In the material, we will understand the concept of EMF induction in situations of its occurrence. We also consider inductance as a key parameter for the occurrence of a magnetic flux when an electric field appears in a 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 as an example: if a conductor frame is placed in an electric field with induction and moved from top to bottom along magnetic field lines or to the right or left perpendicular to them, then the magnetic flux passing through the frame will be constant.

When the frame rotates around its axis, then after a while the magnetic flux will change by a certain amount. As a result, an EMF of induction arises in the frame and an electric current appears, which is called induction.

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 so that the field lines enter in its direction, and the thumb indicates the direction of movement of the conductor, then the remaining four fingers indicate 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 is 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.

Electromotive Force (EMF)- in a device that performs forced separation of positive and negative charges (generator), a value numerically equal to the potential difference between the generator terminals in the absence of current in its circuit is measured in Volts.

Sources of electromagnetic energy (generators)- devices that convert energy of any non-electric form into electrical energy. Such sources are, for example:

generators at power plants (thermal, wind, nuclear, hydroelectric power plants) that convert mechanical energy into electrical energy;

galvanic cells (batteries) and accumulators of all types that convert chemical energy into electrical energy, etc.

EMF is numerically equal to the work that external forces do when moving a unit positive charge inside the source or the source itself, conducting a unit positive charge through a closed circuit.

The electromotive force EMF E is a scalar quantity that characterizes the ability of an external field and an induced electric field to induce an electric current. EMF E is numerically equal to the work (energy) W in joules (J) expended by this field to move a unit of charge (1 C) from one point of the field to another.

The unit of measure for EMF is the volt (V). Thus, the EMF is equal to 1 V if, when a charge of 1 C is moved along a closed circuit, work of 1 J is performed: [E] = I J / 1 C = 1 V.

The movement of charges around the site is accompanied by the expenditure of energy.

The value numerically equal to the work done by the source, conducting a single positive charge through a given section of the circuit, is called voltage U. Since the circuit consists of external and internal sections, the concepts of voltages in the external Uin and internal Uvt sections are distinguished.

From what has been said, it is obvious that The EMF of the source is equal to the sum of the voltages on the external U and internal U sections of the circuit:

E \u003d Uvsh + Uvt.

This formula expresses the law of conservation of energy for an electrical circuit.

It is possible to measure voltages in various parts of the circuit only when the circuit is closed. EMF is measured between the source terminals with an open circuit.

The direction of the EMF is the direction of the forced movement of positive charges inside the generator from minus to plus under the action of a nature other than electrical.

The internal resistance of the generator is the resistance of the structural elements inside it.

Ideal EMF source- a generator, which is equal to zero, and the voltage at its terminals does not depend on the load. The power of an ideal EMF source is infinite.

Conditional image (electric circuit) of an ideal EMF generator with a value of E shown in fig. 1, a.

A real EMF source, unlike an ideal one, contains an internal resistance Ri and its voltage depends on the load (Fig. 1., b), and the source power is finite. The electrical circuit of a real EMF generator is a series connection of an ideal EMF generator E and its internal resistance Ri.

In practice, in order to bring the operating mode of a real EMF generator closer to the ideal operating mode, they try to make the internal resistance of a real generator Ri as small as possible, and the load resistance Rn must be connected with a value of at least 10 times greater than the internal resistance of the generator , i.e. condition must be met: Rn >> Ri

In order for the output voltage of a real EMF generator not to depend on the load, it is stabilized using special electronic voltage stabilization circuits.

Since the internal resistance of a real EMF generator cannot be made infinitely small, it is minimized and performed as a standard for the possibility of a consistent connection of energy consumers to it. In radio engineering, the standard output impedance of EMF generators is 50 ohms (industrial standard) and 75 ohms (household standard).

For example, all television receivers have an input impedance of 75 ohms and are connected to the antennas with a coaxial cable of just such a wave impedance.

To approach ideal EMF generators, the supply voltage sources used in all industrial and household radio-electronic equipment are performed using special electronic output voltage stabilization circuits that allow you to maintain an almost constant output voltage of the power source in a given range of currents consumed from the EMF source (sometimes it called a voltage source).

On electrical circuits, EMF sources are depicted as follows: E - a source of constant EMF, e (t) - a source of harmonic (variable) EMF in the form of a function of time.

The electromotive force E of a battery of identical cells connected in series is equal to the electromotive force of one cell E multiplied by the number of cells n of the battery: E = nE.

Third-party (non-potential) forces in the sources of post. or altern. current; in a closed conducting circuit is equal to the work of these forces to move the unit put. charge along the entire circuit. If through Egr we denote the field strength of external forces, then emf? in a closed loop L is equal to

where dl is the contour length element.

Pot. electrostatic forces. fields cannot support post. of these forces on a closed path is zero. The passage of current through the conductors is accompanied by the release of energy - heating of the conductors. Third-party forces lead to charge. h-tsy inside the generators, galvanic. elements, accumulators and other current sources. The origin of external forces can be different: in generators, these are forces from the vortex electric. the field that occurs when the magnetic field changes. field with time, or Lorentz, acting from the magnetic. fields on e-ns in a moving conductor; in galvanic cells and batteries - this is a chemical. forces, etc. The source emf is equal to the electrical voltage at its terminals with an open circuit. EMF determines the strength of the current in the circuit for a given resistance (see OHMA LAW). It is measured, as well as electric. , in volts.

Physical Encyclopedic Dictionary. - M.: Soviet Encyclopedia. . 1983 .

ELECTROMOTIVE FORCE

(emf) - phenomenological characteristic of current sources. Introduced by G. Ohm in 1827 for DC circuits. current and defined by G. Kirchhoff (G. Kirchhoff) in 1857 as the work of "external" forces during the transfer of a single electric. charge along a closed loop. Then the concept of emf began to be interpreted more broadly - as a measure of specific (per unit charge carried by the current) energy transformations carried out in quasi-stationary [see. Quasi-stationary (quasi-static) approximation]electric circuits not only by "third-party" sources (galvanic batteries, batteries, generators, etc.), but also by "load" elements (electric motors, batteries in charging mode, chokes, transformers, etc.).

Full name magnitude - E. s. - associated with mechanical. analogies of processes in electric. chains and rarely used; more common is the abbreviation - emf. In SI, emf is measured in volts (V); in the Gaussian system (CGSE) unit emf spec. has no name (1 SGSE 300 V).

In the case of a quasi-linear post. current in a closed (without branching) circuit of the total influx of el.-mag. energy generated by sources is completely spent on heat generation (see. Joule losses):

where is the emf in the conducting circuit, I-current, R- resistance (the sign of the emf, as well as the sign of the current, depends on the choice of the direction of bypass along the circuit).

When describing quasi-stationary processes in electric. chains in ur-nii energetic. balance (*) it is necessary to take into account changes in the accumulated magnetic Wm and electrical We energies:

When changing the magnetic field in time there is a vortex electric. E s , the circulation of which along the conducting circuit is usually called emf electromagnetic induction:

Electrical changes. energies are significant, as a rule, in cases where the circuit contains a large electric. capacity, eg. capacitors. Then dW e /dt = D U. I where D U- potential difference between the capacitor plates.

However, other interpretations of the energetics are also possible. conversion to electricity. chains. So, for example, if in the AC circuit. harmonic current connected with inductance L then mutual transformations of electric. and magn. energies in it can be characterized as emf el.-magn. induction and voltage drop across the effective reactance Z L(cm. Impedance): In moving in magn. field of bodies (eg, in the armature of a unipolar inductor), even the work of resistance forces can contribute to the emf.

In branched circuits of quasi-linear currents, the relationship between emf and voltage drops in the sections of the circuit that make up a closed circuit is determined by the second Kirchhoff rule.

EMF is an integral characteristic of a closed circuit, and in the general case it is impossible to strictly indicate the place of its "application". However, quite often, the emf can be considered approximately localized in certain devices or circuit elements. In such cases, it is customary to consider it a characteristic of the device (galvanic battery, battery, dynamo, etc.) and determine it through the potential difference between its open poles. According to the type of energy conversion in these devices, the following types of emf are distinguished: chemical and mimic emf in galvanic. batteries, baths, accumulators, during corrosive processes (galvanic effects), photoelectric emf (photo emf) at external. and ext. photoelectric effect (photocells, photodiodes); elec tro magnetic induction (dynamos, transformers, chokes, electric motors, etc.); elec tro static emf arising, for example, during mechanical. friction (electrophore machines, electrification of thunderclouds, etc.); piezoelectric emf - when squeezing or stretching piezoelectrics (piezoelectric sensors, hydrophones, frequency stabilizers, etc.); thermoionic emf associated with thermionic charge. particles from the surface of heated electrodes; thermoelectric emf ( thermopower)- on contacts of dissimilar conductors ( Seebeck effect And Peltier effect) or in sections of the circuit with a non-uniform temperature distribution ( Thomson effect). Thermopower is used in thermocouples, pyrometers, refrigerators.

M. A. Miller, G. V. Permitin.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .

See what the "ELECTRIC DRIVE FORCE" is in other dictionaries:

electromotive force- A scalar value that characterizes the ability of an external field and an induced electric field to cause an electric current. Note - The electromotive force is equal to the linear integral of the strength of the external field and the induced ... ... Technical Translator's Handbook The Modern Encyclopedia is a scalar value that characterizes the ability of an external field and an induced electric field to cause an electric current ...

What's happened EMF(electromotive force) in physics? Electric current is not understood by everyone. Like space distance, only under the very nose. In general, it is not fully understood by scientists either. Enough to remember Nikola Tesla with his famous experiments, centuries ahead of their time and even today remaining in a halo of mystery. Today we are not solving big mysteries, but we are trying to figure out what is emf in physics.

Definition of EMF in physics

EMF is the electromotive force. Denoted by letter E or the small Greek letter epsilon.

Electromotive force- scalar physical quantity characterizing the work of external forces ( forces of non-electric origin) operating in electrical circuits of alternating and direct current.

EMF, like voltage e, measured in volts. However, EMF and voltage are different phenomena.

Voltage(between points A and B) - a physical quantity equal to the work of the effective electric field performed when transferring a unit test charge from one point to another.

We explain the essence of EMF "on the fingers"

To understand what is what, we can give an analogy example. Imagine that we have a water tower completely filled with water. Compare this tower with a battery.

Water exerts maximum pressure on the bottom of the tower when the tower is full. Accordingly, the less water in the tower, the weaker the pressure and pressure of the water flowing from the tap. If you open the tap, the water will gradually flow out at first under strong pressure, and then more and more slowly until the pressure weakens completely. Here stress is the pressure that the water exerts on the bottom. For the level of zero voltage, we will take the very bottom of the tower.

It's the same with the battery. First, we include our current source (battery) in the circuit, closing it. Let it be a clock or a flashlight. While the voltage level is sufficient and the battery is not discharged, the flashlight shines brightly, then gradually goes out until it goes out completely.

But how to make sure that the pressure does not run out? In other words, how to maintain a constant water level in the tower, and a constant potential difference at the poles of the current source. Following the example of the tower, the EMF is presented as a pump, which ensures the influx of new water into the tower.

The nature of the emf

The reason for the occurrence of EMF in different current sources is different. According to the nature of occurrence, the following types are distinguished:

• Chemical emf. Occurs in batteries and accumulators due to chemical reactions.
• Thermo EMF. Occurs when contacts of dissimilar conductors at different temperatures are connected.
• EMF of induction. Occurs in a generator when a rotating conductor is placed in a magnetic field. EMF will be induced in a conductor when the conductor crosses the lines of force of a constant magnetic field or when the magnetic field changes in magnitude.
• Photoelectric EMF. The occurrence of this EMF is facilitated by the phenomenon of an external or internal photoelectric effect.
• Piezoelectric emf. EMF occurs when a substance is stretched or compressed.

Dear friends, today we have considered the topic "EMF for Dummies". As you can see, the EMF force of non-electric origin, which maintains the flow of electric current in the circuit. If you want to know how problems with EMF are solved, we advise you to contact our authors– scrupulously selected and proven specialists who will quickly and clearly explain the course of solving any thematic problem. And by tradition, at the end we invite you to watch the training video. Happy viewing and good luck with your studies!

At the height of the school year, many scientists need an emf formula for various calculations. Experiments related to also need information about the electromotive force. But for beginners, it is not so easy to understand what it is.

The formula for finding emf

Let's deal with the definition first. What does this abbreviation mean?

EMF or electromotive force is a parameter characterizing the work of any forces of a non-electric nature operating in circuits where the current strength, both direct and alternating, is the same along the entire length. In a coupled conductive circuit, the EMF is equated to the work of these forces in moving a single positive (positive) charge along the entire circuit.

The figure below shows the emf formula.

Ast - means the work of external forces in joules.

q is the transferred charge in coulombs.

Third party forces- these are the forces that carry out the separation of charges in the source and, as a result, form a potential difference at its poles.

For this force, the unit of measurement is volt. It is denoted in the formulas by the letter « E".

Only at the moment of the absence of current in the battery, the electromotive si-a will be equal to the voltage at the poles.

EMF induction:

EMF of induction in a circuit havingNturns:

When moving:

Electromotive force induction in a circuit rotating in a magnetic field at a speedw:

Table of values

A simple explanation of the electromotive force

Suppose there is a water tower in our village. It is completely filled with water. Let's think that this is an ordinary battery. The tower is a battery!

All the water will put a lot of pressure on the bottom of our turret. But it will be strong only when this structure is completely filled with H 2 O.

As a result, the less water, the weaker the pressure will be and the pressure of the jet will be less. Opening the tap, we note that every minute the jet range will be reduced.

As a result:

1. Tension is the force with which the water presses on the bottom. That is pressure.
2. Zero voltage is the bottom of the tower.

The battery is the same.

First of all, we connect a source of energy to the circuit. And we close it accordingly. For example, insert a battery into a flashlight and turn it on. Initially, note that the device is lit brightly. After a while, its brightness will noticeably decrease. That is, the electromotive force has decreased (leaked when compared with water in the tower).

If we take a water tower as an example, then the EMF is a pump that constantly pumps water into the tower. And it never ends there.

EMF of a galvanic cell - formula

The electromotive force of a battery can be calculated in two ways:

• Perform the calculation using the Nernst equation. It will be necessary to calculate the electrode potentials of each electrode included in the GE. Then calculate the EMF using the formula.
• Calculate the EMF using the Nernst formula for the total current generating the reaction that occurs during the operation of the GE.

Thus, armed with these formulas, it will be easier to calculate the electromotive force of the battery.

Where are different types of EMF used?

1. Piezoelectric is used when a material is stretched or compressed. With the help of it, quartz energy generators and various sensors are made.
2. Chemical is used in and batteries.
3. Induction appears at the moment the conductor crosses the magnetic field. Its properties are used in transformers, electric motors, generators.
4. Thermoelectric is formed at the moment of heating contacts of different types of metals. It has found its application in refrigeration units and thermocouples.
5. Photo electric is used to produce photovoltaic cells.