Electromagnetic induction was discovered by Michael Faraday in 1831. He discovered that the electromotive force arising in a closed conducting circuit is proportional to the rate of change of the magnetic flux through the surface bounded by this circuit. The magnitude of the EMF does not depend on whether the cause of the flux change is a change in the magnetic field itself or the movement of the circuit (or part of it) in the magnetic field. The electric current caused by this emf is called induced current.
Faraday's law According to Faraday's law of electromagnetic induction, the electromotive force acting along an arbitrarily chosen circuit The minus sign in the formula reflects Lenz's rule, named after the Russian physicist E. H. Lenz: The induction current arising in a closed conducting circuit has the following direction, that the magnetic field it creates counteracts the change in magnetic flux that caused the current.
Magnetic flux In a uniform magnetic field, the magnitude of the induction vector is equal to B, a flat closed loop of area S is placed. The normal n to the contour plane makes an angle a with the direction of the magnetic induction vector B (see Fig. 1). The magnetic flux through the surface is the quantity Ф, determined by the relation: Ф = В·S·cos a. The unit of measurement of magnetic flux in the SI system is 1 Weber (1 Wb).
Induction emf in a moving conductor Let a conductor of length L move with speed V in a uniform magnetic field, crossing lines of force. The charges in the conductor move along with the conductor. A charge moving in a magnetic field is acted upon by the Lorentz force. Free electrons are displaced to one end of the conductor, and uncompensated positive charges remain at the other. A potential difference arises, which represents the induced emf ei. Its value can be determined by calculating the work done by the Lorentz force when moving a charge along a conductor: ei = A/q = F·L/q. It follows that ei = B·V·L·sin a.
Self-induction Self-induction is a special case of various manifestations of electromagnetic induction. Let's consider a circuit connected to a current source (Fig. 6). Electric current I flows along the circuit. This current creates a magnetic field in the surrounding space. As a result, the circuit is penetrated by its own magnetic flux F. Obviously, the own magnetic flux is proportional to the current in the circuit that created the magnetic field: Ф = L·I. The proportionality factor L is called the loop inductance. Inductance depends on the size, shape of the conductor, and the magnetic properties of the medium. The SI unit of inductance is 1 Henry (H). If the current in the circuit changes, then the intrinsic magnetic flux Fs also changes. A change in the value of Fs leads to the appearance of an induction emf in the circuit. This phenomenon is called self-induction, and the corresponding value is the self-induction emf eiс. From the law of electromagnetic induction it follows that eiс = dФс/dt. If L = const, then eiс= - L·dI/dt.
Transformer A transformer is a static electromagnetic device with two (or more) windings, most often designed to convert alternating current of one voltage into alternating current of another voltage. Energy conversion in a transformer is carried out by an alternating magnetic field. Transformers are widely used in transmitting electrical energy over long distances, distributing it between receivers, as well as in various rectifying, amplifying, signaling and other devices.
Power transformers Power transformers convert alternating current of one voltage into alternating current of another voltage to supply consumers with electricity. Depending on the purpose, they can be increasing or decreasing. In distribution networks, as a rule, three-phase two-winding step-down transformers are used, converting voltages of 6 and 10 kV to a voltage of 0.4 kV.
Current Transformer A current transformer is an auxiliary device in which the secondary current is practically proportional to the primary current and is designed to connect measuring instruments and relays to alternating current electrical circuits. Current transformers are used to convert current of any value and voltage into a current convenient for measuring with standard instruments (5 A), powering current windings of relays, disconnecting devices, as well as isolating devices and their operating personnel from high voltage.
Instrument voltage transformers Instrument voltage transformers are intermediate transformers through which measuring instruments are switched on at high voltages. Thanks to this, the measuring instruments are isolated from the network, which makes it possible to use standard instruments (with their scale re-graded) and thereby expands the limits of the measured voltages. Voltage transformers are used both for measuring voltage, power, energy, and for powering automation circuits, alarms and relay protection of power lines from ground faults. In some cases, voltage transformers can be used as low-power step-down power transformers or as step-up test transformers (for testing the insulation of electrical devices)
Classification of voltage transformers Voltage transformers differ: a) by the number of phases - single-phase and three-phase; b) according to the number of windings, two-winding and three-winding; c) according to the accuracy class, i.e. according to the permissible error values; d) by cooling method, transformers with oil cooling (oil), with natural air cooling (dry and with cast insulation); e) by type of installation for indoor installation, for outdoor installation and for complete switchgear (switchgear)
Classification of current transformers Current transformers are classified according to various criteria: 1. According to their purpose, current transformers can be divided into measuring, protective, intermediate (for including measuring instruments in relay protection current circuits, for equalizing currents in differential protection circuits, etc.) and laboratory (high accuracy, as well as with many transformation ratios). 2. According to the type of installation, current transformers are distinguished: a) for outdoor installation (in open switchgears); b) for indoor installation; c) built into electrical devices and machines: switches, transformers, generators, etc.; d) overhead covers placed on top of the bushing (for example, on the high-voltage input of a power transformer); e) portable (for control measurements and laboratory tests). 3. According to the design of the primary winding, current transformers are divided into: a) multi-turn (coil, loop-winding and figure-of-eight winding); b) single-turn (rod); c) tires.
4. According to the installation method, current transformers for indoor and outdoor installation are divided into: a) feed-through; b) supporting. 5. Based on insulation, current transformers can be divided into groups: a) with dry insulation (porcelain, bakelite, cast epoxy insulation, etc.); b) with paper-oil insulation and with capacitor paper-oil insulation; c) filled with compound. 6. According to the number of transformation stages, there are current transformers: a) single-stage; b) two-stage (cascade). 7. Transformers are distinguished by operating voltage: a) for rated voltage above 1000 V; b) for rated voltage up to 1000 V.
Electrical energy generators Electrical current is generated in generators - devices that convert energy of one kind or another into electrical energy. Generators include galvanic cells, electrostatic machines, thermopiles, solar panels, etc. The scope of application of each of the listed types of electricity generators is determined by their characteristics. Thus, electrostatic machines create a high potential difference, but are unable to create any significant current in the circuit. Galvanic cells can produce a large current, but their duration of action is short. The predominant role in our time is played by electromechanical induction alternating current generators. In these generators, mechanical energy is converted into electrical energy. Their action is based on the phenomenon of electromagnetic induction. Such generators have a relatively simple design and make it possible to obtain large currents at a sufficiently high voltage
Alternating Current Generator An alternating current generator (alternator) is an electromechanical device that converts mechanical energy into alternating current electrical energy. Generators include galvanic cells, electrostatic machines, thermopiles, solar panels, etc. The scope of application of each of the listed types of electricity generators is determined by their characteristics. Thus, electrostatic machines create a high potential difference, but are unable to create any significant current in the circuit.
Test 11-1(electromagnetic induction)
Option 1
1. Who discovered the phenomenon of electromagnetic induction?
A. X. Oersted. B. Sh. Pendant. V. A. Volta. G. A. Ampere. D. M. Faraday. E . D. Maxwell.
2. The leads of the copper wire coil are connected to a sensitive galvanometer. In which of the following experiments will the galvanometer detect the occurrence of an emf of electromagnetic induction in the coil?
A permanent magnet is removed from the coil.
A permanent magnet rotates around its longitudinal axis inside the coil.
A. Only in case 1. B. Only in case 2. C. Only in case 3. D. In cases 1 and 2. E. In cases 1, 2 and 3.
3.What is the name of the physical quantity equal to the product of the module B of the magnetic field induction by the area S of the surface penetrated by the magnetic field and the cosine
angle a between the vector B of induction and the normal n to this surface?
A. Inductance. B. Magnetic flux. B. Magnetic induction. D. Self-induction. D. Magnetic field energy.
4. Which of the following expressions determines the induced emf in a closed loop?
A. B. IN. G. D.
5. When a strip magnet is pushed into and out of a metal ring, an induced current occurs in the ring. This current creates a magnetic field. Which pole faces the magnetic field of the current in the ring towards: 1) the retractable north pole of the magnet and 2) the retractable north pole of the magnet.
6. What is the name of the unit of measurement of magnetic flux?
7. The unit of measurement of what physical quantity is 1 Henry?
A. Magnetic field induction. B. Electrical capacitances. B. Self-induction. D. Magnetic flux. D. Inductance.
8. What expression determines the connection between the magnetic flux through a circuit and inductance L circuit and current strength I in the circuit?
A. LI . B. . IN. LI ‘ . G. LI 2 . D.
9. What expression determines the relationship between the self-induction emf and the current strength in the coil?
A. B . IN . LI . G . . D. LI ‘ .
10. Properties of various fields are listed below. Which of them has an electrostatic field?
Tension lines are not associated with electric charges.
The field has energy.
The field has no energy.
A. 1, 4, 6. B. 1, 3, 5. IN. 1, 3, 6. G. 2, 3, 5. D. 2, 3, 6. E. 2, 4, 6.
11. A circuit with an area of 1000 cm 2 is in a uniform magnetic field with an induction of 0.5 T, the angle between the vector IN
A. 250Wb. B. 1000 Wb. IN. 0.1 Wb. G. 2,5 · 10 -2 Wb. D. 2.5 Wb.
12. What current strength in a circuit with an inductance of 5 mH creates a magnetic flux 2· 10 -2 Wb?
A. 4 mA. B. 4 A. C. 250 A. D. 250 mA. D. 0.1 A. E. 0.1 mA.
13. The magnetic flux through the circuit in 5 · 10 -2 s uniformly decreased from 10 mWb to 0 mWb. What is the value of the EMF in the circuit at this time?
A. 5 · 10 -4 V.B. 0.1 V.V. 0.2 V.G. 0.4 V.D. 1 V.E. 2 V.
14. What is the value of the energy of the magnetic field of a coil with an inductance of 5 H when the current in it is 400 mA?
A. 2 J. B. 1 J. B. 0.8 J. G. 0.4 J. D. 1000 J. E. 4 10 5 J.
15. A coil containing n turns of wire is connected to a direct current source with voltage U at the exit. What is the maximum value of the self-inductive emf in the coil when the voltage at its ends increases from 0 V to U IN?
A, U V, B. nU V.V. U /P U ,
16. Two identical lamps are connected to a DC source circuit, the first in series with a resistor, the second in series with a coil. In which of the lamps (Fig. 1) will the current strength, when switch K is closed, reach its maximum value later than the other?
A. In the first one. B. In the second. B. In the first and second at the same time. D. In the first, if the resistance of the resistor is greater than the resistance of the coil. D. In the second, if the coil resistance is greater than the resistor resistance.
17. A coil with an inductance of 2 H is connected in parallel with a resistor with an electrical resistance of 900 Ohms, the current in the coil is 0.5 A, the electrical resistance of the coil is 100 Ohms. What electric charge will flow in the circuit of the coil and resistor when they are disconnected from the current source (Fig. 2)?
A. 4000 Cl. B. 1000 Cl. V. 250 Cl. G. 1 10 -2 Cl. D. 1.1 10 -3 Cl. E. 1 10 -3 Cl.
18. An airplane flies at a speed of 900 km/h, the module of the vertical component of the induction vector of the Earth’s magnetic field is 4 10 5 Tesla. What is the potential difference between the ends of the airplane's wings if the wingspan is 50 m?
A. 1.8 B. B. 0.9 C. C. 0.5 C. D. 0.25 C.
19. What must be the current strength in the armature winding of an electric motor in order for a force of 120 N to act on a section of the winding of 20 turns 10 cm long, located perpendicular to the induction vector in a magnetic field with an induction of 1.5 Tesla?
A. 90 A. B. 40 A. C. 0.9 A. D. 0.4 A.
20. What force must be applied to a metal jumper to move it uniformly at a speed of 8 m/s along two parallel conductors located at a distance of 25 cm from each other in a uniform magnetic field with an induction of 2 Tesla? The induction vector is perpendicular to the plane in which the rails are located. The conductors are closed by a resistor with an electrical resistance of 2 Ohms.
A. 10000 N. B. 400 N. C. 200 N. G. 4 N. D. 2 N. E. 1 N.
Test 11-1(electromagnetic induction)
Option 2
1. What is the name of the phenomenon of the occurrence of electric current in a closed circuit when the magnetic flux through the circuit changes?
A. Electrostatic induction. B. The phenomenon of magnetization. B. Ampere force. G. Lorentz force. D. Electrolysis. E. Electromagnetic induction.
2. The leads of the copper wire coil are connected to a sensitive galvanometer. In which of the following experiments will the galvanometer detect the occurrence of an emf of electromagnetic induction in the coil?
A permanent magnet is inserted into the coil.
The coil is placed on a magnet.
3) The coil rotates around a magnet located
inside her.
A. In cases 1, 2 and 3. B. In cases 1 and 2. C. Only in case 1. D. Only in case 2. E. Only in case 3.
3. Which of the following expressions determines magnetic flux?
A. BScosα. B. . IN. qvBsinα. G. qvBI. D. IBlsina .
4. What does the following statement express: the induced emf in a closed loop is proportional to the rate of change of the magnetic flux through the surface bounded by the loop?
A. The law of electromagnetic induction. B. Lenz's rule. B. Ohm's law for a complete circuit. D. The phenomenon of self-induction. D. Law of electrolysis.
5. When a strip magnet is pushed into and out of a metal ring, an induced current occurs in the ring. This current creates a magnetic field. Which pole faces the magnetic field of the current in the ring towards: 1) the retractable south pole of the magnet and 2) the retractable south pole of the magnet.
A. 1 - northern, 2 - northern. B. 1 - southern, 2 - southern.
B. 1 - southern, 2 - northern. G. 1 - northern, 2 - southern.
6. The unit of measurement of what physical quantity is 1 Weber?
A. Magnetic field induction. B. Electrical capacitances. B. Self-induction. D. Magnetic flux. D. Inductance.
7. What is the name of the unit of measurement of inductance?
A. Tesla. B. Weber. V. Gauss. G. Farad. D. Henry.
8. What expression determines the relationship between the energy of the magnetic flux in the circuit and the inductance L circuit and current strength I in the circuit?
A . . B . . IN . LI 2 , G . LI ‘ . D . LI.
9.What is the physical quantity X is determined by the expression x= for a coil of P turns .
A. Induction emf. B. Magnetic flux. B. Inductance. D. EMF of self-induction. D. Magnetic field energy. E. Magnetic induction.
10. Properties of various fields are listed below. Which of them does a vortex induction electric field have?
Tension lines are necessarily associated with electric charges.
Tension lines are not associated with electric charges.
The field has energy.
The field has no energy.
The work done by forces to move an electric charge along a closed path may not be equal to zero.
The work done by forces to move an electric charge along any closed path is zero.
A. 1, 4, 6. B. 1, 3, 5. C. 1, 3, c. G. 2, 3, 5. D. 2, 3, 6. E. 2, 4, 6.
11. A circuit with an area of 200 cm 2 is in a uniform magnetic field with an induction of 0.5 T, the angle between the vector IN induction and a normal to the contour surface of 60°. What is the magnetic flux through the loop?
A. 50 Wb. B. 2 · 10 -2 Wb. V. 5 · 10 -3 Wb. G. 200 Wb. D. 5 Wb.
12. A current of 4 A creates a magnetic flux of 20 mWb in the circuit. What is the inductance of the circuit?
A. 5 Gn. B. 5 mH. V. 80 Gn. G. 80 mH. D. 0.2 Gn. E. 200 Gn.
13. The magnetic flux through the circuit in 0.5 s uniformly decreased from 10 mWb to 0 mWb. What is the value of the EMF in the circuit at this time?
A. 5 10 -3 B. B. 5 C. C. 10 C. D. 20 V. D. 0.02 V. E. 0.01 V.
14. What is the value of the energy of the magnetic field of a coil with an inductance of 500 mH when the current in it is 4 A?
A. 2 J. B. 1 J. C. 8 J. D. 4 J. D. 1000 J. E. 4000 J.
15. Coil containing P turns of wire, connected to a DC source with voltage U on the way out. What is the maximum value of the self-inductive emf in the coil when the voltage at its ends decreases from U V to 0 V?
A. U V.B. nU V.V. U / n V.G. Maybe many times more U , depends on the rate of change of current and on the inductance of the coil.
16. In the electrical circuit shown in Figure 1, there are four keys 1, 2, 3 And 4 closed. Opening which of the four will provide the best opportunity to detect the phenomenon of self-induction?
A. 1. B. 2. V. 3. G. 4. D. Any of the four.
17. A coil with an inductance of 2 H is connected in parallel with a resistor with an electrical resistance of 100 Ohms, the current in the coil is 0.5 A, the electrical resistance of the coil is 900 Ohms. What electric charge will flow in the circuit of the coil and resistor when they are disconnected from the current source (Fig. 2)?
A. 4000 Cl. B. 1000 Cl. V. 250 Cl. G. 1 10 -2 Cl. D. 1.1 10 -3 Cl. E. 1 10 -3 Cl.
18. An airplane flies at a speed of 1800 km/h, the module of the vertical component of the induction vector of the Earth’s magnetic field is 4 10 -5 Tesla. What is the potential difference between the ends of the airplane's wings if the wingspan is 25 m?
A. 1.8 B. B. 0.5 B. C. 0.9 V. D. 0.25 V.
19. Rectangular frame with areaS With electric shockI placed in magnetic induction fieldIN . What is the moment of force acting on the frame if the angle between the vectorIN and the normal to the frame is a?
A. IBS sin a. B. IBS. IN. IBS cos a. G. I 2 B.S. sin a. D. I 2 B.S. cos a. .
Option 2
Several types of electric current phenomena are known, differing depending on the type of substance in which it occurs under appropriate conditions.
Electrical conductivity is the ability of a substance to conduct electric current.
All substances are divided into three classes: conductors, semiconductors and dielectrics. Conductors are of the first and second kind: in conductors of the first kind (metals) the current is created by electrons and conductivity is called electronic; in conductors of the second kind (solutions of salts, acids, alkalis) the current is created by ions.
The phenomenon of directed movement of free electric charge carriers in a substance or in a vacuum is called conduction current.
The intensity of an electric current is measured by a physical quantity called electric current strength. The magnitude of the conduction current is determined by the electric charge of all particles passing through the cross section of the conductor per unit time:
In practical calculations, the concept of electric current density is used (numerically determined by the ratio of the current strength to the cross-sectional area of the conductor):
;
Experiments have established that the intensity of the electric current is proportional to the electric field strength and depends on the properties of the conductive substance. The dependence of current on the properties of a substance is called conductivity, and its inverse value is called resistance.
;
G – conductivity;
R= 1\ G - resistance;
Resistance depends on temperature: ;
α – temperature coefficient of resistance.
Semiconductors occupy an intermediate position between conductors and dielectrics; their molecules are connected by covalent bonds. These bonds can be destroyed under certain conditions: we add either an impurity of electrons or an impurity of positive ions, and then the possibility of obtaining electron or hole conductivity arises. To provide current in a semiconductor, a potential difference must be applied.
The electrical conductivity of dielectrics is practically zero due to very strong bonds between electrons and the nucleus. If a dielectric is placed in an external electric field, polarization of atoms will occur due to the displacement of positive charges in one direction and negative charges in the other. With a very strong external electric field, atoms can be torn apart, and a breakdown current occurs.
In addition to the conduction current, there is also a displacement current. The displacement current is caused by a change in the electric field strength vector over time.
Electric current can only flow in a closed system.
Topic 1.2 Simple and complex electrical circuits
An electrical circuit is a set of devices and objects that ensures the flow of electric current from source to consumer.
An element of an electrical circuit is a separate object or device. The main elements of an electrical circuit are: source of electrical energy, consumers, devices for transmitting electrical energy. IN sources of electrical energy various types of non-electrical energy are converted into electrical energy. IN consumers Electrical energy is converted into heat, light and other non-electrical types of energy. Devices for transmitting electrical energy from sources to consumers are power lines. All basic elements of electrical circuits have electrical resistance and affect the amount of current in the electrical circuit.
In addition to the main elements, electrical circuits contain auxiliary elements: fuses, switches, switches, measuring instruments and more.
The electrical circuit is called simple, if it consists of one closed loop. The electrical circuit is called complex(branched), if it consists of several closed contours.
Charge in motion. It can take the form of a sudden discharge of static electricity, such as lightning. Or it could be a controlled process in generators, batteries, solar or fuel cells. Today we will look at the very concept of “electric current” and the conditions for the existence of electric current.
Electric Energy
Most of the electricity we use comes in the form of alternating current from the electrical grid. It is created by generators that work according to Faraday's law of induction, due to which a changing magnetic field can induce an electric current in a conductor.
Generators have rotating coils of wire that pass through magnetic fields as they rotate. As the coils rotate, they open and close relative to the magnetic field and create an electric current that changes direction with each turn. The current passes through a full cycle back and forth 60 times per second.
Generators can be powered by steam turbines heated by coal, natural gas, oil or a nuclear reactor. From the generator, the current passes through a series of transformers, where its voltage increases. The diameter of the wires determines the amount and intensity of current they can carry without overheating and losing energy, and the voltage is limited only by how well the lines are insulated from ground.
It is interesting to note that the current is carried by only one wire and not two. Its two sides are designated as positive and negative. However, since the polarity of alternating current changes 60 times per second, they have other names - hot (main power lines) and ground (running underground to complete the circuit).
Why is electric current needed?
There are many uses for electric current: it can light up your home, wash and dry your clothes, lift your garage door, make water boil in a kettle and enable other household items that make our lives much easier. However, the ability of current to transmit information is becoming increasingly important.
When connecting to the Internet, a computer uses only a small part of the electrical current, but this is something without which modern people cannot imagine their lives.
The concept of electric current
Like a river flow, a flow of water molecules, an electric current is a flow of charged particles. What is it that causes it, and why doesn't it always go in the same direction? When you hear the word "flowing", what do you think of? Perhaps it will be a river. This is a good association because it is for this reason that electric current gets its name. It is very similar to the flow of water, but instead of water molecules moving along a channel, charged particles move along a conductor.
Among the conditions necessary for the existence of electric current, there is a point that requires the presence of electrons. Atoms in a conductive material have many of these free charged particles floating around and between the atoms. Their movement is random, so there is no flow in any given direction. What is needed for electric current to exist?
Conditions for the existence of electric current include the presence of voltage. When it is applied to a conductor, all free electrons will move in the same direction, creating a current.
Curious about electric current
What's interesting is that when electrical energy is transferred through a conductor at the speed of light, the electrons themselves move much slower. In fact, if you walked slowly next to a conductive wire, your speed would be 100 times faster than the electrons. This is due to the fact that they do not need to travel huge distances to transfer energy to each other.
Direct and alternating current
Today, two different types of current are widely used - direct and alternating. In the first, electrons move in one direction, from the “negative” side to the “positive” side. Alternating current pushes electrons back and forth, changing the direction of flow several times per second.
Generators used in power plants to produce electricity are designed to produce alternating current. You've probably never noticed that the lights in your home actually flicker because the current direction changes, but it happens too quickly for your eyes to detect.
What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. Back in the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a home was only possible at very high voltages. But the problem was that sending really high voltage was extremely dangerous for people.
The solution to this problem was to reduce the tension outside the home before sending it inside. To this day, direct electric current is used for long distance transmission, mainly due to its ability to be easily converted into other voltages.
How does electric current work?
The conditions for the existence of electric current include the presence of charged particles, a conductor, and voltage. Many scientists have studied electricity and discovered that there are two types of electricity: static and current.
It is the second that plays a huge role in the daily life of any person, since it represents an electric current that passes through the circuit. We use it daily to power our homes and much more.
What is electric current?
When electrical charges circulate in a circuit from one place to another, an electric current is created. The conditions for the existence of electric current include, in addition to charged particles, the presence of a conductor. Most often this is a wire. Its circuit is a closed circuit in which current passes from the power source. When the circuit is open, he cannot complete the journey. For example, when the light in your room is off, the circuit is open, but when the circuit is closed, the light is on.
Current power
The conditions for the existence of electric current in a conductor are greatly influenced by voltage characteristics such as power. This is a measure of how much energy is used over a certain period of time.
There are many different units that can be used to express this characteristic. However, electrical power is almost measured in watts. One watt is equal to one joule per second.
Electric charge in motion
What are the conditions for the existence of electric current? It can take the form of a sudden discharge of static electricity, such as lightning or a spark from friction with woolen fabric. More often, however, when we talk about electric current, we're talking about a more controlled form of electricity that makes lights burn and appliances work. Most of the electrical charge is carried by negative electrons and positive protons within an atom. However, the latter are mainly immobilized inside atomic nuclei, so the work of transferring charge from one place to another is done by electrons.
Electrons in a conducting material such as a metal are largely free to move from one atom to another along their conduction bands, which are the highest electron orbits. Sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to flow through a conductor in the form of an electric current.
If we draw an analogy with water, then take, for example, a pipe. When we open the valve at one end to allow water to flow into the pipe, we do not have to wait for that water to make its way all the way to the end. We get water at the other end almost instantly because the incoming water pushes the water that is already in the pipe. This is what happens when there is an electric current in a wire.
Electric current: conditions for the existence of electric current
Electric current is usually thought of as a flow of electrons. When the two ends of a battery are connected to each other using a metal wire, this charged mass passes through the wire from one end (electrode or pole) of the battery to the opposite. So, let's name the conditions for the existence of electric current:
- Charged particles.
- Conductor.
- Voltage source.
However, not all so simple. What conditions are necessary for the existence of electric current? This question can be answered in more detail by considering the following characteristics:
- Potential difference (voltage). This is one of the mandatory conditions. There must be a potential difference between the 2 points, meaning that the repulsive force that is created by the charged particles at one place must be greater than their force at another point. Voltage sources, as a rule, do not occur in nature, and electrons are distributed fairly evenly in the environment. Nevertheless, scientists managed to invent certain types of devices where these charged particles can accumulate, thereby creating the very necessary voltage (for example, in batteries).
- Electrical resistance (conductor). This is the second important condition that is necessary for the existence of electric current. This is the path along which charged particles travel. Only those materials that allow electrons to move freely act as conductors. Those who do not have this ability are called insulators. For example, a metal wire will be an excellent conductor, while its rubber sheath will be an excellent insulator.
Having carefully studied the conditions for the emergence and existence of electric current, people were able to tame this powerful and dangerous element and direct it for the benefit of humanity.
The phenomenon of the occurrence of electric current in a closed conducting circuit when the magnetic flux covered by this circuit changes is called electromagnetic induction.
It was discovered by Joseph Henry (observations made in 1830, results published in 1832) and Michael Faraday (observations made and results published in 1831).
Faraday's experiments were carried out with two coils inserted into each other (the outer coil is constantly connected to the ammeter, and the inner one, through a key, to the battery). The induction current in the outer coil is observed:
| A |
| V |
| b |
When closing and opening the circuit of the internal coil, motionless relative to the external one (Fig. a);
When moving the internal coil with direct current relative to the external one (Fig. b);
When moving relative to the outer coil of a permanent magnet (Fig. c).
Faraday showed that in all cases of the occurrence of an induced current in the external coil, the magnetic flux through it changes. In Fig. The outer coil is shown as one turn. In the first case (Fig. a), when the circuit is closed, a current flows through the internal coil, a magnetic field arises (changes) and, accordingly, a magnetic flux through the external coil. In the second (Fig. b) and third (Fig. c) cases, the magnetic flux through the external coil changes due to a change in the distance from it to the internal coil with current, or to the permanent magnet, during the movement.
| A |
| V |
| b |
| I |
| I |
| I |
In 1834, Emilius Christianovich Lenz experimentally established a rule that allows one to determine the direction of the induction current: the induction current is always directed so as to counteract the cause that causes it; the induced current always has such a direction that the increment in the magnetic flux it creates and the increment in the magnetic flux that caused this induced current are opposite in sign. This rule is called Lenz's rule.
Law of Electromagnetic Induction can be formulated in the following form: the emf of electromagnetic induction in a circuit is equal to the rate of change with time of the magnetic flux through the surface bounded by this circuit, taken with a minus sign
Here dФ = is the scalar product of the magnetic induction vector and the vector of the surface area. Vector , where is the unit vector () of the normal to an infinitesimal surface area of area .
The minus sign in the expression is associated with the rule for choosing the direction of the normal to the contour that bounds the surface, and the positive direction of traversing along it. In accordance with the definition, magnetic flux Ф through a surface of area S
depends on time if the following changes over time: surface area S;
magnetic induction vector module B; angle between vectors and normal .
If a closed loop (coil) consists of turns, then the total flux through the surface bounded by such a complex contour is called flux linkage and is defined as
where Ф i is the magnetic flux through the i turn. If all the turns are the same, then
where Ф is the magnetic flux through any turn. In this case
| I |
| I |
| I |
| N turns |
| 1 turn |
| 2 turns |
The expression allows you to determine not only the magnitude, but also the direction of the induction current. If the values of the emf and, therefore, the induced current are positive values, then the current is directed along the positive direction of the circuit, if negative - in the opposite direction (the direction of the positive circuit is determined by choosing the normal to the surface bounded by the circuit)
