaning of the minus sign in this formula?
III. Learning new material
Let's take a transformer. By connecting one of the windings to the AC network, we obtain current in the other coil. Free charges are affected by an electric field.
Electrons in a stationary conductor are driven by an electric field, and the electric field is directly generated by an alternating magnetic field. Changing over time, the magnetic field generates an electric field. The field moves the electrons in the conductor and thereby reveals itself. The electric field that arises when the magnetic field changes has a different structure than the electrostatic one. It is not associated with charges, it does not begin anywhere and does not end anywhere. Represents closed lines. It is called the vortex electric field. But unlike a stationary electric field, the work of a vortex field along a closed path is not zero.
Induction current in massive conductors is called Foucault currents.
Application: melting metals in vacuum.
Harmful effect: unnecessary loss of energy in transformer cores and generators.
EMF when a conductor moves in a magnetic field
When moving the jumperUThe Lorentz force acts on the electrons and does work. Electrons move from C to L. The jumper is the source of the emf, therefore,
The formula is used in any conductor moving in a magnetic field ifIf between vectorsis the angle α, then the formula is used:
Because
That
Cause of EDC- Lorentz force. The sign e can be determined by the right hand rule.
IV. Reinforcing the material learned
Which field is called an induction or vortex electric field?
What is the source of the inductive electric field?
What are Foucault currents? Give examples of their use. In what cases do you have to deal with them?
What distinctive properties does an inductive electric field have compared to a magnetic field? Stationary or electrostatic field?
V. Summing up the lesson
Homework
paragraph 12; 13.
The following can occur through a circuit: 1) in the case of a stationary conducting circuit placed in a time-varying field; 2) in the case of a conductor moving in a magnetic field, which may not change over time. The value of the induced emf in both cases is determined by the law (2.1), but the origin of this emf is different.
Let us first consider the first case of the occurrence of an induction current. Let's place a circular wire coil of radius r in a time-varying uniform magnetic field (Fig. 2.8). Let the magnetic field induction increase, then the magnetic flux through the surface limited by the coil will increase with time. According to the law of electromagnetic induction, an induced current will appear in the coil. When the magnetic field induction changes according to a linear law, the induction current will be constant.
What forces make the charges in the coil move? The magnetic field itself, penetrating the coil, cannot do this, since the magnetic field acts exclusively on moving charges (this is how it differs from the electric one), and the conductor with the electrons in it is motionless.
In addition to the magnetic field, charges, both moving and stationary, are also affected by an electric field. But those fields that have been discussed so far (electrostatic or stationary) are created by electric charges, and the induced current appears as a result of the action of a changing magnetic field. Therefore, we can assume that electrons in a stationary conductor are driven by an electric field, and this field is directly generated by a changing magnetic field. This establishes a new fundamental property of the field: changing over time, the magnetic field generates an electric field . This conclusion was first reached by J. Maxwell.
Now the phenomenon of electromagnetic induction appears before us in a new light. The main thing in it is the process of generating an electric field by a magnetic field. In this case, the presence of a conducting circuit, for example a coil, does not change the essence of the process. A conductor with a supply of free electrons (or other particles) plays the role of a device: it only allows one to detect the emerging electric field.
The field sets electrons in motion in the conductor and thereby reveals itself. The essence of the phenomenon of electromagnetic induction in a stationary conductor is not so much the appearance of an induction current, but rather the appearance of an electric field that sets electric charges in motion.
The electric field that arises when the magnetic field changes has a completely different nature than the electrostatic one.
It is not directly connected with electric charges, and its lines of tension cannot begin and end on them. They do not begin or end anywhere at all, but are closed lines, similar to magnetic field induction lines. This is the so called vortex electric field (Fig. 2.9).
The faster the magnetic induction changes, the greater the electric field strength. According to Lenz's rule, with increasing magnetic induction, the direction of the electric field intensity vector forms a left screw with the direction of the vector. This means that when a screw with a left-hand thread rotates in the direction of the electric field strength lines, the translational movement of the screw coincides with the direction of the magnetic induction vector. On the contrary, when the magnetic induction decreases, the direction of the intensity vector forms a right screw with the direction of the vector.
The direction of the tension lines coincides with the direction of the induction current. The force acting from the vortex electric field on the charge q (external force) is still equal to = q. But in contrast to the case of a stationary electric field, the work of the vortex field in moving the charge q along a closed path is not zero. Indeed, when a charge moves along a closed line of electric field strength, the work on all sections of the path has the same sign, since the force and movement coincide in direction. The work of a vortex electric field when moving a single positive charge along a closed stationary conductor is numerically equal to the induced emf in this conductor.
Induction currents in massive conductors. Induction currents reach a particularly large numerical value in massive conductors, due to the fact that their resistance is low.
Such currents, called Foucault currents after the French physicist who studied them, can be used to heat conductors. The design of induction furnaces, such as microwave ovens used in everyday life, is based on this principle. This principle is also used for melting metals. In addition, the phenomenon of electromagnetic induction is used in metal detectors installed at the entrances to airport terminal buildings, theaters, etc.
However, in many devices the occurrence of Foucault currents leads to useless and even unwanted energy losses due to heat generation. Therefore, the iron cores of transformers, electric motors, generators, etc. are not made solid, but consist of separate plates isolated from each other. The surfaces of the plates must be perpendicular to the direction of the vortex electric field intensity vector. The resistance to electric current of the plates will be maximum, and the heat generation will be minimal.
Application of ferrites. Electronic equipment operates in the region of very high frequencies (millions of vibrations per second). Here, the use of coil cores from separate plates no longer gives the desired effect, since large Foucault currents arise in the calede plate.
In § 7 it was noted that there are magnetic insulators - ferrites. During magnetization reversal, eddy currents do not arise in ferrites. As a result, energy losses due to heat generation in them are minimized. Therefore, cores of high-frequency transformers, magnetic antennas of transistors, etc. are made from ferrites. Ferrite cores are made from a mixture of powders of starting substances. The mixture is pressed and subjected to significant heat treatment.
With a rapid change in the magnetic field in an ordinary ferromagnet, induction currents arise, the magnetic field of which, in accordance with Lenz's rule, prevents a change in the magnetic flux in the coil core. Because of this, the flux of magnetic induction practically does not change and the core does not remagnetize. In ferrites, eddy currents are very small, so they can be quickly remagnetized.
Along with the potential Coulomb electric field, there is a vortex electric field. The intensity lines of this field are closed. The vortex field is generated by a changing magnetic field.
1. What is the nature of external forces that cause the appearance of induced current in a stationary conductor!
2. What is the difference between a vortex electric field and an electrostatic or stationary one!
3. What are Foucault currents!
4. What are the advantages of ferrites compared to conventional ferromagnets!
Myakishev G. Ya., Physics. 11th grade: educational. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; edited by V. I. Nikolaeva, N. A. Parfentieva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.
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