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Any current source is characterized by electromotive force, or an abbreviated EMF. So, on a circular battery for a pocket flashlight: 1.5 V.
What does it mean?
If you connect two variestly charged balls to the conductor, then the charges are quickly neutralized each other, the potentials of the balls will become the same, and the electric field will disappear (Fig. 15.9, a).
Third-party.
In order for the current to be constant, it is necessary to maintain a constant voltage between the balls. For this, the device (current source), which would move the charges from one ball to another in the direction opposite to the direction of forces acting on these charges from the side of the electric field of balls. In such a device on charges, except for the electrical forces, the forces of non-electrostatic origin should be used (Fig. 15.9, b). Only the electric field of charged particles ( coulomb field) It is not capable of maintaining a permanent current in the chain.
Any forces acting on electrically charged particles, with the exception of power forces of electrostatic origin (i.e. Coulomb), called third-party forces.
The conclusion about the need for third-party forces to maintain direct current in the chain will become even more clear if you turn to the law of energy conservation.
The electrostatic field is potentially. The operation of this field when moving in it charged particles along a closed electrical circuit is zero. The passage of current on the conductors is accompanied by the release of energy - the conductor heats up. Therefore, in the chain there must be some source of energy supplying it into the chain. In addition, in addition to the Coulomb forces, third-party, non-optical forces must act. The work of these forces along the closed contour should be different from zero.
It is in the process of performing the work by these forces the charged particles are purchased within the current source and then give it to conductors of the electrical circuit.
Third-party forces lead the charged particles within all sources of current: in generators on power plants, in electroplating elements, batteries, etc.
When the circuit is closed, the electric field is created in all chain conductors. Inside the current source, charges are moving under the action third-party forces against the Coulomb forces (electrons from a positively charged electrode to negative), and in the external circuit they drive the electric field (see Fig. 15.9, b).
Nature of third-party strength.
The nature of third-party strength can be diverse. In power station generators, third-party power is the forces acting from the magnetic field to electrons in a moving conductor.
In the galvanic element, for example, in the Volta element, chemical forces act.
The volt element consists of zinc and copper electrodes placed in a solution of sulfuric acid. Chemical forces cause dissolution of zinc in acid. The solution is transmitted positively charged zinc ions, and the zinc electrode itself is negatively charged. (Copper is very little dissolved in sulfuric acid.) There is a potential difference between the zinc and copper electrodes, which causes the current in the external electrical circuit.
The effect of third-party forces is characterized by an important physical value called electromotive power (Abbreviated EMF).
Electromotive force The source of current is equal to the ratio of the work of third-party forces when moving the charge on a closed contour to the absolute value of this charge:
The electromotive force as well as voltage is expressed in volts.
The potential difference at the battery terminals with an open circuit is equal to the electromotive force. EMF of one battery element usually 1-2 V.
You can also speak about the electromotive strength and on any section of the chain. This is a specific work of third-party forces (work on the movement of a single charge) not in the entire contour, but only in this area.
The electromotive power of the galvanic element is the value that is 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.
What EMF. (electromotive force) in physics? Electric current is not understood by everyone. As a cosmic distance, only under the nose. In general, he and scientists are not clear to the end. It is enough to remember with his famous experiments, for centuries ahead of their time and even these days remaining secrets remaining in Oleole. Today we do not break the big secrets, but we are trying to figure out what is EDF in physics.
Definition of EMF in physics
EMF. - Electrical power. Denotes letter E. Or a small Greek letter Epsilon.
Electromotive force - a scalar physical value that characterizes the work of third-party forces ( forces of non-electric origin) acting in electrical circuits of AC and DC.
EMF.like I. tensione, measured in volts. However, EDC and tension are different phenomena.
Voltage (Between points A and b) is a physical value equal to the operation of an effective electric field performed when transferring a single test charge from one point to another.
Explain the essence of EDS "on the fingers"
To sort out that there is something, you can give an example analogy. Imagine that we have a water tower, completely filled with water. Compare this tower with a battery.
Water has a maximum pressure on the bottom of the tower, when the tower is full completely. Accordingly, the smaller the water in the tower, the weaker the pressure and pressure arising from the water crane. If you open a crane, water will gradually flow first under strong pressure, and then everything is slower, while the pressure does not weaken at all. Here the voltage is the pressure that water has on the bottom. For the level of zero voltage, we will take the bottom of the Tower itself.
The same with the battery. First we turn on our current source (battery) to the chain, closing it. Let it be a clock or flashlight. While the voltage level is sufficient and the battery is not discharged, the flashlight shines brightly, then gradually goes out until it goes away.
But how to make the pressure not dried? In other words, how to maintain a permanent water level in the tower, and on the poles of the current source - a constant potential difference. According to the example of the EMF Tower, it seems like a pump that provides a flow into the tower of new water.
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.
Dear friends, today we have reviewed the topic "EMF for teapots". As you can see, EDC - the power of non-electric originwhich supports electric current in the chain. If you want to know how tasks with EDC are solved, we advise you to turn to - scrupulously selected and verified specialists who quickly and intelligibly explain the course of solving any thematic task. And by tradition at the end, we suggest you to watch a training video. Pleasant viewing and success in school!
The cause of the electromotive force can be a change in the magnetic field in the surrounding space. This phenomenon is called electronagnetic induction. The value of EMF induction in the circuit is determined by the expression
where - the flow of the magnetic field through a closed surface limited by the contour. The sign "-" before the expression shows that the induction current created by the EMF induction prevents the change in the magnetic flux in the circuit (see the Lenz rule).
41. Inductance, its unit C. Long solenoid inductance.
Inductance (or self-induction coefficient) - the proportionality ratio between electric tokomflowing in any closed loop and magnetic flowcreated by this current through the surface , the edge of which is this contour. .
In the formula
Magnetic flow, - current in the circuit, - inductance.
Often talking about the inductance of a straight long wire ( cm.). In this case, others (especially in non-compliant quasi-stationary approximation) cases when a closed circuit is not easy to adequately and unambiguously indicate the definition below requires special refinements; Partly useful for this is an approach (referred to below), binding inductance with the magnetic field energy.
Through inductance is expressed EMF self-induction In the circuit that occurs when changing current :
.
From this formula it follows that the inductance is numerically equal EMF self-inductionarising in the circuit when the current is changed by 1 and for 1 s.
With a given strength of current, inductance determines eNERGY magnetic field created by this current :
Measurement and units
In the system of SI, inductance is measured in Henry, abbreviated GN, in the SGS system - in centimeters (1 mb \u003d 10 9 cm). The outline has inductance in one Henry, if when changing the current by one ampere per second at the outputs of the circuit will occur in one volt. Real, not superconducting, the contour has an ohmic resistance R, so it will further arise the voltage U \u003d I * R, where I is the current current flowing along the contour to this moment of time.
The symbol used to designate inductance was taken in honor of Lenza Emilia Christianovich (Heinrich Friedrich Emil Lenz) [ the source is not specified 1017 days ]. The unit of inductance is named after Joseph Henry. The term inductance itself was proposed by Oliver Heaviside (Oliver Heaviside) in February 1886 [ the source is not specified 1017 days ] .
Electric current that flows in a closed loop creates around itself a magnetic field, the induction of which, according to the Bio-Savara-Laplace law, is proportional to the current. The magnetic flux F demolled with the circuit is so directly proportional to the current I in the circuit: (1) where the proportionality L ratio is called inductance contour. When changing the current force circuit will also change the magnetic flux with it; So, the circuit will induce E.D.S. The emergence of E.D.S. induction in a conductive circuit when the current is changed in it is called self-induction. From the expression (1) the unit of inductance is set henry (GG): 1 GG - the inductance of the contour, the magnetic flow of self-induction of which at a current of 1 A is 1 WB: 1 GG \u003d 1 WB / s \u003d 1 V
Calculate the inductance of an infinitely long solenoid. Full magnetic stream Through the solenoid (stream) is μ 0 μ (N 2 I / l.) S. Substituting in (1), we find (2) i.e. the inductance of the solenoid depends on the length l. Solenife, the number of its turns n, it, s and magnetic permeability μ of the substance from which the solenoid core is manufactured. It is proved that the inductance of the contour depends generally only on the geometric shape of the circuit, its size and magnetic permeability of the medium in which it is located, and an analogue of the inductance of the contour with the electrical capacity of a secluded conductor, which also depends only on the shape of the conductor, its size and The dielectric permeability of the medium. Find, applying the Faraday law to the phenomenon of self-induction, that E.D.S. self-induccus is equal if the contour does not undergo deformations and the magnetic permeability of the medium remains unchanged (in the future it will be shown that it does not always be fulfilled), then L \u003d const and (3) where the minus sign defined by the Lenz rule says that the presence of inductance in the circuit leads to a slowdown in the current change in it. If the current increases over time, then (di / dt<0) и ξ s >0 i.e., the self-induction current is directed towards the current caused by an external source, and slows its increase. If the current is reduced over time, then (di / dt\u003e 0) and ξ s<0 т. е. индукционный ток имеет такое же направление, как и уменьшающийся ток в контуре, и замедляет его уменьшение. Значит, контур, обладая определенной индуктивностью, имеет электрическую инертность, заключающуюся в том, что любое изменение тока уменьшается тем сильнее, чем больше индуктивность контура.
42. Current when opening and closing the chain.
With any change in the strength of the current in the conductive circuit, er arises. d. s. self-induction, with the result that additional currents appear in the circuit, called extractacles of self-induction. Extroctures of self-induction, according to Lenz rule, are always directed to prevent changes in the circuit in the chain, i.e., directed opposite to the current created by the source. When the source is turned off, the extracture current has the same direction as the weakening current. Consequently, the presence of inductance in the chain leads to a slowdown in the disappearance or setting the current in the chain.
Consider the process of turning off the current in the chain containing the current source with EDs. , resistance resistor R. and inductance coil L.. Under the action of external er d. s. In the circuit flows constant current
(internal resistance of the current source neglege).
At the time of time t.\u003d 0 Turn off the current source. Current in the coil inductance L. will begin to decrease, which will lead to the emergence of ED. self-induction obstructive, according to Lenz rule, reducing current. At every moment of time, the current in the chain is determined by the law I.= s. / R., or
Dividing in the expression (127.1) variables, we obtain integrating this equation I. (from I. 0 BE I.) I. t. (from 0 to t.), we find ln ( I. /I. 0) = – RT/ L., or
where \u003d. L./ R. - Permanent, called relaxation time. From (127.2) it follows that is the time during which the strength of the current is reduced in a time.
Thus, in the process of turning off the current source, the current will decrease under exponential law (127.2) and the curve is determined 1 In fig. 183. The greater the inductance of the chain and the less of its resistance, the more and, therefore, the slower the current in the chain during its opening is reduced.
When closing a chain, in addition to external er. d. s. Arises e. d. s. self-induction preventing, according to Lenz rule, increasing current. According to the law of Ohm, or
Entering a new variable we transform this equation to the form
where is the relaxation time.
At the time of closure ( t.\u003d 0) current I. \u003d 0 I. u. \u003d -. Therefore, integrating on and (from to IR– ) I. t. (from 0 to t.), we find ln [( IR– )]/–= - t./ , or
where is the established current (when t.).
Thus, in the process of incorporating the current source, the increase in the current strength in the chain is set by the function (127.3) and the curve 2 is determined in Fig. 183. The strength of the current increases from the initial value I.= 0 and asymptotically tends to the established meaning . The rate of increasing current is determined by the same relaxation time = L./ R., as decreasing current. Current sets is the faster than the smaller the inductance of the chain and more of its resistance.
Establish the value of E.D.S. self-induction arising from the instantaneous increase in the constant current chain resistance from R. 0 BE R.. Suppose we blur the contour when the established current flows. When opening the circuit, the current varies by formula (127.2). Substitute an expression for it for I. 0 I. , get
E.D.S. self-induction
i.e. with a significant increase in chain resistance (R./ R. 0 \u003e\u003e 1), which has a large inductance, E.D. self-induction can many times higher than ED. Current source included in the chain. Thus, it is necessary to take into account that the contour containing inductance cannot be blocked sharply, as this (the occurrence of significant ED self-induction) can lead to a breakdown of isolation and the conclusion of measuring instruments. If in the contour resistance to introduce gradually, then E.D.S. self-induction will not reach large values.
43. The phenomenon of mutual induction. Transformer.
Consider two fixed circuits (1 and 2), which are located close enough from each other (Fig. 1). If a current I 1 flows in contour 1, then the magnetic flux that is created by this current (the field creates this thread is shown in the figure with solid lines), directly proportional to I 1. Denote by Ф 21 part of the flow, permeating circuit 2. Then (1) where L 21 is the proportionality coefficient.
Fig.1
If the current i 1 changes its value, then E.D. is induced in circuit 2. ξ I2, which, according to the Faraday law, will be equal and opposite by the sign of the rate of change of magnetic flux F 21, which is created in the current in the first circuit and permeates the second: in a similar way, when flowing in circuit 2 of the current I 2, the magnetic flux (its field is shown in Fig. 1 strokes) permeates the first contour. If F 12 is part of this stream, which permeates the circuit 1, then if the current i 2 changes its value, then in circuit 1 is induced by EDs. ξ i1, which is equal and opposite by the sign of the rate of change of the magnetic flux F 12, which is created by the current in the second circuit and permeates the first: phenomenon of the occurrence of ED. In one of the contours, when the current change in the other is called mutual induction. Proportionality coefficients L 21 and L 12 are called mutual inductance of contours. Calculations that are confirmed by experience show that L 21 and L 12 are equal to each other, i.e. (2) the proportionality coefficients L 12 and L 21 depend on the size, geometric shape, the relative position of the contours and the magnetic permeability of the environment surrounding the circuit . The unit of mutual inductance is the same as for inductance - Henry (GG). Find the mutual inductance of two coils that are wound on a common toroidal core. This case has a large practical value (Fig. 2). The magnetic induction of the field, which is created by the first coil with the number of turns N 1, current i 1 and the magnetic permeability μ of the core, B \u003d μμ 0 (N 1 i 1 / l.) where l. - Core length for midline. Magnetic flow Through one round of the second coil f 2 \u003d bs \u003d μμ 0 (N 1 i 1 / l.) S.
It means that the full magnetic flux (streaming) through the secondary winding, which contains N 2 of the turns, the flow ψ is created by the current i 1, therefore, using (1), we find (3) if you calculate the magnetic stream, which is created by the coil 2 through the coil 1, then For L 12, we obtain an expression in accordance with formula (3). It means that the mutual inductance of two coils, which are wound on a common toroidal core,
Transformer (from lat. tRANSFORMO. - convert) - this is a static electromagnetic device having two or more inductively connected windings on any magnetic conductory and intended for conversion by means electromagnetic induction one or more systems (voltages) of alternating current into one or more of other systems (voltages) of alternating current without changing the frequency of the system (voltage) of alternating current
The electromotive force, in the people of EDS, as well as the voltage is measured in volts, but is very different.
EMF in terms of hydraulics
I think you already familiar to the water tower from the past article about
Suppose that the tower is completely filled with water. From the bottom of the tower we drilled a hole and cut the pipe along which the water runs to your home.
The neighbor wanted to pour the cucumbers, you decided to wash the car, mother started washing and voila! The flow of water has become less and less, and soon completely dried ... What happened? The water ended in the tower ...
The time that will need to devastate the tower depends on the tank of the tower itself, as well as how many consumers will use water.
All the same can be said about the radioElement condenser:
Suppose we charged it from 1.5 volts batteries and he accepted the charge. Draw a charged condenser like this:
But as soon as we cling to it a load (let the LED be loaded) by closing the key S, in the first fractions of seconds the LED will glow brightly, and then quietly fade ... and until it is completely woined. The time of extinction of the LED will depend on the capacitance of the capacitor, as well as on which load we cling to the charged condenser.
As I said, it is equivalent to a simple placed tower and consumers who use water.
But why then in our towers water never ends? Yes, because it works Water supply pump! And where does this pump take water? From a well, which is drilled for the extraction of groundwater. Sometimes it is also called Artesian.
As soon as the tower is completely filled with water, the pump turns off. In our hydrodes, the pump always supports the maximum water level.
So let's remember what tension is? By analogy with hydraulics, it is the water level in the water. The total tower is the maximum water level, which means the maximum voltage. No in the tower of water - voltage zero.
EMF electric current
How do you remember from past articles, water molecules are "electrons". For the occurrence of electric current, electrons must move in one direction. But that they move in one direction, there must be voltage and some load. That is, water in the tower is a tension, and people who spend water for their needs is a load, as they create a stream of water from the pipe, which is located at the foot of the waterfront. And the flow is nothing more than the current.
The condition should also be observed that water should always be at the maximum mark, regardless of how many people spend it for their needs at the same time, otherwise the tower will devastly. For water, this saving means is water pump. And for electric current?
For electric current there must be some force that would pushed electrons in one direction for a long time. That is, this power must move the electrons! Electromotive force! Yes exactly! ELECTROMOTIVE FORCE! You can call it abbreviated EMF - E.lens D.seeing FROMel. It is measured in volts, as well as voltage, and is indicated in the main letter E..
So, in our batteries, too, is such a "pump"? There is, and it would be better to call it a "electron feed pump"). But, of course, no one says. They say simple - EDC. I wonder where this pump is hidden in the battery? This is simply an electrochemical reaction, due to which the "water level" is kept in the battery, but then still this pump is wearing and the voltage in the battery starts to see, because the "pump" does not have time to swing water. In the end, it is completely broken and the voltage on the battery is almost zero.
Real source of EDS
The source of electrical energy is the source of EDC with the internal resistance R HV. These can be any chemical nutrition elements, like batteries and batteries.
Their inner structure from the point of view of EMF looks something like this:
Where E. - this is EMF, and R at - This is the internal resistance of the battery
So, what conclusions can be made of this?
If no load is cling to the battery, such as incandescent lamp, etc., as a result, the current strength in such a circuit will be zero. The simplified scheme will be like this:
But if we still attach the incandescent bulb to our battery, then we will have a closed chain and in the chain will flow current:
If you draw a graph of the dependence of the force in the circuit of the current from the voltage on the battery, it will look like this:
What is the conclusion? In order to measure the EMF batteries, it is enough for us to just take a good multimeter with high input resistance and measure the voltage at the battery terminals.
The perfect source of EMS.
Suppose, let our battery have zero internal resistance, then it turns out that R h \u003d 0.
It is not difficult to guess that in this case the drop voltage on the zero resistance will also be zero. As a result, our schedule will take this kind:
As a result, we got just the source of EDS. Therefore, the source of EDC is the perfect power source, which does not depend on the strength of the current in the circuit. That is, what load we would not be cling to such a source of EDC, we will still have an accumulating stress without drawdown. The source itself of EMF is indicated like this:
In practice, the ideal source of EDS does not exist.
Types of EMS.
– electrochemical (EMF batteries and batteries)
– photo effect (Electric current from solar energy)
– induction (Generators using the principle of electromagnetic induction)
– Seebek or thermoem effect (The occurrence of electric current in a closed chain consisting of sequentially connected heterogeneous conductors, contacts between which are at different temperatures)
– piezoDes. (Getting EMF from)
Summary
EMF is the power of non-electric origin, which makes the electric current in the chain.
Real The source of EDC has internal resistance inside, ideal The source of EMF internal resistance is zero.
The ideal source of EMF always has a constant voltage value on its terminals regardless of the load in the chain.
We find out what value is the main characteristic of the current source. Any current source has two poles: positive and negative. So that he had these poles, it is necessary inside it to collect free positive charges on one pole, and negative on the other. For this you need to work. This work cannot accomplish electrostatic forces, since the variemless charges are attracted, and they must be disconnected. Work on the accumulation of charges is made not by electrostatic forces, but by third-party. The nature of the latter can be different. For example, in electric current generators, the separation of charges is carried out by the forces of the magnetic field, in batteries and electroplating elements - chemical. The study of current sources shows that the ratio of the work of a third-party force to the charge accumulated on the pole, for this current source, there is a constant value and is called the electromotive power source force:
Electrical power source power
The scalar value, which is the characteristic of the current source and is measured by the work performed by a third-party force inside its accumulation at each pole of 1 to charge, is called the electromotive power source power. Charge B. 1 toaccumulated on the current source pole, has a potential electrical energy, numerically equal to e. d. s. Source.
Unit e. d. s.
Meanimi e. d. s. Current source. To the demonstration electroplating element by connecting a voltmeter (Fig. 75, a) *. By changing the mutual location of the electrodes in the electrolyte, as well as the magnitude of their immersion in the electrolyte, we see that the testimony of the voltmeter ( 1.02 B.) Do not change. E. d. S. It does not depend on the size of the current source. It depends only on the nature of third-party forces, causing the accumulation of charges on the poles. Each current source has its own er. d. s.
* (With this measure e. d. s. The voltmeter reading will be slightly smaller than the value of er. d. s. The greater the resistance of the voltmeter coil compared to the internal resistance of the source, the less this difference will be, which is observed in the described experience.)
When the electrical circuit is closed, the current source forms a stationary electric field in the wires and transmits it with energy accumulated by charges on its poles. Due to this energy, the stationary field performs work on the current formation, transmitting its energy to it, which consumer converts to other types of energy.
The inner part of the chain is the current source, like any conductor, has resistance; it's called internal resistance of the current source R. The current generator internal resistance is the resistance of an anchor winding, in chemical sources electrolyte resistance.
When closing the circuit, the electric field, moving the charge 1 to From the point A to the point in the external section of the chain (Fig. 75, b), it makes a job that is numerically equal to the voltage U on this site. Having reached the pole in, charge 1 to must go to the inner plot of the chain and move to the pole A. So that he again turned out to be on the pole A and had the same energy E, as when leaving the point A, the third-party source forces should work on it, equal to work spent on its Move through the external section of the chain, which is numerically equal to the voltage U in this area, plus the work spent on overcoming the internal resistance R source. The latter is numerically equal to the voltage U on the inner segment of the chain. Consequently, e. d. s. The source is numerically equal E \u003d U + U.The electromotive force is numerically equal to the work that the current source performs by moving charge 1 to the entire chain.
We measure the voltage at the external and inner sections; Chains (Fig. 75, c) *. Voltmeter A shows the voltage at the external resistance R, and the voltmeter in - on the internal; Resistance r. Changing the amount of resistance of the external chain; We notice that at the same time the voltage in the plots of the chain (Table 4) changes.
* (Properties 1 and 2 are made of thick copper wire in chlorvinyl insulation, which is cut from the side located to the middle of the vessel. Properties come into contact with electrodes with electrodes.)
We see that the sum of stresses at the external and inner sections of the chain - the value is constant (within the limits of the errors of the experience) and equal to e. d. s. Source. It shows the magnitude of the energy that the current source is able to transfer to the electrical circuit when moving throughout the charge chain in 1 to.