What is emf in physics in simple words. Electromotive force

EMF is understood as the specific work of external forces to move a single charge in the circuit of an electrical circuit. This concept in electricity involves many physical interpretations related to various fields 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 during reactions accompanied by the separation of electrical charges. In physics, it corresponds to the electromotive force created at the ends of an electrical 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 they are still somewhat different. In short, the EMF is on the power source without a load, and when a load is connected to it, it is already a voltage. Because the number of volts on the power supply under load is almost always slightly 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 is described by the law. Physical meaning this phenomenon is the ability of electrical magnetic field induce an emf in a nearby conductor. In this case, either the field must change, for example, in the 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 that is similar in meaning - mutual induction. It lies in the fact that a change in the direction and strength of the current of one coil induces an emf at the terminals of a nearby coil; it is widely used in various areas equipment, 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 electrical engineering, a physical effect called EMF is used in the manufacture of special AC converters that provide the required values ​​of effective quantities (current and voltage). Thanks to the phenomena of induction, engineers have been able to develop many electrical devices: from the usual (inductor) to the transformer.

The concept of mutual induction refers only to alternating current, the flow of which in a circuit or conductor changes the magnetic flux.

An electric current of constant direction is characterized by other manifestations of this force, such as, for example, a potential difference at the poles of a galvanic cell, which we will discuss later.

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 processes taking place, it is enough to remember that the induced emf is induced when a 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 current consumed by the motor when the load increases 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 torque on the rotor axis.

Unfortunately, within the scope of this article we will not delve into this topic - write in the comments if you are interested in it, 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. Pass through the rotor windings electricity, a magnetic field appears 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.

A little more theory

When designing such circuits, the current distribution and voltage drop across individual elements are taken into account. To calculate the distribution of the first parameter, the sum known from physics is used - the sum of the 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 the complete circuit, the formula which is given below:

I=E/(R+r),

WhereE – emf,R – load resistance,r is the resistance of the power source.

The internal resistance of the power source 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, must be taken into account. A more accurate approach, taking into account large values ​​of operating currents, takes into account the resistance of each connecting conductor.

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 flowing 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 EMF decreases slightly, and the internal resistance increases noticeably.

As a result, if you measure the voltage on a AA battery that is not connected to anything, you see the normal 1.5V (or so), 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 voltmeter’s internal resistance is many times higher than the internal resistance of the battery, then you measured its EMF. When the battery began to supply current to the load at its terminals, it became not 1.5V, but, say, 1.2V - the device did not have enough voltage or current for normal operation. It was precisely this 0.3V that dropped on the internal resistance of the galvanic element. If the battery is very old and its electrodes are destroyed, then there may be no electromotive force or voltage at all at the battery terminals - i.e. zero.

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

You can find out 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 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 value 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 electrical charges through it.
2. EMF, like voltage, is measured in Volts.
3. The differences between EMF and voltage are that the first is measured without a load, and the second with a load, while the internal resistance of the power source is taken into account and influences.

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

Materials

In this lesson we will take a closer look at the mechanism for providing 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: Laws direct current
Lesson: Electromotive force

In one of the previous topics (conditions for the existence of electric current), the issue of the need for a power source to maintain the existence of electric current for a long time was already touched upon. The current itself, of course, can be obtained without such power sources. For example, a capacitor discharges when the camera flashes. But such a current will be too fleeting (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 entire 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 are forces of non-electrical 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. They ensure the existence of current, since the work of electrical forces in a closed circuit is always zero.

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

Extraneous forces inside the source do work against the electrical forces, spreading the charges in directions opposite to their natural course (as they move in the external circuit) (Fig. 2).

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

An analogue of the action of a power source can be considered a water pump, which releases water against its natural flow (from bottom to top, into apartments). The water naturally flows back down under the influence of gravity, but for continuous operation of the water supply to the apartment, continuous operation of the pump is necessary.

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

Unit of measurement:

Insert. EMF of open and closed circuit

Consider the following circuit (Fig. 3):

Rice. 3.

With the switch open and an ideal voltmeter (resistance is infinitely high), there will be no current in the circuit, and only work on separating charges will be done 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 a closed loop:

Here: - voltage on the external circuit (on 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.: Mnemosyne, 2012.
2. Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Ilexa, 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 external 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. *The emf of a flashlight battery is 4.5 V. Will a light bulb designed for 4.5 V burn at full intensity from this battery? Why?

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

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

generators at power plants (thermal, wind, nuclear, hydro), converting mechanical energy into electrical energy;

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

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

Electromotive EMF force E is a scalar quantity characterizing the ability of an external field and induced electric field cause 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 in the field to another.

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

The movement of charges across an area is accompanied by the expenditure of energy.

Magnitude, numerically equal to work, which the source makes by 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 Uvsh 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 = Uin + Uin.

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

It is possible to measure voltages in different 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 forced movement positive charges inside the generator from minus to plus under the influence of a nature other than electrical.

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

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

Conventional image (electrical diagram) of an ideal EMF generator of magnitude 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 power of the source 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 operating mode of an ideal one, they try to make the internal resistance of the real generator Ri as small as possible, and the load resistance Rн must be connected with a value no less than 10 times greater than the internal resistance of the generator , i.e. the following condition must be met: Rн >> Ri

To ensure that the output voltage of a real EMF generator does not 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 made standard for the possibility of coordinated connection of energy consumers to it. In radio engineering, the standard output resistance 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 exactly this impedance.

To get closer to ideal EMF generators, supply voltage sources used in all industrial and household electronic equipment are made using special electronic output voltage stabilization circuits, which make it possible to maintain an almost constant output voltage of the power source in a given range of currents consumed from the EMF source (sometimes its called a voltage source).

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

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

In this lesson we will take a closer look at the mechanism for providing 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: Laws of direct current
Lesson: Electromotive force

In one of the previous topics (conditions for the existence of electric current), the issue of the need for a power source to maintain the existence of electric current for a long time was already touched upon. The current itself, of course, can be obtained without such power sources. For example, a capacitor discharges when the camera flashes. But such a current will be too fleeting (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 entire 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 are forces of non-electrical 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. They ensure the existence of current, since the work of electrical forces in a closed circuit is always zero.

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

Extraneous forces inside the source do work against the electrical forces, spreading the charges in directions opposite to their natural course (as they move in the external circuit) (Fig. 2).

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

An analogue of the action of a power source can be considered a water pump, which releases water against its natural flow (from bottom to top, into apartments). The water naturally flows back down under the influence of gravity, but for continuous operation of the water supply to the apartment, continuous operation of the pump is necessary.

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

Unit of measurement:

Insert. EMF of open and closed circuit

Consider the following circuit (Fig. 3):

Rice. 3.

With the switch open and an ideal voltmeter (resistance is infinitely high), there will be no current in the circuit, and only work on separating charges will be done 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 a closed loop:

Here: - voltage on the external circuit (on 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.: Mnemosyne, 2012.
2. Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Ilexa, 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 external 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. *The emf of a flashlight battery is 4.5 V. Will a light bulb designed for 4.5 V burn at full intensity from this battery? Why?

In electrical engineering, power supplies for electrical circuits are characterized 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 can move charges from lower to higher potential. This force of non-electrical origin is the emf of the source. For example, the emf of a galvanic cell.

According to this, EMF (E) can be calculated as:

E=A/q, Where:

• A – work in joules;
• q is the charge in coulombs.

The magnitude 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 along an electrical circuit

The circuit of a closed electrical circuit includes an external part, characterized by resistance R, and an internal part with source resistance Rin. A continuous current (In) in the circuit will flow as a result of the action of the EMF, which overcomes both the external and internal resistance of the circuit.

The current in the circuit is determined by the formula (Ohm's law):

In= E/(R+Rin).

In this case, the voltage at the source terminals (U 12) will differ from the EMF by the amount of the voltage drop across the internal resistance of the source.

U 12 = 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.

Developers of power supplies try to reduce the internal resistance Rin, as this may allow more current to be received from the source.

Where is it used?

Various types of EMF are used in technology:

• Chemical. Used in batteries and accumulators.
• Thermoelectric. Occurs when 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, and transformers.
• Photovoltaic. Used to create photocells.
• Piezoelectric. When the material is stretched or compressed. Used for the manufacture of sensors and quartz oscillators.

Thus, EMF is necessary to maintain constant current and is used in various types technology.