EMF. Ohma law for a full chain - materials for preparation for the exam in physics

EMF. Ohm law for full chain

Author of the article - Professional Tutor, Author of Tutorials for Preparations for Ent Igor Vyacheslavovich Yakovlev

Themes of the EGE codifier : Electric power, internal resistance of the current source, Ohma law for a complete electrical circuit.

Until now, when studying electric current, we considered the directional movement of free charges in external chain , that is, in conductors connected to the current source terminals.

As we know, a positive charge Q.:

• goes to an external chain with a positive source terminal;

• moves in the outer chain under the action of a stationary electric field created by other moving charges;

• Comes to a negative source terminal, completing its path in the external chain.

Now our positive charge Q.You need to close your trajectory and return to the positive terminal. To do this, he needs to overcome the final segment of the path - inside the current source from the negative terminal to the positive. But think: go there he does not want to go there! The negative terminal attracts him to himself, the positive terminal pushes him from himself, and as a result, electric power acts inside the source inside the source \ Vec {F_E}aimed vs Charge traffic (i.e. against current direction).

Third-party power

However, the current on the chain goes; Therefore, there is a force, "fluttering" charge through the source contrary to the opposition of the electric field of the terminals (Fig. 1).

Fig. 1. Fredder

This power is called Fidder power ; It is due to it that the current source is functions. Third-party power \ Vec {F_ {CT}}has nothing to do with the stationary electric field - she is said to Neelectric origin; In batteries, for example, it arises due to the flow of relevant chemical reactions.

Denote by A_ {CT}The work of a third-party force to move the positive charge q inside the current source from the negative terminal to positive. This work is positive, since the direction of third-party strength coincides with the direction of movement of the charge. Operation of third-party power A_ {CT}called also called operation of the current source .

In the external chain, third-party power is absent, so the work of a third-party force to move the charge in the outer chain is zero. Therefore, the work of a third-party force to move charge Q.Around the entire chain boils down to work on the movement of this charge only inside the current source. In this way, A_ {CT}- It is also the work of a third-party power to move charge throughout the chain .

We see that third-party power is unprofitable - its work when moving the charge on a closed path is not zero. It is this non-opticality that provides circulation of electric current; The potential electric field, as we have said earlier, cannot maintain a permanent current.

Experience shows that work A_ {CT}Directly proportional to the moved charge Q.. Therefore, the attitude A_ {CT} / QNo longer depends on the charge and is a quantitative characteristic of the current source. This ratio is designated \ Mathcal E.:

\ Mathcal E = \ FRAC {\ DisplayStyle A_ {CT}} {\ DisplayStyle Q \ vphatom {1 ^ A}}. (one)

This value is called electromotive power (EMF) current source. As you can see, the EMF is measured in volts (B), so the name "Electrical Force" is extremely unsuccessful. But it has long been rooted, so you have to come to terms.

When you see the inscription on the battery: "1.5 V", then know that this is the EDC. Is this voltage value that creates a battery in an external circuit? It turns out no! Now we will understand why.

Ohm law for full chain

Any source of current has its resistance R.called internal resistance of this source. Thus, the current source has two important characteristics: EMF and internal resistance.

Let the current source with EMF equal \ Mathcal E., and internal resistance R.Connected to resistor R.(which is called in this case External resistor , or External load , or Payload ). All this together is called full chain (Fig. 2).

Fig. 2. Full chain

Our task is to find current strength I.in chains and tension U.on resistor R..

During T.The chain is charged Q = IT.. According to the formula (one) The current source performs the work:

A_ {Ct} = Eq = EIT. (2)

Since the current is constant, the operation of the source is entirely turning into heat, which is highlighted on the resistances R. и R.. This amount of heat is determined by the law of Joule-Lenza:

Q = I ^ 2RT + I ^ 2RT = I ^ 2 (R + R) T. (3)

So, A_ {ct} = qand we equate the right parts of the formula (2) и (3) :

\ Mathcal e it = i ^ 2 (R + R) T.

After cutting on IT.We get:

\ Mathcal E = i (R + R).

So we found a current in the chain:

I = \ FRAC {\ DisplayStyle \ Mathcal E} {\ DisplayStyle R + R \ VPHANTOM {1 ^ A}}. (four)

Formula (four) called Ohm's law for full chain .

If you connect the source terminals with a negligible resistance wire (R = 0), then it turns out short circuit . Through the source, the maximum current will flow - Short circuit current :

I_ {k3} = \ FRAC {\ DisplayStyle \ Mathcal E} {\ DisplayStyle R \ VPHANTOM {1 ^ A}}.

Due to the smallness of the internal resistance of the short circuit, it can be very large. For example, the finger battery warms up so that it burns hands.

Knowing current strength (formula (four) ), we can find the voltage on the resistor R.Using the Ohm law for the chain section:

U = Ir = \ FRAC {\ DisplayStyle \ Mathcal E R} {\ DisplayStyle R + R \ VPHANTOM {1 ^ A}}. (five)

This tension is the difference of potentials between points A. и B.(Fig. 2). Potential Point A.equal to the potential of the positive source terminal; Potential Point B.It is equal to the potential of the negative terminal. Therefore, tension (five) called also called voltage at the source terminals .

We see from the formula (five) that in the real chain will be U <\ Mathcal E- after all \ Mathcal E.multiplied by fraction, less units. But there are two cases when U = \ mathcal e.

one. Perfect current source . So called a source with zero internal resistance. For R = 0formula (five) Gives U = \ mathcal e.

2. Open circuit . Consider the current source by itself, outside the electrical circuit. In this case, we can assume that the external resistance is infinitely large: R = \ INFTY. Then the value R + R.indistinguishable R., and formula (five) again gives us U = \ mathcal e.

The meaning of this result is simple: If the source is not connected to the chain, then the voltmeter connected to the poles of the source will show its EMF .

Efficiency electric chain

It is not difficult to understand why the resistor R.called payload. Imagine that this is a light bulb. The heat released on the light bulb is Useful Since thanks to this warmth, the light bulb performs its purpose - gives light.

The amount of heat released on the payload R.during T.Denote Q_ {Polezn}.

If the current current in the circuit is equal I.T.

Q_ {polezn} = i ^ 2rt.

A certain amount of heat is also highlighted at the current source:

Q_ {IST} = i ^ 2RT.

The total amount of heat that is highlighted in the chain is:

Q_ {POLN} = Q_ {Polezn} + Q_ {IST} = I2RT + I2RT = I2 (R + R) T.

Efficiency electric chain - This is the ratio of useful heat to full:

\ eta = \ frac {\ displaystyle q_ {polezn}} {\ displaystyle q_ {POLN} \ VPHANTOM {1 ^ a}} = \ FRAC {\ DisplayStyle I ^ 2RT} {\ DisplayStyle I ^ 2 (R + R) T \ vphatom {1 ^ a}} = \ FRAC {\ DisplayStyle R} {\ DisplayStyle R + R \ VPHANTOM {1 ^ A}}.

The CPD of the chain is equal only if the current source is perfect (r = 0).

Ohm law for inhomogeneous plot

Simple law Ohm U = Ir.It is valid for the so-called homogeneous section of the chain - that is, the site on which there are no current sources. Now we will get more common relations, of which it follows as the Ohm law for a homogeneous site, and the above-mentioned law of Oma for the total chain.

Plot chain called inhomogeneous If there is a current source on it. In other words, an inhomogeneous plot is a plot with EMF.

In fig. 3R.and current source. EMF of the source is equal \ Mathcal E., its internal resistance is considered equal to zero (the internal resistance of the source is equal R., you can simply replace the resistor R.on resistor R + R.).

Fig. 3. EMF "helps" current: \ varphi_a - \ varphi_b + \ mathcal e = ir

The strength of the current on the plot is equal I., current flows from point A.To point B.. This current is not necessarily caused by the source only. \ Mathcal E.. The section under consideration, as a rule, is part of a certain chain (not shown in the figure), and other sources of current may be present in this chain. Therefore, current I.is the result of the cumulative action All Sources available in the chain.

Let the potentials of the points A. и B.equal accordingly \ Varphi_A. и \ Varphi_B.. We emphasize once again that we are talking about the potential of a stationary electric field generated by the action of all sources of the chain - not only the source belonging to this section, but also possibly available outside this area.

Voltage on our site is: U = \ Varphi_a - \ Varphi_B. During T.through the plot is charged Q = IT.At the same time, the stationary electric field makes a job:

A_ {POL} = Uq = uit.

In addition, the current is positive operation (after all charge Q.passed through it!):

A_ {CT} = \ Mathcal EQ = \ Mathcal EIT.

Current strength is constant, so the total work to promote charge Q.committed on the site by the stationary eletrical field and third-party source forces, turns into heat: A_ {POL} + A_ {CT} = Q.

We substitute expressions here for A_ {POL}, A_ {CT}And the law of Joule-Lenza:

UIT + \ Mathcal Eit = I ^ 2RT.

Cutting by IT.Receive Ohm law for heterogeneous section of chain :

U + \ Mathcal E = IR, (6)

Or, what is the same:

\ Varphi A - \ Varphi B + \ Mathcal E = IR. (7)

Note: before \ Mathcal E.There is a sign "plus". The reason for this we have already indicated - the current source in this case commits Positive work, "pulling" inside the charge Q.from a negative terminal to positive. Simply put, the source "helps" flow to flow from point A.To point B..

Note two consequences of derived formulas (6) и (7) .

1. If the plot is homogeneous, then \ Mathcal E = 0. Then from formula (6) we get U = Ir.- Ohm law for a homogeneous section of the chain.

2. Suppose that the current source has internal resistance R.. This, as we have already mentioned, is equivalent to replacement. R.on the R + R.:

\ Varphi_a - \ varphi_b + \ mathcal e = i (R + R).

Now we will clicter our site by connecting the points A. и B.. We obtain the complete chain discussed above. It turns out that \ varphi_a = \ varphi_b,And the previous formula will turn into the Ohm law for the full chain:

\ Mathcal E = i (R + R).

Thus, the Ohm law for a homogeneous site and the Ohm law for the total chain of both flows out of the Ohm law for an inhomogeneous site.

Maybe another case of connection when the source \ Mathcal E."Prevents" current to go on the site. This situation is shown in Fig. 4. Here current coming from A. к B.Directed against the action of third-party source forces.

Fig. 4. EMF "interferes" current: \ Varphi_a - \ Varphi_B - \ Mathcal E = IR

How is this possible? Very simple: other sources existing in the chain outside the section under consideration, "overpower" the source on the site and force the current to flow against \ Mathcal E.. This is how it happens when you put the phone for charging: the adapter connected to the outlet causes the movement of charges against the action of third-party phone battery forces, and the battery is thereby charging!

What will change now in the withdrawal of our formulas? Only one thing - the work of third force will be negative:

A_ {ct} = \ mathcal e q = \ mathcal eit.

Then the Ohm law for an inhomogeneous site will take the form:

\ Varphi_a - \ varphi_b - \ Mathcal E = IR, (eight)

or:

U - \ Mathcal E = IR,

where is still U = \ Varphi_a - \ Varphi_B- Voltage on the site.

Let's collect together formulas (7) и (eight) and write the law of Oma for a plot with EMF as follows:

\ varphi_a - \ varphi_b \ pm \ mathcal e = ir.

Current while flowing from point A.To point B.. If the current direction coincides with the direction of third-party forces, then before \ Mathcal E.put "plus"; If these directions are opposite, then "minus" is put.

The electromotive force or the EMF is reduced is the ability of the current source of the yield in a different feed element, create a potential difference in the electrical circuit. Power elements are batteries or batteries. This is a scalar physical value equal to the work of third-party forces to move one charge with a positive value. This article will consider the theoretical issues of EDC, as it is formed, as well as for which it can be used in practice and where they are used, and most importantly, how to calculate it. Formula EDC.

Formula EDC.

What is EDF: an explanation of simple words

Under EMF means the specific work of third-party forces for moving a single charge in the circuit electrical chain . This concept in electricity involves many physical interpretations relating to various fields of technical knowledge. In electrical engineering, this is the specific work of third-party forces appearing in inductive windings when a variable field is hovering. In chemistry, it means the difference in potentials resulting in electrolysis, as well as with reactions accompanied by the separation of electrical charges.

In physics, it corresponds to the electromotive strength created at the ends of the electrical thermocouple, for example. To explain the essence of EDS with simple words - it will be necessary to consider each of the options for its interpretation. Before moving to the main part of the article, we note that the EMF and the stress are very close to the meaning of the concept, but still somewhat different. If you say briefly, the EMF is on the power supply without load, and when the load is connected to it - this is already a voltage. Because the amount of volts on the PI under load is almost always somewhat less than without it. This is due to the presence of internal resistance of such power supplies, such as transformers and electroplating elements.

Additional material on the topic: Simple words about voltage converters.

Electrical force (EMF), a physical value that characterizes the effect of third-party (non-optical) forces in the sources of direct or alternating current; In a closed conductive circuit, the operation of these forces on the movement of a single positive charge along the contour is equal. If a third-party field strength is denoted, then the EMF in the closed circuit (L) is equal to where DL is an element of the circuit length. The potential forces of electrostatic (or stationary) fields cannot maintain a permanent current in the chain, since the work of these forces on the closed path is zero. The passage of current on the conductors is accompanied by the release of energy - heating the conductors.

Third-party forces lead charged particles within the current sources: generators, galvanic elements, batteries, etc. The origin of third-party forces may be different. In the generators, third-party forces are the forces by the vortex electric field arising from the change in the magnetic field with time, or the Lorentz force acting from the magnetic field side to electrons in a moving conductor; In electroplating elements and batteries, this is chemical forces, etc. EMF determines the current strength in the chain with a predetermined resistance (see Ohma law). EMF is measured, as well as voltage, in volts. What is EDF.

What is EDF.

Nature EMF.

The cause of the emergence of EDC in different current sources is different. By nature, the following types are distinguished:

  • Chemical EMF. It occurs in batteries and batteries due to chemical reactions.
  • Thermo EMF. It occurs when the contacts of heterogeneous conductors are connected at different temperatures.
  • EMF induction. It occurs in the generator when placing a rotating conductor into a magnetic field. EMF will induce the conductor when the conductor crosses the power lines of the constant magnetic field or when the magnetic field varies in size.
  • Photoelectric EMF. The emergence of this EDC contributes to the phenomenon of an external or internal photo effect.
  • Piezoelectric EMF. EMF occurs when stretching or squeezing substances.

Electromagnetic induction (self-induction)

Let's start with electromagnetic induction. This phenomenon describes the law of electromagnetic induction of Faraday. The physical meaning of this phenomenon is the ability of the electromagnetic field to bring EMF in a nearby conductor. In this case, the field should be changed, for example, by magnitude and direction of vectors, or move relative to the conductor, or the conductor should move relative to this field. At the ends of the conductor in this case, the potential difference occurs.

Experience demonstrates the appearance of EMF in the coil when exposed to a changing magnetic field of a permanent magnet. There is another similar in meaning of the phenomenon - mutual induction. It lies in the fact that changing the direction and strength of the current of one coil induces EMF at the conclusions of the coil located nearby, is widely used in various fields of technology, including electrician and electronics. It is based on the operation of transformers, where the magnetic stream of one winding reserves the current and voltage into the second. What is self-induction.

What is self-induction.

In an electrician, the physical effect called EMF is used in the manufacture of special AC transducers that provide the desired values ​​of active values ​​(current and voltage). Thanks to induction and self-induction phenomena, engineers managed to develop multiple electrical devices: from a conventional inductance coil (choke) and up to the transformer. The concept of mutually induction concerns the alternating current only when the magnetic flux changes in the circuit or conductor. Electrical power induction

Table of the parameters of the electromotive power of induction.

EMF in everyday life and units of measurement

Other examples are found in the practical life of any ordinary person. Such familiar things like small batteries, as well as other miniature batteries fall under this category. In this case, the working EMF is formed due to the chemical processes flowing within the sources of constant voltage. When it occurs on terminals (poles) of the battery due to internal changes - the element is fully ready for operation. Over time, the value of the EMF is somewhat reduced, and the internal resistance increases markedly.

As a result, if you measure the voltage to not connected to any to the finger battery, you see normal for it 1.5V (or so), but when the load is connected to the battery, let's say, you installed it in some device - it does not work. Why? Because if you assume that a voltmeter has internal resistance many times higher than the internal resistance of the battery - then you measured its EMF. When the battery starts to give the current in the load at its outputs, it became not 1.5V, and, let's say, 1.2V - the device is not a voltage, no current for normal operation. Calculation of EDS.

Calculation of EDS.

Just this 0.3 B and fell on the inner resistance of the electroplating element. If the battery is completely old and its electrodes are destroyed, then there may be no electromotive force or voltage at the battery terminals at all - i.e. zero. A very small magnitude of the electromotive force is indoor and within the receiver antenna, which is then enhanced by special cascades, and we get our television, radio and even Wi-Fi signal.

Material on the topic: Select a digital-analog converter.

How the EMF is formed

The ideal source of EDS is a generator whose inner resistance is zero, and the voltage on its clips does not depend on the load. The power of the ideal source of EMF is infinite. The real source of EMF, in contrast to the ideal, contains the inner resistance Ri and its voltage depends on the load (Fig. 1., b), and the source power is finite. The electrical circuit of the actual EMF generator is a serial connection of the ideal generator of EDS E and its internal resistance RI.

In practice, to bring the mode of operation of the actual EDC generator to the mode of operation of the ideal, the internal resistance of the real generator Ri is trying to do as little as possible, and the resistance of the RN load must be connected to a value of at least 10 times the greater the internal resistance of the generator, i.e. It is necessary to perform condition: RN >> Ri

In order for the output voltage of the actual EMF generator does not depend on the load, it will stabilize it with the use of special electronic stabilization schemes. Since the internal resistance of the actual EMF generator cannot be performed infinitely small, it is minimized and carried out by standard for the possibility of a consistent connection to it consumers of energy. In the radio engineering, the magnitude of the standard output resistance of the EDC generators is 50 ohms (industrial standard) and 75 ohms (household standard).

For example, all television receivers have an input resistance of 75 ohms and connected to the antennas with a coaxial cable of precisely such a wave resistance. To approach the ideal EDC generators, the supply voltage sources used in all industrial and household radio electronic equipment are performed using special electronic output voltage stabilization schemes that allow you to withstand the almost unchanged output voltage of the power supply in a given range of currents consumed from the EMF source (sometimes it refer to the voltage source).

On electrical circuits, the sources of EMF are depicted as follows: E is the source of the constant EMF, E (T) is the source of the harmonic (variable) EMF in the form of a time function. The electromotive force of the battery sequentially connected identical elements is equal to the electromotive force of one element E, multiplied by the number of battery n elements: E = N. Permanent current and EMF.

Permanent current and EMF.

Electrical power (EMF) of the source of energy

To maintain electric current in the conductor, an external source of energy is required, creating a potential difference between the ends of this conductor. Such sources of energy were called sources of electrical energy (or current sources). Sources of electrical energy have a certain electromotive force (abbreviated EMF), which creates and for a long time supports the potential difference between the sections of the conductor.

Lagutin Vitaly Sergeevich

Engineer in the specialty "Software Computer Engineering and Automated Systems", MEPhI, 2005-2010

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Sometimes they say that EMF creates an electric current in the chain. It must be remembered about the conventions of this definition, since we have already found it higher that the cause of the occurrence and existence of an electric current is an electric field.

The source of electrical energy produces a certain work by moving electrical charges across the closed chain. The unit of measurement of the electromotive force is accepted (abbreviated volt is denoted by the letter V or V - "WE" Latin). EMF of the source of electrical energy is equal to one Volta, if when moving one cooler of electricity across the entire closed, the circuit source of electrical energy makes a job equal to one Joule: Electrical power (EMF) of the source of energy.

Electrical power (EMF) of the source of energy.

In practice, the EMF measurement is used both larger and smaller units, namely:

  • 1 kilovolt (KV, KV), equal to 1000 V;
  • 1 Millivolt (MV, MV), equal to one thousandth volt of Volta (10-3 V),
  • 1 microvolt (MKV, μV) equal to one million dollars (10-6 V).

Obviously, 1 kV = 1000 V; 1 B = 1000 mV = 1 000 000 μV; 1 mV = 1000 μV.

At present, there are several types of sources of electrical energy. For the first time, an electroplating battery was used as a source of electrical energy, consisting of several zinc and copper circles, between which the skin was laid, moistened in acidic water. In the electroplating battery, the chemical energy turned into an electric (it will be described in more detail in chapter XVI). The electroplating battery was obtained by the electroplating battery named Italian physiologist Luigi Galvani (1737-1798), one of the founders of the teachings about electricity.

Numerous experiments on the improvement and practical use of galvanic batteries were held by Russian scientists Vasily Vladimirovich Petrov. Even at the beginning of the last century, he created the world's largest electroplating battery and used it for a number of brilliant experiments. Electrical sources operating on the principle of transformation of chemical energy into electrical are called chemical sources of electrical energy.

It is useful to know: how to calculate the power of the electric current.

Another major source of electrical energy that has been widely used in electrical engineering and radio engineering is the generator. In the generators, mechanical energy is converted into electrical. Chemical sources of electrical energy and generators have an electromotive force manifests itself equally, creating potential difference in the source and supporting it for a long time.

These clamps are called the Poles of the source of electrical energy. One pole of the source of electrical energy has a positive potential (disadvantage of electrons), is denoted by the plus (+) sign and is called a positive pole.

Another pole has a negative potential (excess electrons), is denoted by a minus (-) sign and is called a negative pole. From electrical sources, electrical energy is transmitted by wires to its consumers (electrical lamps, electric motors, electric arcs, electric heating devices, etc.).

How the EMF is formed.

Examples of solving problems

To each position of the first column, select the relevant position of the second:

Solution: The electromotive power of the galvanic element is the value, numerically equal to the work of third-party forces when moving a single positive charge inside the element from one pole to another.

The work of third-party forces cannot be expressed through the potential difference, since the third-party forces are noteparted and their work depends on the form of the trajectory of the charges of charges.

EMF is determined by the formula:

What is an electromotive force (EMF) and how to calculate it

The current is determined by the formula:

What is an electromotive force (EMF) and how to calculate it

Resistance is determined by the formula: What is an electromotive force (EMF) and how to calculate it

What is an electromotive force (EMF) and how to calculate it

The difference in potentials is determined by the formula:

What is an electromotive force (EMF) and how to calculate it

Correct answer:

Physical quantities Formulas
Electromotive force What is an electromotive force (EMF) and how to calculate it
Tok Power What is an electromotive force (EMF) and how to calculate it
Resistance What is an electromotive force (EMF) and how to calculate it
Potential difference What is an electromotive force (EMF) and how to calculate it

What is an electromotive force?

This is the ratio of the work of third-party forces when moving the charge on a closed contour to the absolute value of this charge.

What is an electrical chain?

A set of devices that are connected by conductors designed to flow current.

How does Oma law sound for a complete chain?

The strength of the current in the total chain is equal to the ratio of the EDC chain to its full resistance.

Conclusion

Lagutin Vitaly Sergeevich

Engineer in the specialty "Software Computer Engineering and Automated Systems", MEPhI, 2005-2010

Ask a Question

If you create an electric field in the conductor and do not maintain this field, then the movement of the current media will result in the field inside the conductor disappear, and the current will stop. In order to maintain a current in the chain, it is necessary to carry out the movement of charges on a closed trajectory, that is, to make the DC lines closed. Consequently, in a closed chain there should be sections on which charge carriers will move against the power of the electrostatic field, that is, from points with less potential to points with high potential. This is possible only in the presence of non-electrical forces, called third-party forces. By third-party forces are the forces of any nature, except for Coulomb.

For more information about the subject of the article, you can learn from the "Electroforming Power in Electric Current" file. And also in our group VK publishes interesting materials with which you can get acquainted first. To do this, we invite readers to subscribe and join the group.

In conclusion I want to express my gratitude to the sources from where the material to prepare the article:

www.booksite.ru.

www.scsiexplorer.com.ua

www.samelectrik.ru.

www.electricalschool.info.

www.sxemotehnika.ru.

www.zaochnik.ru.

www.ido.tsu.ru.

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To maintain an electric current in the conductor for a long time, it is necessary that there is always a positive charge from the end of the conductor with a lower potential), and the charges delivered to the current are constantly removed, while the charges were constantly tied to the end with high potential. That is, you should provide a circulation of charges. In this cycle, charges must move along a closed path. The movement of current carriers is implemented using non-electrostatic forces. Such forces are referred to as third parties. It turns out that, to maintain current, third-party forces are needed, which operate throughout the chain or in separate sections of the chain.

Formula finding EMF.

First, we'll figure it out with the definition. What does this abbreviation mean?

EMF or an electromotive force is a parameter that characterizes the work of any power of non-electrical nature, working in chains where the current is both a constant and alternating is the same throughout the length. In the adhesive conductive eds circuit, the operation of these forces on the movement of a single positive (positive) charge along the entire contour is equated.

Below in the figure shows EMF formula.

AST - means the work of third-party forces in Joules.

Q is a portable charge in the coulons.

Thirdness - This is the forces that perform the separation of charges in the source and in the end form the difference in potentials on its poles.

For this force, the unit of measure is volt . Refers to the formulas she letter «E ".

Only at the moment of lack of current in the battery, the electromotive CA will be equal to the voltage on the poles.

EMF induction:

EMF induction in a circuit having N turns:

turns

When moving:

in move

Electromotive force induction in the circuit, spinning in a magnetic field at speed w:

Table of values

Table Velchin

EMF and Ohm's Law [| ]

The electromotive power of the source is associated with an electric current flowing into the chains, the ratios of the Ohm law. Ohma law for Inhomogeneous plot of chain

It has the form [1]: φ 1 - φ 2 + E = IR, {\ DisplayStyle \ Varphi _ {1} - \ varphi _ {2} + {\ mathcal {e}} = ir,} where φ 1 - φ 2 {\ displayStyle \ varphi _ {1} - \ varphi _ {2}} - the difference between the values ​​of the potential at the beginning and at the end of the chain section, I {\ DisplayStyle I} is the current current by the section, and R {\ displaystyle r } - plot resistance.

If points 1 and 2 coincide (the circuit is closed), then φ 1 - φ 2 = 0 {\ displaystyle \ varphi _ {1} - \ varphi _ {2} = 0} and the previous formula moves into the formula of the Ohm law for Closed chain

[1]: E = i R, {\ DisplayStyle {\ Mathcal {E}} = IR,} Where now R {\ DisplayStyle R} - Full

Resistance to the entire chain.

In general, the total chain resistance is made up of external resistance to the circuit area source (R E {\ DisplayStyle R_ {E}) and the internal resistance of the current source itself (R {\ DisplayStyle R}). Taking into account this follows:

E = i R E + I R. {\ displaystyle {\ mathcal {e}} = Ir_ {E} + Ir.}

Easy explanation of the electromotive force

Suppose that there is a water tower in our village. It is completely filled with water. We will think that this is a regular battery. The tower is a battery!

All water will have a strong pressure on the bottom of our turret. But it will be strong only when this building is fully filled with H2O.

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

As a result:

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

With the battery, everything is similar.

First of all, we connect the source with the energy in the chain. And accordingly clicch it. For example, insert the battery into the flashlight and turn it on. Initially, we note that the device is burning brightly. After some time, its brightness will noticeably decrease. That is, the electromotive force has decreased (leaked to compare with water in the tower).

If you take an example of the water tower, then the EMF is a pump swinging water into the tower constantly. And she never ends there.

EMF current source [| ]

If there are no third-party strength on the chain area ( homogeneous plot of chain

) And, it means that there is no source of current on it, then, as it follows the Ohm's law for an inhomogeneous section of the chain, it is performed: φ 1 - φ 2 = i r. {\ displayStyle \ Varphi _ {1} - \ varphi _ {2} = Ir.} So, if you select the source anode as a point 1, it is its cathode, then for the difference between the potentials of the anode φ a {\ DisplayStyle \ Varphi _ {a}} and cathode φ k {\ displayStyle \ Varphi _ {k}} can be written:

φ a - φ k = i r e, {\ displaystyle \ varphi _ {a} - \ varphi _ {k} = ir_ {e},}

where, as before, R e {\ displayStyle R_ {E}} is the resistance of the external section of the chain.

From this ratio and the law of Oma for a closed circuit recorded in the form E = i R E + i R {\ displayStyle {\ Mathcal {E}} = Ir_ {E} + Ir} It is not difficult to get

φ a - φ k e = r e r e + r {\ displaystyle {\ frac {\ varphi _ {a} - \ varphi _ {k}} {\ mathcal {e}}} = {\ FRAC {R_ {E }} {R_ {E} + R}}} and then φ a - φ k = r e R e + R e. {\ displaystyle \ varphi _ {a} - \ varphi _ {k} = {\ FRAC {R_ {E}} {R_ {E} + R}} {\ Mathcal {E}}.}

From the obtained ratio follow two outputs:

  1. In all cases, when the circuit flows the current, the potential difference between the terminals of the current source φ a - φ k {\ displaystyle \ varphi _ {a} - \ varphi _ {k}} is less than the source EMF.
  2. In the limiting case, when R e {\ displayStyle R_ {E}} is infinite (the circuit is broken), E = φ A is φ k. {\ displaystyle {\ mathcal {e}} = \ varphi _ {a} - \ varphi _ {k}.}

Thus, the emf of the current source is equal to the potential difference between its terminals in a state when the source is disabled from the chain [1].

EMF Galvanic Element - Formula

The electromotive strength of the battery can be calculated in two ways:

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

Nernsta equation

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

Faraday and Lenza laws

Electrical currents create magnetic effects. Is it possible for the magnetic field to generate electrical? Faraday found that the desired effects arise due to a change in MP in time.

When the conductor intersects with a variable magnetic flux, the electromotive force causing electrically motors is induced. The system that generates the current may be a permanent magnet or an electromagnet.

The phenomenon of electromagnetic induction is regulated by two laws: Faraday and Lenza.

Lenza law allows you to characterize the electromotive force regarding its direction.

Important! The direction of the induced EMF is such that the current caused by it seeks to withstand its reason.

Faradays noted that the intensity of the induced current is growing when the number of power lines crossing the contour changes faster. In other words, EMF electromagnetic induction is directly dependent on the speed of a moving magnetic flux.

EMF inductionEMF induction

Formula EMF induction is defined as:

E = - DF / DT.

The sign "-" shows how the polarity of an induced EMF is associated with a flow sign and changing speed.

A general formulation of the law of electromagnetic induction was obtained, from which it is possible to derive expressions for special cases.

Where are different types of EDS?

  1. Piezoelectric is used when tensile or compression of the material. With the help of it, quartz energy generators and different sensors are manufactured.
  2. Chemical is used in galvanic elements and batteries.
  3. Induction appears at the time of the intersection of the magnetic field. Its properties are used in transformers, electrical engines, generators.
  4. The thermoelectric is formed at the time of heating contacts of differentty-metal metals. It has found its application in refrigeration plants and thermocouples.
  5. Photography is used to produce photocells.

Non-electricostatic EMF character [| ]

Inside the source of EDS, the current flows in the direction opposite to normal. This is not possible without an additional force of non-electrostatic nature, overcoming the power of electrical repulsion as shown in the figure, electric current, the normal direction of which is from the "plus" to "minus", inside the EDC source (for example, inside the galvanic element) flows in the opposite direction. The direction from the "plus" to "minus" coincides with the direction of electrostatic force acting on positive charges. Therefore, in order to force the current to flow in the opposite direction, an additional force of non-electrostatic nature is necessary (centrifugal force, Lorentz power, the strength of chemical nature, the power of the vortex electric field) which would overcome the power from the electrostatic field. Dissipative forces, although they counteract the electrostatic field, cannot force the current to flow in the opposite direction, so they are not included in third-party forces, the work of which is used in the definition of EDC.

Rotating coil

Provide the optimal arrangement of functional components while simultaneously movement, it is difficult to use the direct wire represented in the example. However, having bent the frame, you can get the simplest generator of electricity. The maximum effect ensures an increase in the number of conductors per unit of work volume. The design corresponding to the marked parameters is a coil, a typical element of the modern alternator of the AC.

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

where S is the area of ​​the working surface under consideration.

Explanation. With uniform rotation of the rotor, the corresponding cyclic sinusoidal change of the magnetic flux occurs. Similarly, the amplitude of the output signal changes. From the figure it is clear that a certain value is a gap between the main functional components of the design.

EMF self-induction

Magnetic induction lines

When an alternating current passes through the coil, it generates a variable MP, which has a changing magnetic flow induced by EMF. This effect is called self-induction.

Since MP is proportional to the current intensity, then:

F = l x i,

where L is the inductance (GG), determined by geometrical values: the amount of turns per unit length and the size of their cross-section.

For EMF induction, the formula takes the form:

E = - L x di / dt.

Wire Movement in Magnetic Field

Electromagnetic induction phenomenon

When the Leng Length L wire moves into a MP, which has induction in, an EDC will induce inside it, proportional to its linear velocity V. To calculate the EMF, the formula is applied:

  • In the case of the conductor movement, perpendicular to the direction of the magnetic field:

E = - in x l x v;

  • In case of movement at a different angle α:

E = - in x L x V x Sin α.

The induced EMF and the current will be directed aside, which we find, using the rule of right hand: by placing the hand perpendicular to the power lines of the magnetic field and pointing to a thumb in the direction of moving the conductor, you can find out the direction of EDC for the remaining four straightened fingers.

Moving Wires in MPMoving Wires in MP

Constraction

Resonant Frequency: Formula

If two coils are located nearby, then they are reducing EMF of mutual induction, depending on the geometry of both schemes and their orientation relative to each other. When the separation of the chains increases, the intedentality decreases, since the magnetic flux connecting them decreases.

ConstractionConstraction

Let there be two coils. On the wire of one coil with the N1 with turns, current flows I1, creating a MP passing through the coil with N2 with turns. Then:

  1. The interdigabilities of the second coil relatively first:

M21 = (n2 x f21) / i1;

  1. Magnetic flow:

Ф21 = (m21 / n2) x i1;

  1. We find induced EMF:

E2 = - N2 X DF21 / DT = - M21X DI1 / DT;

  1. Identically in the first coil induced by EMF:

E1 = - m12 x di2 / dt;

Important! The electromotive force caused by mutually induction in one coil is always proportional to the change in the electrotock in another.

Mutual inductance can be recognized as equal:

M12 = M21 = M.

Accordingly, E1 = - M X Di2 / DT and E2 = M x Di1 / DT.

M = K √ (L1 x L2),

where k is the communication coefficient between two inductances.

The mutual induction phenomenon is used in transformers - electrical appliances that allow you to change the value of the voltage of the variable electrotock. The device is two coils wound around one core. The current present in the first creates a changing MP in the magnetic circuit and the electrotocks in another coil. If the number of turns of the first winding is less than the other, the voltage increases, and vice versa.

In addition to generating, electricity transformation magnetic induction is used in other devices. For example, in magnetic levitational trains, which are not moving in direct contact with rails, and several centimeters are higher due to the electromagnetic power of the repulsion.

INDUCTANCE

(from Lat. Inductio - guidance, motivation), the value characterizing Magn. SV-VA electric. chains. The current current in the conductive circuit creates in the surrounding pr-ve Magn. The field, and the magnetic flux F, piercing the contour (linked to it), is directly proportional to the current I: F = Li. Coeff. Proportionality l Naz. I. or coeff. self-induction contour. I. Depends on the size and shape of the contour, as well as from the magnetic permeability of the environment. In SI I. Measured in Henry, in the Gauss system of units, it has the dimension of length (1 GG = 109 cm).

Through I. expresses EMF self-induction? In the circuit, which occurs when the current changes in it:

(DI Change current during DT). I. Defines the energy of W Magn. Current fields i:

W = Li2 / 2.

If you draw an analogy between the electric. and mechanical. phenomena, then Magn. Energy should be compared with kinetich. The energy of the body T = MV2 / 2 (M is the mass of the body, V is its speed), while I. will play the role of mass, and the current - speed. T. about., I. Defines Inertz. CV current.

To increase I. Apply inductors with iron cores; As a result, the dependence of Magn. Permeability M ferromagnets from the tension of Magn. The fields (and, consequently, from current) I. such coils depends on I. I. Long solenoid from N turns with cross-sectional area S and L length in medium with Magn. permeability M is equal to (in units):

L = mm0n2s / l,

where M0- Magn. Permeability of vacuum.

Source: Physical Encyclopedic Dictionary on GUFO.ME

Values ​​in other dictionaries

  1. Inductance - (from Lat. Inductio - guidance, motivation) The physical quantity characterizing the magnetic properties of the electrical circuit. The current current in the conductive circuit creates a magnetic field in the surrounding space, and the magnetic stream ... Big Soviet Encyclopedia
  2. Inductance - Indust'fulness, inductance, MN. No, · wives (· Book. Spec.). · Distractors. SUD To inductive. Eindormum evidence. Explanatory Dictionary Ushakov
  3. Inductance - Industry / IVN / ASS /. Morphemno-spell dictionary
  4. Inductance - inductance I. Distractors. SUD by arrival Inductive I 2. II. The physical quantity characterizing the magnetic properties of electrical circuits. Explanatory dictionary Efremova
  5. Inductance - ORF. inductance, and spent spelling dictionary
  6. inductance - -I, g. Log., Piz. Property for meaning. arr. inductive. Inductance of evidence. Inductance of the conductor. Small Academic Dictionary
  7. Inductance - inductance, inductance, inductance, inductance, inductance, inductance, inductance, inductance, inductance, inductance, inductance, inductors Grammar dictionary
  8. Inductance - inductance - a physical quantity characterizing the magnetic properties of electrical circuits and equal to the ratio of the flow F of magnetic induction crossing the surface bounded by a conductive circuit to the current in this circuit, which creates F; In si measured in Henry. Big Encyclopedic Dictionary
  9. Inductance - inductance, property of an electrical circuit or a chain element, creating an electromotive force (EMF) when the electric current changes. In the system of the system, Henry is served. Scientific and Technical Dictionary
  10. Inductance - SUMS., Number of synonyms: 1 Inductance 1 Dictionary of synonyms of the Russian language
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Formula EMF.

\ [\ Epsilon = \ FRAC {A} {Q} \]

Here \ Epsilon.- EMF, A.- work of third-party forces, Q.- Charge value.

Voltage measurement unit - In (volt) .

EMF is a scalar value. In the closed circuit, EDC is equal to the work of forces to move a similar charge throughout the contour. At the same time, the current in the circuit and within the current source will flow in opposite directions. External work, which creates EDF, should not be electric origin (Lorentz power, electromagnetic induction, centrifugal force, force arising during chemical reactions). This work is needed to overcome the strength of the current of the current carriers within the source.

If the circuit goes current, the EMF is equal to the sum of the stress drops in the entire chain.

Examples of solving problems on the topic "Electrical Force"

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In the midst of the school year, many scientists are required by EMF formula for different calculations. Experiments related to the galvanic element also need information about the electromotive force. But for beginners, it is not so easy to understand what it is.

Formula finding EMF.

First, we'll figure it out with the definition. What does this abbreviation mean?

EMF or an electromotive force is a parameter that characterizes the work of any power of non-electrical nature, working in chains where the current is both a constant and alternating is the same throughout the length. In the adhesive conductive eds circuit, the operation of these forces on the movement of a single positive (positive) charge along the entire contour is equated.

Below in the figure shows EMF formula.

Formula EMF.

AST - means the work of third-party forces in Joules.

Q is a portable charge in the coulons.

Thirdness - This is the forces that perform the separation of charges in the source and in the end form the difference in potentials on its poles.

For this force, the unit of measure is volt . Refers to the formulas she letter «E ".

Only at the moment of lack of current in the battery, the electromotive CA will be equal to the voltage on the poles.

EMF induction:

induction

EMF induction in a circuit having Nturns:

turns

When moving:

in move

Electromotive force induction in the circuit, spinning in a magnetic field at speed w:

F5.

Table of values

Table Velchin

Easy explanation of the electromotive force

Suppose that there is a water tower in our village. It is completely filled with water. We will think that this is a regular battery. The tower is a battery!

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

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

As a result:

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

With the battery, everything is similar.

First of all, we connect the source with the energy in the chain. And accordingly clicch it. For example, insert the battery into the flashlight and turn it on. Initially, we note that the device is burning brightly. After some time, its brightness will noticeably decrease. That is, the electromotive force has decreased (leaked to compare with water in the tower).

If you take an example of the water tower, then the EMF is a pump swinging water into the tower constantly. And she never ends there.

EMF Galvanic Element - Formula

The electromotive strength of the battery can be calculated in two ways:

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

Nernsta equation

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

Where are different types of EDS?

  1. Piezoelectric is used when tensile or compression of the material. With the help of it, quartz energy generators and different sensors are manufactured.
  2. Chemical is used in galvanic elements and batteries.
  3. Induction appears at the time of the intersection of the magnetic field. Its properties are used in transformers, electrical engines, generators.
  4. The thermoelectric is formed at the time of heating contacts of differentty-metal metals. It has found its application in refrigeration plants and thermocouples.
  5. Photography is used to produce photocells.

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