What is emf in what units is it measured. EMF. Ohm's law for a complete circuit. Real EMF source

What 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 watch 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!

EMF is understood as the specific work of external forces to move a unit charge in the circuit of an electric circuit. This concept in electricity involves many physical interpretations related to various areas of technical knowledge. In electrical engineering, this is the specific work of external forces that appears in inductive windings when an alternating field is induced in them. In chemistry, it means the potential difference that occurs during electrolysis, as well as in reactions accompanied by the separation of electric charges. In physics, it corresponds to the electromotive force generated at the ends of an electric thermocouple, for example. To explain the essence of EMF in simple words, you will need to consider each of the options for its interpretation.

Before moving on to the main part of the article, we note that EMF and voltage are very similar concepts in meaning, but still somewhat different. In short, the EMF is on the power source without load, and when a load is connected to it, this is already voltage. Because the number of volts on the IP under load is almost always somewhat less than without it. This is due to the internal resistance of power sources such as transformers and galvanic cells.

Electromagnetic induction (self-induction)

Let's start with electromagnetic induction. This phenomenon describes the law. The physical meaning of this phenomenon is the ability of an electromagnetic field to induce an EMF in a nearby conductor. In this case, either the field must change, for example, in magnitude and direction of the vectors, or move relative to the conductor, or the conductor must move relative to this field. In this case, a potential difference arises at the ends of the conductor.

There is another phenomenon similar in meaning - mutual induction. It lies in the fact that a change in the direction and current strength of one coil induces an EMF at the terminals of a nearby coil, which is widely used in various fields of technology, including electrical and electronics. It underlies the operation of transformers, where the magnetic flux of one winding induces current and voltage in the second.

In electrics, a physical effect called EMF is used in the manufacture of special AC converters that provide the desired values ​​​​of effective quantities (current and voltage). Thanks to the phenomena of induction and engineers, it was possible to develop many electrical devices: from a conventional one (choke) to a transformer.

The concept of mutual inductance applies only to alternating current, during the flow of which the magnetic flux changes in the circuit or conductor.

For an electric current of constant directivity, other manifestations of this force are characteristic, such as, for example, the potential difference at the poles of a galvanic cell, which we will discuss below.

Electric motors and generators

The same electromagnetic effect is observed in the design or, the main element of which is inductive coils. His work is described in accessible language in many textbooks related to the subject called "Electrical Engineering". To understand the essence of the ongoing processes, it is enough to recall that the induction EMF is induced when the conductor moves inside another field.

According to the law of electromagnetic induction mentioned above, a counter EMF is induced in the armature winding of the motor during operation, which is often called "back EMF", because when the motor is running, it is directed towards the applied voltage. This also explains the sharp increase in the current consumed by the motor when the load is increased or the shaft is jammed, as well as starting currents. For an electric motor, all the conditions for the appearance of a potential difference are obvious - a forced change in the magnetic field of its coils leads to the appearance of a torque on the rotor axis.

Unfortunately, within this article we will not delve into this topic - write in the comments if it is of interest to you, and we will tell you about it.

In another electrical device - a generator, everything is exactly the same, but the processes occurring in it have the opposite direction. An electric current is passed through the rotor windings, a magnetic field arises around them (permanent magnets can be used). When the rotor rotates, the field, in turn, induces an EMF in the stator windings - from which the load current is removed.

Some more theory

When designing such circuits, the distribution of currents and the voltage drop on individual elements are taken into account. To calculate the distribution of the first parameter, a well-known from physics is used - the sum of voltage drops (taking into account the sign) on all branches of a closed circuit is equal to the algebraic sum of the EMF of the branches of this circuit), and to determine their values, they use for a section of the circuit or Ohm's law for a complete circuit, the formula which is given below:

I=E/(R+r),

whereE - EMF,R is the load resistance,r is the power supply resistance.

The internal resistance of the power supply is the resistance of the windings of generators and transformers, which depends on the cross section of the wire with which they are wound and its length, as well as the internal resistance of galvanic cells, which depends on the state of the anode, cathode and electrolyte.

When carrying out calculations, the internal resistance of the power source, considered as a parallel connection to the circuit, is necessarily taken into account. In a more precise approach, taking into account the large values ​​of operating currents, the resistance of each connecting conductor is taken into account.

EMF in everyday life and units of measurement

Other examples are found in the practical life of any ordinary person. This category includes such familiar things as small batteries, as well as other miniature batteries. In this case, the working EMF is formed due to chemical processes occurring inside DC voltage sources.

When it occurs at the terminals (poles) of the battery due to internal changes, the element is completely ready for operation. Over time, the value of the EMF decreases somewhat, and the internal resistance increases markedly.

As a result, if you measure the voltage on an AA battery that is not connected to anything, you see 1.5V (or so) normal for it, but when a load is connected to the battery, let's say you installed it in some device - it does not work.

Why? Because if we assume that the internal resistance of the voltmeter is many times higher than the internal resistance of the battery, then you measured its EMF. When the battery began to give off current in the load, its terminals became not 1.5V, but, say, 1.2V - the device does not have enough voltage or current for normal operation. Just these 0.3V fell on the internal resistance of the galvanic cell. If the battery is very old and its electrodes are destroyed, then there may be no electromotive force or voltage at the battery terminals at all - i.e. zero.

This example clearly demonstrates the difference between EMF and voltage. The author says the same at the end of the video, which you can see below.

You can learn more about how the EMF of a galvanic cell occurs and how it is measured in the following video:

A very small electromotive force is also induced within the receiver antenna, which is then amplified by special cascades, and we receive our television, radio and even Wi-Fi signal.

Conclusion

Let's summarize and once again briefly recall what EMF is and in what SI units this quantity is expressed.

1. EMF characterizes the work of external forces (chemical or physical) of non-electrical origin in an electrical circuit. This force does the work of transferring electric charges to it.
2. EMF, like voltage, is measured in volts.
3. The differences between EMF and voltage are that the first is measured without load, and the second with load, while taking into account and affecting the internal resistance of the power source.

And finally, to consolidate the material covered, I advise you to watch another good video on this topic:

materials

This publication discusses the basic terms, laws and methods for calculating the EMF of magnetic induction. Using the materials below, you can independently determine the current strength in interconnected circuits, the change in voltage in typical transformers. This information will be useful for solving various electrical problems.

magnetic flux

It is known that the passage of current through a conductor is accompanied by the formation of an electromagnetic field. The operation of speakers, locking devices, relay drives, and other devices is based on this principle. By changing the parameters of the power source, the necessary force efforts are obtained to move (hold) the combined parts with ferromagnetic properties.

However, the opposite is also true. If a frame of conductive material is moved between the poles of a permanent magnet along a corresponding closed circuit, the movement of charged particles will begin. By connecting the appropriate devices, you can register the change in current (voltage). In the course of an elementary experiment, one can find out an increase in the effect in the following situations:

• perpendicular arrangement of the conductor/power lines;
• movement acceleration.

The picture above shows how to determine the direction of current in a conductor using a simple rule.

What is induction emf

The movement of charges noted above creates a potential difference if the circuit is open. The presented formula shows exactly how the EMF will depend on the main parameters:

• vector expression of the magnetic flux (B);
• length (l) and speed (v) of the control conductor;
• angle (α) between motion/induction vectors.

A similar result can be obtained if the system is composed of a stationary conducting circuit, which is affected by a moving magnetic field. By closing the circuit, create suitable conditions for the movement of charges. If you use a lot of conductors (coil) or move faster, the current will increase. The presented principles are successfully used to convert mechanical forces into electrical energy.

Designation and units of measurement

The EMF in the formulas is denoted by the vector E. The tension that is created by external forces is implied. Accordingly, this value can be estimated from the potential difference. According to current international standards (SI), the unit of measurement is one volt. Large and small values ​​are indicated using multiple prefixes: "micro", "kilo", etc.

Faraday and Lenz laws

If electromagnetic induction is considered, the formulas of these scientists help to clarify the mutual influence of significant system parameters. Faraday's definition makes it possible to refine the EMF dependence (E– average value) from changes in the magnetic flux (ΔF) and time (Δt):

E = – ∆F/ ∆t.

Intermediate conclusions:

• the current increases if the conductor crosses a greater number of magnetic lines of force per unit time;
• "-" in the formula helps to take into account the mutual relationship between the polarity E, the speed of the frame, the direction of the induction vector.

Lenz substantiated the dependence of the EMF on any changes in the magnetic flux. When the coil circuit is closed, conditions are created for the movement of charges. In this embodiment, the design is converted into a typical solenoid. A corresponding electromagnetic field is formed next to it.

This scientist substantiated an important feature of induction EMF. The field formed by the coil prevents the external flow from changing.

The movement of a wire in a magnetic field

As shown in the first formula (E = B * l * v * sinα), the amplitude of the electromotive force largely depends on the parameters of the conductor. More precisely, the number of field lines per unit length of the circuit's working area has an effect. A similar conclusion can be drawn taking into account the change in the speed of movement. One should not forget about the relative position of the marked vector quantities (sinα).

Important! Moving the conductor along the lines of force does not provoke the induction of an electromotive force.

Rotating coil

It is difficult to ensure the optimal arrangement of functional components while moving when using the straight wire shown in the example. However, by bending the frame, you can get the simplest generator of electricity. The maximum effect is provided by an increase in the number of conductors per unit of working volume. The design corresponding to the noted parameters is a coil, a typical element of a modern alternating current generator.

To estimate the magnetic flux (F) you can apply the formula:

F = B * S * cosα,

where S is the area of ​​the considered working surface.

Explanation. With uniform rotation of the rotor, a corresponding cyclic sinusoidal change in the magnetic flux occurs. The amplitude of the output signal changes in a similar way. It is clear from the figure that the size of the gap between the main functional components of the structure is of some importance.

EMF self-induction

If an alternating current is passed through the coil, an electromagnetic field with similar (uniformly changing) power characteristics will be formed nearby. It creates a variable sinusoidal magnetic flux, which, in turn, provokes the movement of charges and the formation of an electromotive force. This process is called self-induction.

Given the considered basic principles, it is easy to determine that F = L * l. The value of L (in henry) determines the inductive characteristics of the coil. This parameter depends on the number of turns per unit length (l) and the cross-sectional area of ​​the conductor.

Mutual induction

If you assemble a module of two coils, under certain conditions, you can observe the phenomenon of mutual induction. An elementary measurement will show that as the distance between the elements increases, the magnetic flux decreases. The reverse phenomenon is observed as the gap decreases.

To find suitable components when creating electrical circuits, you need to study thematic calculations:

• you can take for example coils with a different number of turns (n1 and n2);
• mutual induction (M2) when passing through the first current circuitI1 will be calculated as follows:

M2 = (n2 * F)/I1

• after converting this expression, the value of the magnetic flux is determined:

F = (M2/ n2) *I1

• to calculate the emf of electromagnetic induction, the formula is suitable from the description of the basic principles:

E2 = - n2 * ΔF/ Δt = M 2 * ΔI1/ Δt

If necessary, you can find the ratio for the first coil using a similar algorithm:

E1 = - n1 * ΔF/ Δt = M 1 * ΔI2/ Δt.

It should be noted that in this case the force (I2) in the second working circuit matters.

Joint influence (mutual induction - M) is calculated by the formula:

M = K * √(L1 * l2).

A special coefficient (K) takes into account the actual strength of the connection between the coils.

Where are different types of EMF used?

Moving a conductor in a magnetic field is used to generate electricity. The rotation of the rotor is provided by the difference in liquid levels (HPP), wind energy, tides, fuel engines.

A different number of turns (mutual induction) is used to change the voltage in the secondary winding of the transformer as needed. In such designs, mutual coupling is increased using a ferromagnetic core. Magnetic induction is used to create a powerful repulsive force in the creation of ultra-modern highways. The created levitation makes it possible to eliminate the force of friction, significantly increase the speed of the train.

Video

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 functioning, those active substances, due to which electrical energy is released, decompose almost completely as a result of chemical reactions. 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 the temperature is distributed very unevenly at different ends of dissimilar conductors or simply at different parts of the circuit.

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.

In this lesson, we will take a closer look at the mechanism for providing a long-term electric current. Let us introduce the concepts of "power source", "external forces", describe the principle of their operation, and also introduce the concept of electromotive force.

Topic: Direct Current Laws
Lesson: Electromotive force

In one of the previous topics (the conditions for the existence of an electric current), the question of the need for a power source for the long-term maintenance of the existence of an electric current was already raised. By itself, the current, of course, can be obtained without such power sources. For example, discharging a capacitor during a camera flash. But such a current will be too transient (Fig. 1).

Rice. 1. Short-term current during mutual discharge of two oppositely charged electroscopes ()

Coulomb forces always strive to bring opposite charges together, thereby equalizing the potentials throughout the circuit. And, as you know, for the presence of a field and a current, a potential difference is necessary. Therefore, it is impossible to do without any other forces that separate the charges and maintain the potential difference.

Definition. Third-party forces - forces of non-electric origin, aimed at diluting charges.

These forces can be of different nature depending on the type of source. In batteries they are of chemical origin, in electric generators they are of magnetic origin. It is they who ensure the existence of the current, since the work of electric forces in a closed circuit is always equal to zero.

The second task of energy sources, in addition to maintaining the potential difference, is to replenish energy losses in the collision of electrons with other particles, as a result of which the former lose kinetic energy, and the internal energy of the conductor increases.

Third-party forces inside the source do work against electric forces, spreading charges in directions opposite to their natural course (as they move in an external circuit) (Fig. 2).

Rice. 2. Scheme of action of third-party forces

An analogue of the action of the power source can be considered a water pump, which lets water against its natural course (from bottom to top, into apartments). Conversely, the water naturally descends under the action of gravity, but for the continuous operation of the water supply of the apartment, the continuous operation of the pump is necessary.

Definition. Electromotive force - the ratio of the work of external forces to move the charge to the magnitude of this charge. Designation - :

Unit of measurement:

Insert. EMF open and closed circuit

Consider the following circuit (Fig. 3):

Rice. 3.

With an open key and an ideal voltmeter (the resistance is infinitely high), there will be no current in the circuit, and only work on the separation of charges will be performed inside the galvanic cell. In this case, the voltmeter will show the EMF value.

When the key is closed, current will flow through the circuit, and the voltmeter will no longer show the EMF value, it will show the voltage value, the same as at the ends of the resistor. With closed loop:

Here: - voltage on the external circuit (at the load and supply wires); - voltage inside the galvanic cell.

In the next lesson, we will study Ohm's law for a complete circuit.

Bibliography

1. Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemozina, 2012.
2. Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.
3. Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.

1. ens.tpu.ru ().
2. physbook.ru ().
3. electrodynamics.narod.ru ().

Homework

1. What are outside forces, what is their nature?
2. How is the voltage at the open poles of a current source related to its EMF?
3. How is energy converted and transferred in a closed circuit?
4. * Flashlight battery EMF - 4.5 V. Will a 4.5 V light bulb burn with full heat from this battery? Why?