emf changes. Electromotive Force - Knowledge Hypermarket

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 electricity. 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 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 of technology.

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 terms– 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. physical meaning this phenomenon lies in the ability electromagnetic field 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 teaching aids related to the subject called "Electrical Engineering". To understand the essence of the ongoing processes, it is enough to recall that EMF induction 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, the 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 complete chain, the formula of 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. With a more precise approach, taking into account 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. With time EMF value slightly decreases, 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

And what is its relationship with other parameters Everyday life we all successfully use electrical appliances, many laws have been derived empirically and taken as an axiom. This is one of the reasons for the overcomplication of definitions. Unfortunately, even the electromotive force, this basis of electrical engineering, is illuminated in such a way that it is quite difficult for a person unfamiliar with electricity to understand anything. Let's explain this question with the help of clear terms and examples.

In a conductor it is called "electric current". As you know, all objects of our material world consist of atoms. To simplify understanding, we can assume that each atom is represented as a million times smaller in the center, the nucleus is located, and at different distances from it, electrons rotate in circular orbits.

By means of some external influence, an electromotive force is created in the conductor that forms a closed circuit and the action “knocks out” valence electrons from their orbits in atoms, therefore free electrons and positively charged ions are formed.

The electromotive force is necessary in order to "force" the charges to constantly move along the conductor and circuit elements in a certain direction. Without it, the current almost instantly fades away. To understand what an electromotive force is, a comparison of electricity with water will allow. A straight section of pipe is a conductor. With two of its sides, it goes out into the reservoirs. As long as the water levels in the reservoirs are equal and there is no slope, the liquid in the pipe is motionless.

Obviously, there are three ways to make it move: create a height difference (by the slope or the amount of liquid in the reservoirs) or force it to pump. An important point: if we talk about the difference in heights, then tension is implied. For the EMF, the movement is “forced”, since the external forces that have an impact are non-potential.

Any source of electric current has an EMF - the very force that maintains the movement of charged particles (in the above analogy, makes water move). Measured in volts. The name speaks for itself: EMF characterizes the work of external forces applied to a section of the circuit, performing the movement of each unit charge from one pole to another (between the terminals). It is numerically equal to the ratio of the work of the applied external forces to the value of the charge being moved.

Indirectly, the need for an EMF source can be derived from the law of conservation of energy and the properties of a current-carrying conductor. In a closed circuit, the work of the field in moving charges is zero. However, the conductor heats up (and the stronger, the more current passes through it per unit time). Conclusion: there must be a share of third-party energy in the circuit. The indicated external forces are the magnetic field in the generators, which constantly excites the electrons; energy chemical reactions in batteries.

The electromotive force of induction was first discovered experimentally in 1831. He found that an electric current arises in a conductor penetrated by lines of intensity of a changing magnetic field. The action of the field imparts the energy they lack to the outer electrons in the atoms, as a result of which they come off and begin to move (a current appears). Of course, there is no direct motion of particles (how can one not recall the relativity of the axioms of electrical engineering here). Rather, there is an exchange of particles between nearby atoms.

The developed electromotive force is an internal characteristic of any power source.

Lecture Search

EMF. Numerically, the electromotive force is measured by the work done by a source of electrical energy in the transfer of a single positive charge throughout a closed circuit. If the energy source, doing work A, ensures the transfer of charge q throughout the closed circuit, then its electromotive force (E) will be equal to

The SI unit for electromotive force is the volt (v). A source of electrical energy has an emf of 1 volt if, when moving through the entire closed circuit of a charge of 1 coulomb, work is done equal to 1 joule. The physical nature of electromotive forces in different sources is very different.

Self-induction - the occurrence of an EMF of induction in a closed conducting circuit when the current flowing through the circuit changes. When the current I in the circuit changes, the magnetic flux B through the surface bounded by this circuit also changes proportionally. A change in this magnetic flux, due to the law of electromagnetic induction, leads to the excitation of an inductive emf E in this circuit. This phenomenon is called self-induction.

The concept is related to the concept of mutual induction, being its particular case.

Power. Power is the work done per unit of time. Power is the work done per unit of time, i.e. to transfer charge to el. the circuit or in a closed circuit expends energy, which is equal to A \u003d U * Q, since the amount of electricity is equal to the product of the current strength, then Q \u003d I * t, it follows that A \u003d U * I * t. P=A/t=U*Q/t=U*I=I*t*R=P=U*I(I)

1W=1000mV, 1kW=1000V, Pr=Pp+Po power balance formula. Pr-generator power(emf)

Pr=E*I, Pp=I*U useful power, i.e. power that is consumed without loss. Po=I^2*R-lost power. In order for the circuit to function, it is necessary to maintain a balance of power in the electric circuit.

12. Ohm's law for a chain section.

The current strength in the circuit section is directly proportional to the voltage at the ends of this conductor and inversely proportional to its resistance: I \u003d U / R;

1)U=I*R, 2)R=U/R

13. Ohm's law for a complete circuit.

The current strength in the circuit is proportional to the EMF acting in the circuit and inversely proportional to the sum of the circuit resistances and the internal resistance of the source.

EMF of the voltage source (V), - current in the circuit (A), - resistance of all external elements of the circuit (Ohm), - internal resistance of the voltage source (Ohm) .1) E \u003d I (R + r)? 2)R+r=E/I

14. Series, parallel connection of resistors, equivalent resistance. Distribution of currents and voltage.

At serial connection several resistors, the end of the first resistor is connected to the beginning of the second, the end of the second to the beginning of the third, etc. With such a connection, the same current I passes through all elements of the series circuit.

Ue=U1+U2+U3. Therefore, the voltage U at the source terminals is equal to the sum of the voltages across each of the resistors connected in series.

Re=R1+R2+R3, Ie=I1=I2=I3, Ue=U1+U2+U3.

When connected in series, the resistance of the circuit increases.

Parallel connection of resistors. A parallel connection of resistances is such a connection in which the beginnings of the resistances are connected to one terminal of the source, and the ends to the other terminal.

The total resistance of the resistors connected in parallel is determined by the formula

The total resistance of resistors connected in parallel is always less than the smallest resistance included in this connection.

when the resistances are connected in parallel, the voltages across them are equal to each other. Ue=U1=U2=U3 Current I flows into the circuit, and currents I1, I2, I3 flow out of it. Since moving electric charges do not accumulate at a point, it is obvious that the total charge flowing to the branch point is equal to the total charge flowing from it: Ie \u003d I1 + I2 + I3 Therefore, the third property of a parallel connection can be formulated as follows: branched part of the circuit is equal to the sum of the currents in the parallel branches. For two parallel resistors:

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DETERMINATION OF THE EMF AND POWER OF THE CURRENT SOURCE - Megatutorial

CHIPS, branch of USUPS

Department of UND

STUDY OF THE ELECTROSTATIC FIELD

students

Teacher

Chelyabinsk

The purpose of the work: to determine the position of equipotential surfaces and lines of force of the electrostatic field by modeling, to calculate the field strength.

Equipment: a sheet of metal foil with a coordinate grid and electrodes, a VSP-33 power supply, a multimeter, a probe.

CALCULATION FORMULA

An electrostatic field is a form of matter that manifests itself in the action on electric charges. An electrostatic field is created:

The strength characteristic of the field is the intensity. It's a vector defined by...

The energy characteristic of the electrostatic field is the potential. By definition it is...

There is a connection between the two characteristics of the field, strength and potential:

For clarity, the electrostatic field is depicted graphically using force and equipotential lines. These are the lines...

Approximately according to the location of equipotential lines, the intensity can be calculated by the formula:

COMPLETING OF THE WORK

Calculation of tension E=…………………..

Evaluation of the error in measuring the intensity δЕ=

ANSWERS TO CONTROL QUESTIONS

CHIPS, branch of USUPS

Department of UND

DETERMINATION OF THE EMF AND POWER OF THE CURRENT SOURCE

students

Teacher

Chelyabinsk

The purpose of the work: to determine the EMF of a DC source by the compensation method, to determine the useful power and efficiency depending on the load resistance.

Equipment: current source under investigation, stabilized voltage source, resistance box, milliammeter, galvanometer.

CALCULATION FORMULA

Current sources are devices in which various types of energy are converted into ...... ...

The characteristic of the current source is ………… By definition, it is equal to the ratio ………………..

Consider an electrical circuit from a current source with internal resistance r, closed to a load by resistance R. According to the law of conservation of energy, the work of external forces is converted to ……… according to the equation ……………………… From where we get Ohm's law for a closed circuit in the form:

In the compensation method for measuring EMF using the power supply regulator of the PSU, the voltage on the resistance box R is selected exactly equal to …………….. Then the source EMF will be equal to ………..

The useful power of the current source is the thermal power released on the load. According to the Joule-Lenz law ……………………………

Substituting the current strength according to Ohm's law, we obtain the formula for useful power:

The operation of the current source is characterized by the efficiency. This is, by definition……

The formula for the efficiency of the current source is:

COMPLETING OF THE WORK

An example of calculating the EMF E \u003d JR \u003d

Average EMF value<Е> =

Evaluation of the random error in measuring the EMF of the source =

EMF measurement result E =………±……….В Р = 90%.

Calculation example: net power: Рpol =J 2R =

full power Рzatr =<Е>J= Efficiency η

Power

ANSWERS TO CONTROL QUESTIONS

CHIPS, branch of USUPS

Department of UND

megaobuchalka.ru

Relationship formula between EMF (electromotive force) and voltage.

In tasks for electric current, voltage and EMF (electromotive force) are present as given or found. Is there enough simple connection between these options. Let us introduce any chain (Fig. 1).

Rice. 1. Relationship between EMF and voltage

Let a source with emf be given

Voltage in the external circuit. The internal resistance of the source is , and the resistance of the external circuit is . This system is energized. Then: (1) (2)

It is logical to assume that the number of electrons generated by the source is equal to the number of electrons that went into the circuit, then we equate (1) and (2):

Relation (3) - relationship between EMF and voltage in a complete DC circuit.

In an ideal circuit (the internal resistance of the source is zero

), EMF is numerically equal to voltage.

Conclusion: the above ratios help in a number of tasks in which the parameters of the current / voltage source are given, but it is necessary to find the current or voltage on any element of the circuit (resistor, coil, lamp, etc.), and vice versa.

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EMF and voltage

In order for the electric current to pass through the circuit for a long time, it is necessary to continuously maintain a potential difference at the poles of the voltage source. Similarly, if two vessels are connected by a tube with different levels water, then the water will move from one vessel to another until the levels in the vessels are equal. By adding water to one vessel and withdrawing it from another, it is possible to ensure that the movement of water through the tube between the vessels will continue uninterrupted.

When the source of electrical energy is operating, electrons from the anode pass to the cathode.

From this we can conclude that a force acts inside the source of electrical energy, which must continuously maintain the current in the circuit, that is, in other words, must ensure the operation of this source.

The reason that establishes and maintains a potential difference, causes a current in the circuit, overcoming its external and internal resistance, is called electromotive force (abbreviated emf) and is denoted by the letter E.

The electromotive force of electrical energy sources arises under the influence of causes specific to each of them.

In chemical sources of electrical energy (galvanic cells, batteries) e. d.s. obtained as a result of chemical reactions, in generators e. d.s. arises due to electromagnetic induction, in thermoelements - due to thermal energy.

The potential difference that causes the passage of current through the resistance of a section of an electrical circuit is called the voltage between the ends of this section. Electromotive force and voltage are measured in volts. To measure e. d.s. and voltage are devices - voltmeters (Figure 1).

Thousandths of a volt - millivolts - are measured by millivoltmeters, thousands of volts - kilovolts - by kilovoltmeters.

To measure e. d.s. source of electrical energy, it is necessary to connect the voltmeter to the terminals of this source with the external circuit open (Figure 2). To measure the voltage in any section of the electrical circuit, the voltmeter must be connected to the ends of this section (Figure 3).

Video 1. What is electromotive force (emf)

Source: Kuznetsov M.I., "Fundamentals of Electrical Engineering" - 9th edition, revised - Moscow: graduate School, 1964 - 560s.

www.electromechanics.ru

Electromotive force. | Association of teachers of St. Petersburg

Electromotive force.
The role of the current source: to divide the charges due to the performance of work by external forces. Any forces acting on a charge, with the exception of potential forces of electrostatic origin (i.e., Coulomb) are called external forces. (External forces are explained by the electromagnetic interaction between electrons and nuclei)
EMF - energy characteristic of the source. This physical quantity, equal to the ratio of the work done by external forces when moving an electric charge along a closed circuit, to this charge: It is measured in volts (V).
Another characteristic of the source is the internal resistance of the current source: r.
Ohm's law for a complete circuit.
Energy transformations in the circuit: - law of energy conservation (A - the work of external forces; Ext. - the work of the current on the external section of the circuit with resistance R; Aint. - the work of the current on the internal resistance of the source r.)
Ohm's Law: The current in a DC circuit is directly proportional to the EMF of the current source and inversely proportional to the impedance of the electrical circuit.
Consequences:
1. If R>>r, then ε=U. Measure e with a high-resistance voltmeter with the external circuit open.
2.If R<
3. On the inner section of the chain: Aint=U1q, on the outer section of the chain: Aext=U2q. A=Aint+ Aext Then: εq=U1q+U2q. Therefore: ε= U1+U2 The EMF of the current source is equal to the sum of the voltage drops in the external and internal sections of the circuit.
4. If R grows, then I decreases. - when the current in the circuit decreases, the voltage increases!
5. Power: a) Full .. b) Useful. . c) lost. . d) efficiency .
Connection of current sources.
1. Serial connection of sources: the total EMF of the circuit is equal to the algebraic sum of the EMF of individual sources, the total internal resistance is equal to the sum of the internal resistances of all current sources. If all sources are the same and included in the same direction, then . Then s-r Ohm will be written in the form:
2. Parallel connection of sources: one of the sources (with the highest EMF) works as a source, the rest - as consumers (battery charging is based on this principle). Calculation by Kirchhoff's rules (see). If all sources are the same, then Ohm's law will be written in the form:
Ohm's law for an inhomogeneous section of a chain.
- the signs "+" or "-" are selected depending on whether the currents created by the EMF source and the electric field are directed in one or opposite directions.
1. The algebraic sum of the currents in each node (branch point) is equal to 0. - a consequence of the law of conservation of electric charge.
Consequence of Ohm's law for an inhomogeneous section of the chain.
The direction of the currents is chosen arbitrarily. If after calculations the current value is negative, then the direction is opposite. A closed loop is bypassed in one direction. If the bypass direction is the same as the current direction, then IR>0. If during the bypass they come to the "+" of the source, then its EMF is negative. The resulting system of equations should include all EMF and all resistances. That. the system should consist of one equation for currents and the k-1st equation for EMF (k is the number of closed loops).

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What is emf - formula and application

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:

• 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 (U12) will differ from the EMF by the amount of voltage drop across the internal resistance of the source.

U12 = 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 U12.

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 of technology.

elektro.guru

Electromotive Force - WiKi

Electromotive force (EMF) is a scalar physical quantity that characterizes the work of external forces, that is, any forces of non-electric origin acting in quasi-stationary DC or AC circuits. In a closed conducting circuit, the EMF is equal to the work of these forces in moving a single positive charge along the entire circuit.

By analogy with the electric field strength, the concept of external force strength E→ex(\displaystyle (\vec (E))_(ex)) is introduced, which is understood as a vector physical quantity equal to the ratio of the external force acting on the test electric charge to the value of this charge. Then in the closed circuit L(\displaystyle L) the EMF will be equal to:

E=∮L⁡E→ex⋅dl→,(\displaystyle (\mathcal (E))=\oint \limits _(L)(\vec (E))_(ex)\cdot (\vec (dl) ))

where dl→(\displaystyle (\vec (dl))) is the outline element.

EMF, like voltage, is measured in volts in the International System of Units (SI). We can talk about the electromotive force in any part of the circuit. This is the specific work of external forces not in the entire circuit, but only in this section. The EMF of a galvanic cell is the work of external forces when moving a single positive charge inside the cell from one pole to another. The work of external forces cannot be expressed in terms of the potential difference, since external forces are non-potential and their work depends on the shape of the trajectory. So, for example, the work of external forces when moving a charge between the terminals of a current source outside the source itself is equal to zero.

EMF and Ohm's law

The electromotive force of the source is related to the electric current flowing in the circuit by Ohm's law relations. Ohm's law for an inhomogeneous section of the circuit has the form:

φ1−φ2+E=IR,(\displaystyle \varphi _(1)-\varphi _(2)+(\mathcal (E))=IR,)

where φ1−φ2(\displaystyle \varphi _(1)-\varphi _(2)) is the difference between the potential values ​​at the beginning and at the end of the circuit section, I(\displaystyle I) is the current flowing through the section, and R (\displaystyle R) - section resistance.

If points 1 and 2 coincide (the circuit is closed), then φ1−φ2=0(\displaystyle \varphi _(1)-\varphi _(2)=0) and the previous formula becomes the Ohm's law formula for a closed circuit:

E=IR,(\displaystyle (\mathcal (E))=IR,)

where now R(\displaystyle R) is the impedance of the entire circuit.

In general, the circuit impedance is the sum of the resistance of the circuit section external to the current source (Re(\displaystyle R_(e))) and the internal resistance of the current source itself (r(\displaystyle r)). With this in mind, you should:

E=IRe+Ir.(\displaystyle (\mathcal (E))=IR_(e)+Ir.)

EMF current source

If external forces do not act on the circuit section (homogeneous section of the circuit) and, therefore, there is no current source on it, then, as follows from Ohm's law for an inhomogeneous section of the circuit, the following is true:

φ1−φ2=IR.(\displaystyle \varphi _(1)-\varphi _(2)=IR.)

Hence, if we choose the anode of the source as point 1, and its cathode as point 2, then for the difference between the potentials of the anode φa(\displaystyle \varphi _(a)) and the cathode φk(\displaystyle \varphi _(k)) can be written:

φa−φk=IRe,(\displaystyle \varphi _(a)-\varphi _(k)=IR_(e),)

where, as before, Re(\displaystyle R_(e)) is the resistance of the outer section of the circuit.

From this relation and Ohm's law for a closed circuit written as E=IRe+Ir(\displaystyle (\mathcal (E))=IR_(e)+Ir) it is easy to get

φa−φkE=ReRe+r(\displaystyle (\frac (\varphi _(a)-\varphi _(k))(\mathcal (E)))=(\frac (R_(e))(R_(e )+r))) and then φa−φk=ReRe+rE.(\displaystyle \varphi _(a)-\varphi _(k)=(\frac (R_(e))(R_(e)+r) )(\mathcal (E)).)

Two conclusions follow from the obtained relation:

1. In all cases when current flows through the circuit, the potential difference between the terminals of the current source φa−φk(\displaystyle \varphi _(a)-\varphi _(k)) is less than the EMF of the source.
2. In the limiting case when Re(\displaystyle R_(e)) is infinite (the chain is broken), E=φa−φk.(\displaystyle (\mathcal (E))=\varphi _(a)-\varphi _(k ).)

Thus, the EMF of the current source is equal to the potential difference between its terminals in the state when the source is disconnected from the circuit.

EMF induction

The reason for the occurrence of an electromotive force in a closed circuit can be a change in the magnetic field flux penetrating the surface bounded by this circuit. This phenomenon is called electromagnetic induction. The value of the EMF induction in the circuit is determined by the expression

E=−dΦdt,(\displaystyle (\mathcal (E))=-(\frac (d\Phi )(dt)),)

where Φ(\displaystyle \Phi ) is the magnetic field flux through the closed surface bounded by the contour. The "−" sign in front of the expression shows that the induction current created by the induction EMF prevents a change in the magnetic flux in the circuit (see Lenz's rule). In turn, the reason for the change in the magnetic flux can be both a change in the magnetic field and the movement of the circuit as a whole or its individual parts.

Non-electric nature of EMF

Inside the EMF source, the current flows in the opposite direction to the normal one. This is impossible without an additional force of a non-electric nature that overcomes the electrical repulsion force.

As shown in the figure, an electric current, the normal direction of which is from “plus” to “minus”, inside an EMF source (for example, inside a galvanic cell) flows in the opposite direction. The direction from "plus" to "minus" coincides with the direction of the electric force acting on positive charges. Therefore, in order to make the current flow in the opposite direction, an additional force of a non-electric nature (centrifugal force, Lorentz force, chemical nature forces) is needed to overcome the electrical force.

see also

Notes

en-wiki.org

Content:

When the concept of "electron" was born, people immediately associated it with a certain job. Electron is Greek for amber. The fact that the Greeks, in order to find this useless, in general, magical stone, had to travel quite far to the north - such efforts here, in general, do not count. But it was worth doing some work - by rubbing a pebble on a dry woolen cloth with your hands - and it acquired new properties. Everyone knew it. To rub just like that, for the sake of a purely disinterested interest, in order to observe how now small debris begins to be attracted to the “electron”: dust particles, hairs, threads, feathers. In the future, when a whole class of phenomena appeared, later united in the concept of "electricity", the work that must be spent without fail did not give people peace. Since you need to spend it to get a trick with dust particles, it means that it would be nice to somehow save this work, accumulate it, and then get it back.

Thus, from more and more complicated tricks with different materials and philosophical reasoning, they learned to collect this magical power in a jar. And then make it so that it is gradually released from the jar, causing actions that can already be felt, and very soon measured. And they measured it so ingeniously, having only a couple of silk balls or sticks and a spring torsion balance, that even now we quite seriously use all the same formulas for calculating electrical circuits that have now permeated the entire planet, infinitely complex, compared with those first devices. .

And the name of this mighty genie, sitting in a jar, still contains the delight of old discoverers: "Electromotive force." But this force is not electrical at all. On the contrary, it is an extraneous terrible force that makes electric charges move "against their will", that is, overcoming mutual repulsion, and gather somewhere on one side. This results in a potential difference. It can also be used by launching charges in a different way. Where they are "not guarded" by this terrible EMF. And to force, thereby, to do some work.

Principle of operation

EMF is a force of a very different nature, although it is measured in volts:

• Chemical. It comes from the processes of chemical substitution of ions of some metals by ions of others (more active). As a result, extra electrons are formed, tending to "escape" at the edge of the nearest conductor. This process can be reversible or irreversible. Reversible - in batteries. They can be charged by returning the charged ions back into the solution, which makes it more acidic, for example (in acid batteries). The acidity of the electrolyte is the reason for the EMF of the battery, it works continuously until the solution becomes absolutely chemically neutral.

• Magnetodynamic. Occurs when a conductor, in some way oriented in space, is exposed to a changing magnetic field. Or from a magnet moving relative to a conductor, or from the movement of a conductor relative to a magnetic field. Electrons in this case also tend to move in the conductor, which allows them to be captured and placed on the output contacts of the device, creating a potential difference.

• electromagnetic. An alternating magnetic field is created in the magnetic material by an alternating electrical voltage of the primary winding. In the secondary winding, the movement of electrons occurs, and hence the voltage is proportional to the voltage in the primary winding. The EMF symbol can be used to designate transformers in equivalent substitution circuits.

• Photovoltaic. Light, falling on some conductive materials, is able to knock out electrons, that is, to make them free. An excess of these particles is created, which is why the excess ones are pushed to one of the electrodes (anode). There is a voltage that can generate an electric current. Such devices are called photocells. Initially, vacuum photocells were invented, in which the electrodes were installed in a flask with a vacuum. In this case, the electrons were pushed out of the metal plate (cathode) and captured by another electrode (anode). Such photocells have found application in light sensors. With the invention of more practical semiconductor photocells, it became possible to create powerful batteries from them in order to generate a significant voltage by summing the electromotive force of each of them.

• Thermoelectric. If two different metals or semiconductors are soldered at one point, and then heat is delivered to this point, for example, candles, then at the opposite ends of a pair of metals (thermocouples) there is a difference in the density of the electron gas. This difference can accumulate if thermocouples are connected in series, similar to the connection of galvanic cells in a battery or individual photovoltaic cells in a solar battery. ThermoEMF is used in very accurate temperature sensors. This phenomenon is associated with several effects (Peltier, Thomson, Seebeck), which are successfully investigated. It is a fact that heat can be directly converted into electromotive force, i.e. voltage.

• electrostatic. Such sources of EMF were invented almost simultaneously with galvanic cells or even earlier (if we consider rubbing amber with silk as a normal production of EMF). They are also called electrophore machines, or, after the name of the inventor, Wimshurst generators. Although Wimshurst created a clear technical solution that allows the removed potential to be accumulated in a Leiden jar - the first capacitor (moreover, of good capacity). The first electrophore machine can be considered a huge ball of sulfur, mounted on an axis, the apparatus of the Magdeburg burgomaster Otto von Guericke in the middle of the 17th century. The principle of operation is rubbing materials that are easily electrified from friction. True, von Guericke's progress can be called, as the saying goes, driven by laziness, when there is no desire to rub amber or something else by hand. Although, of course, this inquisitive politician of something, but fantasy and activity was not to be occupied. Let us recall at least his well-known experience with two strings of donkeys (or mules) tearing a ball without air by the chains into two hemispheres.

Electrization, as originally assumed, comes precisely from “friction”, that is, by rubbing amber with a rag, we “tear off” electrons from its surface. However, studies have shown that this is not so simple. It turns out that there are always charge irregularities on the surface of dielectrics, and ions from the air are attracted to these irregularities. Such an air-ion coat is formed, which we damage by rubbing the surface.

• Thermionic. When metals are heated, electrons are released from their surface. In a vacuum, they reach another electrode and induce a negative potential there. A very promising direction right now. The figure shows a scheme for protecting a hypersonic aircraft from overheating of body parts by an oncoming air flow, and the thermoelectrons emitted by the cathode (which is then cooled - the simultaneous action of the Peltier and / or Thomson effects) reach the anode, inducing a charge on it. The charge, or rather, the voltage, which is equal to the received EMF, can be used in the consumption circuit inside the device.

1 - cathode, 2 - anode, 3, 4 - cathode and anode taps, 5 - consumer

• Piezoelectric. Many crystalline dielectrics, when they experience mechanical pressure on themselves in any direction, react to it by inducing a potential difference between their surfaces. This difference depends on the applied pressure and is therefore already used in pressure sensors. Piezoelectric gas stove lighters do not require any other source of energy - just pressing a button with your finger. Known attempts to create a piezoelectric ignition system in vehicles based on piezoceramics, receiving pressure from a system of cams associated with the main shaft of the engine. "Good" piezoelectrics - in which the EMF proportionality to pressure is highly accurate - are very hard (for example, quartz), they almost do not deform under mechanical pressure.

• However, long exposure to pressure on them causes their destruction. In nature, thick layers of rock are also piezoelectric, the pressure of the earth's thickness induces huge charges on their surfaces, which generates titanic storms and thunderstorms in the depths of the earth. However, not everything is so terrible. Elastic piezoelectrics have already been developed, and even the manufacture of products based on them (and based on nanotechnology) for sale has already begun.

The fact that the unit of measurement of EMF is the unit of electrical voltage is understandable. Since the most diverse mechanisms that create the electromotive force of the current source all convert their types of energy into the movement and accumulation of electrons, and this ultimately leads to the appearance of such a voltage.

Current arising from EMF

The electromotive force of the current source is the driving force that the electrons from it begin to move if the electrical circuit is closed. EMF forces them to do this, using its non-electric "half" of nature, which does not depend, after all, on the half associated with electrons. Since it is believed that the current in the circuit flows from plus to minus (such a direction was determined before everyone knew that the electron is a negative particle), then inside the device with EMF, the current makes the final movement - from minus to plus. And they always draw at the EMF sign, where the arrow - + is directed. Only in both cases - both inside the EMF of the current source, and outside, that is, in the consuming circuit - we are dealing with electric current with all its mandatory properties. In conductors, current encounters their resistance. And here, in the first half of the cycle, we have the load resistance, in the second, internal, - the source resistance or internal resistance.

The internal process does not work instantly (although very quickly), but with a certain intensity. He does the work of delivering charges from minus to plus, and this also meets with resistance ...

Resistance is of two kinds.

1. Internal resistance works against the forces separating the charges, it has a nature "close" to these separating forces. At least it works with them in a single mechanism. For example, an acid that takes oxygen from lead dioxide and replaces it with SO 4 - ions definitely experiences some chemical resistance. And this just manifests itself as the work of the internal resistance of the battery.
2. When the outer (output) half of the circuit is not closed, the appearance of more and more electrons at one of the poles (and their decrease from the other pole) causes an increase in the electrostatic field strength at the poles of the battery and an increase in repulsion between electrons. This allows the system to “not go haywire” and stop at a certain state of saturation. No more electrons from the battery are taken outside. And it outwardly looks like the presence of a constant electrical voltage between the battery terminals, which is called U xx, the open circuit voltage. And it is numerically equal to EMF - electromotive force. Therefore, the unit of measure for EMF is the volt (in the SI system).

But if you only connect a load of conductors with non-zero resistance to the battery, then a current will immediately flow, the strength of which is determined by Ohm's law.

It would seem that it is possible to measure the internal resistance of the EMF source. It is worth including an ammeter in the circuit and shunting (shorting) the external resistance. However, the internal resistance is so low that the battery will begin to discharge catastrophically, generating a huge amount of heat, both on the external shorted conductors and in the internal space of the source.

However, you can do it differently:

1. Measure E (remember, open circuit voltage, the unit of measurement is volts).
2. Connect some resistor as a load and measure the voltage drop across it. Calculate the current I 1 .
3. You can calculate the value of the internal resistance of the EMF source using the expression for r

Typically, the ability of a battery to produce electricity is measured by its energy "capacity" in ampere hours. But it would be interesting to see what maximum current it can produce. Despite the fact that perhaps the electromotive force of the current source will cause it to explode. Since the idea to arrange a short circuit on it did not seem very tempting, this value can be calculated purely theoretically. EMF is equal to U xx. You just need to draw a graph of the voltage drop across the resistor versus current (and therefore the load resistance) to the point where the load resistance will be zero. This is the point Ikz, intersection of the red line with the coordinate line I , in which the voltage U has become zero, and the entire voltage E of the source will fall on the internal resistance.

Often seemingly simple basic concepts can not always be understood without examples and analogies. What is the electromotive force, and how it works, can only be imagined by considering its many manifestations. And it is worth considering the definition of EMF, as it is given by solid sources through clever academic words - and start all over from the beginning: the electromotive force of the current source. Or just print on the wall in gold letters: