M. Faraday found that the strength of the induction current is proportional to the rate of change of the magnetic flux through the surface bounded by the contour:
I i ~ ΔF/Δt.
The occurrence of a current in a closed circuit means the presence of external forces, the work of which to move a unit charge in the circuit is called the electromotive force (EMF). This means that when the flow changes through the surface bounded by a closed loop, an EMF occurs in the loop ɛ i which is called the EMF of induction. According to Ohm's law for a closed circuit, . Therefore, the induction emf is proportional to ΔF/Δt because the resistance R does not depend on changes in magnetic flux.
Formulated like this:
EMF induction ɛ i in a closed loop is equal in absolute value to the rate of change of the magnetic flux through the surface bounded by the loop:
.
The experiments described above indicate that electromagnetic induction is the occurrence of an electric field and an electric current with a change in time magnetic field or when a conductor moves in a magnetic field. These two types of electromagnetic induction effects differ in the physical nature of the processes responsible for their occurrence. The first type is due to the induction of a vortex electric field by an alternating magnetic field, the second type is due to the action of Lorentz forces on moving charges in a stationary magnetic field. In both cases, the basic law of induction is satisfied, expressed by the formula (
).
In the material, we will understand the concept of EMF induction in situations of its occurrence. Also consider inductance as key parameter the occurrence of a magnetic flux when an electric field appears in the conductor.
Electromagnetic induction is the generation of electric current by magnetic fields that change over time. Thanks to the discoveries of Faraday and Lenz, patterns were formulated into laws, which introduced symmetry into the understanding of electromagnetic flows. Maxwell's theory brought together knowledge about electric current and magnetic fluxes. Thanks to the discovery of Hertz, humanity learned about telecommunications.
An electromagnetic field appears around a conductor with an electric current, however, in parallel, the opposite phenomenon also occurs - electromagnetic induction. Consider the magnetic flux using an example: if a frame from a conductor is placed in electric field with induction and move it from top to bottom along the magnetic lines of force or to the right or left perpendicular to them, then the magnetic flux passing through the frame will be a constant value.
When the frame rotates around its axis, then after a while the magnetic flux will change by a certain amount. As a result, an induction emf arises in the frame and electricity, which is called inductive.
EMF induction
Let us examine in detail what the concept of EMF of induction is. When a conductor is placed in a magnetic field and it moves with the intersection of field lines, an electromotive force appears in the conductor called induction EMF. It also occurs if the conductor remains stationary, and the magnetic field moves and intersects with the conductor lines of force.
When the conductor, where the emf occurs, closes to the external circuit, due to the presence of this emf, an induction current begins to flow through the circuit. Electromagnetic induction involves the phenomenon of inducing an EMF in a conductor at the moment it is crossed by magnetic field lines.
Electromagnetic induction is the reverse process of transforming mechanical energy into electric current. This concept and its laws are widely used in electrical engineering, most electrical machines are based on this phenomenon.
Faraday and Lenz laws
The laws of Faraday and Lenz reflect the patterns of occurrence of electromagnetic induction.
Faraday found that magnetic effects appear as a result of changes in the magnetic flux over time. At the moment of crossing the conductor with an alternating magnetic current, an electromotive force arises in it, which leads to the appearance of an electric current. Both a permanent magnet and an electromagnet can generate current.
The scientist determined that the intensity of the current increases with a rapid change in the number of lines of force that cross the circuit. That is, the EMF of electromagnetic induction is in direct proportion to the speed of the magnetic flux.
According to Faraday's law, the induction EMF formulas are defined as follows:
The minus sign indicates the relationship between the polarity of the induced emf, the direction of the flow, and the changing speed.
According to Lenz's law, it is possible to characterize the electromotive force depending on its direction. Any change in the magnetic flux in the coil leads to the appearance of an EMF of induction, and with a rapid change, an increasing EMF is observed.
If the coil, where there is an EMF of induction, has a short circuit to an external circuit, then an induction current flows through it, as a result of which a magnetic field appears around the conductor and the coil acquires the properties of a solenoid. As a result, a magnetic field is formed around the coil.
E.Kh. Lenz established a pattern according to which the direction of the induction current in the coil and the induction EMF are determined. The law states that the induction EMF in the coil, when the magnetic flux changes, forms a directional current in the coil, in which the given magnetic flux of the coil makes it possible to avoid changes in the extraneous magnetic flux.
Lenz's law applies to all situations of electric current induction in conductors, regardless of their configuration and the method of changing the external magnetic field.
The movement of a wire in a magnetic field
The value of the induced emf is determined depending on the length of the conductor crossed by the field lines of force. With a larger number of field lines, the value of the induced emf increases. With an increase in the magnetic field and induction, a greater value of EMF occurs in the conductor. Thus, the value of the EMF of induction in a conductor moving in a magnetic field is directly dependent on the induction of the magnetic field, the length of the conductor and the speed of its movement.
This dependence is reflected in the formula E = Blv, where E is the induction emf; B - the value of magnetic induction; I - conductor length; v is the speed of its movement.
Note that in a conductor that moves in a magnetic field, the induction EMF appears only when it crosses the magnetic field lines. If the conductor moves along the lines of force, then no EMF is induced. For this reason, the formula applies only in cases where the movement of the conductor is directed perpendicular to the lines of force.
The direction of the induced EMF and electric current in the conductor is determined by the direction of movement of the conductor itself. To identify the direction, the right hand rule has been developed. If you hold the palm of your right hand in such a way that field lines of force enter in its direction, and thumb indicates the direction of movement of the conductor, then the remaining four fingers show the direction of the induced EMF and the direction of the electric current in the conductor.
Rotating coil
The functioning of the electric current generator is based on the rotation of the coil in a magnetic flux, where there is a certain number of turns. EMF is induced in an electric circuit always when it is crossed by a magnetic flux, based on the magnetic flux formula Ф \u003d B x S x cos α (magnetic induction multiplied by the surface area through which the magnetic flux passes, and the cosine of the angle formed by the direction vector and the perpendicular plane lines).
According to the formula, F is affected by changes in situations:
- when the magnetic flux changes, the direction vector changes;
- the area enclosed in the contour changes;
- angle changes.
It is allowed to induce EMF with a stationary magnet or a constant current, but simply when the coil rotates around its axis within the magnetic field. In this case, the magnetic flux changes as the angle changes. The coil in the process of rotation crosses the lines of force of the magnetic flux, as a result, an EMF appears. With uniform rotation, a periodic change in the magnetic flux occurs. Also, the number of field lines that cross every second becomes equal to the values at regular intervals.
In practice, in alternating current generators, the coil remains stationary, and the electromagnet rotates around it.
EMF self-induction
When an alternating electric current passes through the coil, an alternating magnetic field is generated, which is characterized by a changing magnetic flux that induces an EMF. This phenomenon called self-induction.
Due to the fact that the magnetic flux is proportional to the intensity of the electric current, then the self-induction EMF formula looks like this:
Ф = L x I, where L is the inductance, which is measured in H. Its value is determined by the number of turns per unit length and the value of their cross section.
Mutual induction
When two coils are located side by side, they observe the EMF of mutual induction, which is determined by the configuration of the two circuits and their mutual orientation. With increasing separation of the circuits, the value of mutual inductance decreases, since there is a decrease in the total magnetic flux for the two coils.
Let us consider in detail the process of the emergence of mutual induction. There are two coils, current I1 flows through the wire of one with N1 turns, which creates a magnetic flux and goes through the second coil with N2 number of turns.
The value of the mutual inductance of the second coil in relation to the first:
M21 = (N2 x F21)/I1.
Magnetic flux value:
F21 = (M21/N2) x I1.
The induced emf is calculated by the formula:
E2 = - N2 x dФ21/dt = - M21x dI1/dt.
In the first coil, the value of the induced emf:
E1 = - M12 x dI2/dt.
It is important to note that the electromotive force provoked by mutual inductance in one of the coils is in any case directly proportional to the change in electric current in the other coil.
Then the mutual inductance is considered equal to:
M12 = M21 = M.
As a consequence, E1 = - M x dI2/dt and E2 = M x dI1/dt. M = K √ (L1 x L2), where K is the coupling coefficient between the two inductance values.
Mutual inductance is widely used in transformers, which make it possible to change the value of an alternating electric current. The device is a pair of coils that are wound on a common core. The current in the first coil forms a changing magnetic flux in the magnetic circuit and a current in the second coil. With fewer turns in the first coil than in the second, the voltage increases, and, accordingly, with a greater number of turns in the first winding, the voltage decreases.
In addition to generating and transforming electrical energy, the phenomenon of magnetic induction is used in other devices. For example, in magnetic levitation trains moving without direct contact with the current in the rails, but a couple of centimeters higher due to electromagnetic repulsion.
At the ends of the conductor, and hence the current, it is necessary to have external forces of a non-electric nature, with the help of which the separation of electric charges occurs.
Third party forces any forces acting on electrically charged particles in a circuit are called, with the exception of electrostatic (i.e., Coulomb).
Third-party forces set in motion charged particles inside all current sources: in generators, at power plants, in galvanic cells, batteries, etc.
When the circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, the charges move under the action of external forces against the Coulomb forces (electrons move from a positively charged electrode to a negative one), and in the rest of the circuit they are driven by an electric field (see figure above).
In current sources, in the course of work on the separation of charged particles, a transformation occurs different types energy into electricity. According to the type of converted energy, the following types of electromotive force are distinguished:
- electrostatic- in an electrophore machine, in which mechanical energy is converted into electrical energy during friction;
- thermoelectric- in a thermoelement, the internal energy of a heated junction of two wires made of different metals is converted into electrical energy;
- photovoltaic— in a photocell. Here, light energy is converted into electrical energy: when some substances are illuminated, for example, selenium, copper oxide (I), silicon, a loss of a negative electric charge is observed;
- chemical- in galvanic cells, batteries, and other sources in which chemical energy is converted into electrical energy.
Electromotive Force (EMF)- characteristic of current sources. The concept of EMF was introduced by G. Ohm in 1827 for DC circuits. In 1857, Kirchhoff defined EMF as the work of external forces during the transfer of a unit electric charge along a closed circuit:
ɛ \u003d A st / q,
where ɛ - EMF of the current source, A st- the work of external forces, q is the amount of charge transferred.
The electromotive force is expressed in volts.
We can talk about the electromotive force in any part of the circuit. This is the specific work of external forces (the work of moving a unit charge) not in the entire circuit, but only in this area.
Internal resistance of the current source.
Let there be a simple closed circuit consisting of a current source (for example, a galvanic cell, battery or generator) and a resistor with resistance R. The current in a closed circuit is not interrupted anywhere, therefore, it also exists inside the current source. Any source represents some resistance to current. It's called current source internal resistance and is marked with the letter r.
In the generator r- this is the resistance of the winding, in a galvanic cell - the resistance of the electrolyte solution and electrodes.
Thus, the current source is characterized by the values of EMF and internal resistance, which determine its quality. For example, electrostatic machines have a very high EMF (up to tens of thousands of volts), but at the same time their internal resistance is huge (up to hundreds of Mohms). Therefore, they are unsuitable for receiving high currents. In galvanic cells, the EMF is only approximately 1 V, but the internal resistance is also small (approximately 1 ohm or less). This allows them to receive currents measured in amperes.
In electrical engineering, power supplies for electrical circuits are characterized by electromotive force (EMF).
What is EMF
In the external circuit of the electrical circuit, electric charges move from the plus of the source to the minus and create an electric current. To maintain its continuity in the circuit, the source must have a force that could move charges from a lower to a higher potential. Such a force of non-electric origin is the EMF of the source. For example, the EMF of a galvanic cell.
According to this, the EMF (E) can be calculated as:
E=A/q, where:
- A is work in joules;
- q - charge in pendants.
The value of EMF in the SI system is measured in volts (V).
Formulas and calculations
EMF is the work done by external forces to move a unit charge through an electrical circuit.
The circuit of a closed electric circuit includes an external part, characterized by resistance R, and an internal part with source resistance Rin. Continuous current (In) in the circuit will flow as a result of the action of the EMF, which overcomes both external and internal resistance of the circuit.
The current in the circuit is determined by the formula (Ohm's law):
In \u003d E / (R + Rin).
In this case, the voltage at the source terminals (U 12) will differ from the EMF by the amount of voltage drop across the internal resistance of the source.
U 12 \u003d E - In * Rin.
If the circuit is open and the current in it is 0, then the EMF of the source will be equal to the voltage U 12.
Power supply designers are trying to reduce the internal resistance Rin, as this can allow more current to be drawn from the source.
Where applicable
In technology, various types of EMF are used:
- Chemical. Used in batteries and accumulators.
- Thermoelectric. Occurs when the contacts of dissimilar metals are heated. Used in refrigerators, thermocouples.
- Induction. Formed when a conductor crosses a magnetic field. The effect is used in electric motors, generators, transformers.
- Photovoltaic. It is used to create photocells.
- Piezoelectric. When the material is stretched or compressed. Used for the manufacture of sensors, quartz oscillators.
Thus, EMF is necessary to maintain a constant current and finds applications in various types technology.
In physics, the concept electromotive force(abbreviated - EMF) is used as the main energy characteristic of current sources.
Electromotive Force (EMF)
Electromotive force (EMF) - the ability of the energy source to create and maintain a potential difference on the clamps.
EMF- measured in volts
The voltage at the source terminals is always less EMF by the voltage drop.
Electromotive force
U RH = E – U R0
U RH is the voltage at the source terminals. Measured with the external circuit closed.
E - EMF - measured at the factory.
Electromotive force (EMF) is a physical quantity, which is equal to the quotient of the division of the work that, when moving an electric charge, is performed by external forces in a closed circuit, to this charge itself.
It should be noted that electromotive force in the current source also occurs in the absence of the current itself, that is, when the circuit is open. This situation is usually called "idle", and the value itself EMF when it is equal to the difference in those potentials that are available at the terminals of the current source.
Chemical electromotive force
Chemical electromotive force is present in batteries, galvanic batteries in the course of corrosion processes. Depending on the principle on which the operation of a particular power source is built, they are called either batteries or galvanic cells.
One of the main distinguishing characteristics of galvanic cells is that these current sources are, so to speak, disposable. During their operation, active substances, due to which electrical energy is released, as a result of chemical reactions, they decay almost completely. That is why if the galvanic cell is completely discharged, then it is no longer possible to use it as a current source.
Unlike galvanic cells, batteries are reusable. This is possible because the chemical reactions that take place in them are reversible.
electromagnetic electromotive force
electromagnetic EMF occurs during the operation of such devices as dynamos, electric motors, chokes, transformers, etc.
Its essence is as follows: when conductors are placed in a magnetic field and they are moved in it in such a way that the magnetic lines of force intersect, guidance occurs. EMF. If the circuit is closed, then an electric current occurs in it.
In physics, the phenomenon described above is called electromagnetic induction. electromotive force, which is induced in this case, is called EMF induction.
It should be noted that pointing EMF Induction occurs not only in those cases when the conductor moves in a magnetic field, but also when it remains stationary, but at the same time the magnitude of the magnetic field itself changes.
Photoelectric electromotive force
This variety electromotive force occurs when there is either an external or internal photoelectric effect.
In physics, the photoelectric effect (photoelectric effect) means that group of phenomena that occurs when light acts on a substance, and at the same time electrons are emitted in it. This is called the external photoelectric effect. If, however, it appears electromotive force or the electrical conductivity of a substance changes, then they speak of an internal photoelectric effect.
Now, both external and internal photoelectric effects are very widely used to design and manufacture a huge number of such light radiation receivers that convert light signals into electrical ones. All these devices are called photocells and are used both in technology and in various scientific research. In particular, photocells are used to make the most objective optical measurements.
Electrostatic driving force
As for this type electromotive force, then it, for example, occurs during mechanical friction that occurs in electrophore units (special laboratory demonstration and auxiliary devices), it also takes place in thunderclouds.
Wimshurst generators (this is another name for electrophore machines) use such a phenomenon as electrostatic induction for their operation. During their operation, electric charges accumulate at the poles, in Leyden jars, and the potential difference can reach very substantial values (up to several hundred thousand volts).
The nature of static electricity is that it occurs when, due to the loss or acquisition of electrons, intramolecular or intraatomic equilibrium is disturbed.
Piezoelectric electromotive force
This variety electromotive force occurs when either squeezing or stretching of substances called piezoelectrics occurs. They are widely used in designs such as piezoelectric sensors, crystal oscillators, hydrophones, and some others.
It is the piezoelectric effect that underlies the operation of piezoelectric sensors. They themselves belong to the sensors of the so-called generator type. In them, the input is the applied force, and the output is the amount of electricity.
As for devices such as hydrophones, their operation is based on the principle of the so-called direct piezoelectric effect, which piezoceramic materials have. Its essence lies in the fact that if sound pressure is applied to the surface of these materials, then a potential difference appears on their electrodes. Moreover, it is proportional to the magnitude of the sound pressure.
One of the main areas of application of piezoelectric materials is the production of quartz oscillators, which have quartz resonators in their design. Such devices are designed to receive oscillations of a strictly fixed frequency, which are stable both in time and with temperature changes, and also have a very low level of phase noise.
Thermionic electromotive force
This variety electromotive force occurs when thermal emission of charged particles occurs from the surface of heated electrodes. Thermionic emission is used quite widely in practice, for example, the operation of almost all radio tubes is based on it.
Thermoelectric electromotive force
This variety EMF occurs when at different ends of dissimilar conductors or simply in different parts of the circuit, the temperature is distributed very non-uniformly.
thermoelectric electromotive force used in devices such as pyrometers, thermocouples and refrigeration machines. Sensors whose operation is based on this phenomenon are called thermoelectric, and are, in fact, thermocouples consisting of electrodes soldered together, made of different metals. When these elements are either heated or cooled, a EMF, which is proportional to the change in temperature.
