7-9 class students in tasks sometimes meet the concept of EDC. And immediately the question: "What is it?"
If you take to any current source: the battery (galvanic element), the power supply, etc., - see it, for example, the inscription "4.5 V". You call this source voltage. But in fact it is EMF - electromotive force. It is indicated by ℰ, measured in volts (B).
If the electrical resistance of the source can be neglected (i.e., the problem does not say anything about this resistance or it is written that the source is perfect), then the EMF and the source voltage are equal.
In this way,
EMF is one of the characteristics of the current source.
Typically, to solve problems in 7-9 classes of this.
Level A.
In high school classes, the concept of EMF requires a more detailed consideration.
Thirdness
Consider two examples.
1. Mass ball m. fixed at some point BUT above the table (Fig. 1, a).
2. Ball with charge q. 1 (and low mass) fixed at some point BUT A short distance from the second fixed charge q. 2 (Fig. 1, b).
Fig. one
What happens to the balls if they are released?
1. Mass ball m. Start fall, and if you do not catch it, falls on the table. The ball makes the power of gravity. In this case, they say that the strength of gravity (or gravitational field) makes work.
2. Ball with charge q. 1 will start moving to charge q. 2, and if you do not catch it, it will face him. The ball makes moving the strength of attraction to the second ball ( coulomb force). In this case, they say that the Coulomb force (or electric field) makes work.
Is it possible to return the balls to the point a?
It is possible, but for this you need to make extra force.
In the first example, we can throw the ball up. We will spend our own energy to force the ball to move in the right direction.
The second example will consider in more detail. The ball can be made to move left one more charge q. 3, big value than charge q. 2. But it will also be the same Coulomb force. You can also apply mechanical strength, you can inform the ball with additional energy (for example, light, chemical, etc.) so that it can overcome the charge attraction q. 2 .
Forces acting on the charge, with the exception of Coulomb, are called third-party. Inside any source of current, charges are moving under the action of third-party forces.
In all cases, if the power causes the body to move in the right direction, it makes work. So, third-party forces make work on the movement of the charge, which is called third party.
EMF.
The ratio of the work of third-party forces to move the charge to the magnitude of this charge is EMF (electromotive force).
Denote the work of third-party forces - A. CT, portable charge - q., then from the definition it follows that EDC
Based on this formula, you can give another definition:
EMF is a physical scalar value, numerically equal to the work of third-party forces on the movement of a single positive charge.
Thus, the EMF characterizes the effect of third-party strength and is not a force in the usual understanding of this word. Here again is used not very successful, but historically established terminology.
From this formula, it is clear that EMF is measured in volts (B).
.
In this lesson, we will describe the mechanism for providing long-term electric current. We introduce the concepts of the "power source", "third-party strength", we describe the principle of their action, as well as we introduce the concept of electromotive force.
Subject: DC laws
Lesson: Electrical Force
In one of the previous topics (the conditions of the electric current), the question of the need for a power source for long-term maintenance of the existence of an electric current was already affected. The current itself, of course, can be obtained without such power supplies. For example, the discharge of the capacitor when the camera is outbreak. But this current will be too vehicles (Fig. 1).
Fig. 1. Short-term current with mutual discharge of two variemlessly charged electroscopes ()
Coulomb forces always seek to reduce multi-way charges, thereby aligning the potentials throughout the chain. And, as is well known, the difference in potentials is necessary for the presence of fields and current. Therefore, it is impossible to do without any other forces that build charges and supporting the potential difference.
Definition. Third-party forces - forces of non-electric origin, aimed at breeding charges.
These forces can be of varying nature depending on the type of source. In batteries, they are chemical origin, in electrical generators - magnetic. They provide the existence of current, since the work of the electric forces on a closed contour is always zero.
The second task of energy sources, in addition to maintaining the potential difference, is the replenishment of energy losses on the collision of electrons with other particles, as a result of which the first loses the kinetic energy, and the internal energy of the conductor rises.
Third-party forces inside the source perform work against electric forces, spreading charges on the parties opposite to their natural move (as they move in the outer chain) (Fig. 2).
Fig. 2. Third-party action scheme
An analogue of the power supply can be considered a water pump that allows water against its natural stroke (bottom up, to the apartment). The back of the water is naturally under the action of gravity goes down, but for the continuous operation of the water supply of the apartment requires continuous operation of the pump.
Definition. The electromotive force is the ratio of the work of third-party charges for the movement of the charge to the magnitude of this charge. Designation -:
Unit of measurement:
Insert. EMF open and closed chain
Consider the following chain (Fig. 3):
Fig. 3.
With an open key and an ideal voltmeter (resistance is infinitely large) there will be no current in the chain, and only work on the separation of charges will be carried out inside the galvanic element. In this case, the voltmeter will show the value of the EDC.
When the key is closed around the chain, there will be a current, and the voltmeter will no longer show the value of the EMF, it will show the voltage value, the same as at the ends of the resistor. With a closed loop:
Here: - voltage on the external chain (on load and supply wires); - Voltage inside the galvanic element.
In the next lesson, we will study the Ohm law for the full chain.
List of references
- Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemozina, 2012.
- Gentendestein L.E., Dick Yu.I. Physics 10 class. - M.: Ilex, 2005.
- Myakyshev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.
- ens.TPU.ru ().
- physbook.ru ().
- electrodynamics.narod.ru ().
Homework
- What is third-party strength, what is their nature?
- How is the voltage on open poles of the current source with its EMF?
- How is the energy in a closed circuit turning and transmitted?
- * EMF lantern batteries - 4.5 V. Will this battery burn a light bulb, calculated on 4.5 V? Why?
At the ends of the conductor, which means that the current requires the existence of third-party forces of non-electric nature, with which the separation of electrical charges occurs.
Third-party forces Any forces acting on electrically charged particles in the chain are called, with the exception of electrostatic (i.e. Coulomb).
Third-party forces lead the charged particles of all sources of current: in the generators, on power plants, in electroplating elements, batteries, etc.
When the circuit is closed, an electric field is created in all pro-vodkers chains. Inside the source of current, the charges are moving under the action of third-party forces against the Coulomb forces (electrons are moving from a positively charged electrode to negative), and in the entire remaining chain they lead and move the electric field (see Fig. Above).
In current sources in the process of work on the separation of charged particles, there are transformation of different types of energy into electric-kui. By type of transformed energy, the following types of electromotive power distinguish:
- Electrostatic - in the electrophore machine, in which the transformation of mechanical energy during friction into electric;
- thermoelectric - in the thermoelement - the inner energy of the heated spa of two wires made from different metals turns into electric;
- Photoelectric - In the photocell. Here there is a conversion of light energy into electric: with the illumination of some substances, for example, selenium, copper oxide (I), silicon is observed a loss of negative electrical charge;
- Chemical - in electroplating elements, batteries, etc. Sources in which the transformation of chemical energy into electric.
Electromotive Force (EMF) - Characteristics of current sources. The concept of EMF was introduced by G. Om in 1827 for DC circuits. In 1857, Kirchhoff defined the EMF as a work of third-party forces when transferring a single electric charge along a closed loop:
ɛ \u003d A ST / Q,
where ɛ - EMF of the current source, And Art - work of third-party forces, q. - Number of displaced charge.
Electrical power expressed in volts.
You can talk about the electromotive strength on any section of the chain. This is a specific work of third-party strength (work on the movement of a single charge) not in the entire contour, but only in this area.
Internal resistance of the current source.
Suppose there is a simple closed circuit consisting of a current source (for example, a galvanic element, battery or generator) and resistor resistance R.. The current in the closed chain is not interrupted anywhere, therefore, it exists within the current source. Any source is a certain current of the current. It's called internal resistance of the current source And denotes the letter r..
In the generator r. - This is the resistance of the winding, in the electroplating element - the resistance of the electrolyte solution and electrodes.
Thus, the current source is characterized by the values \u200b\u200bof EDC and internal resistance that determine its quality. For example, electrostatic machines have a very large EMF (up to tens of thousands of volts), but their internal resistance is huge (up to the one-year). Therefore, they are unsuitable for rapid currents. In the electroplating elements of the EMF, only approximately 1 B, but the internal resistance is small (approximately 1 ohm and less). This allows them to receive currents measured by ampere.
The electric current does not proceed in the copper wire for the same reason that the fixed water remains in the horizontal tube. If one end of the pipe is connected to the tank in such a way that the pressure difference formed, the fluid will flow from one end. Similarly, to maintain direct current, there is an external impact moving charges. This impact is called an electromotive force or EMF.
Between the end of the XVIII and the beginning of the XIX century, the work of such scientists, like a pendant, Lagrange and Poisson, laid the mathematical foundations for determining electrostatic values. Progress in the understanding of electricity at this historical stage is obvious. Franklin has already introduced the concept of "the number of electrical substance", but still he, nor his successors could not measure it.
Following the experiments of Galvani, Volta tried to find confirmation that the "electroplating liquids" of the animal was one nature with static electricity. In search of truth, he found that when two electrodes from different metals are in contact through the electrolyte, both charge and remain charged despite the circuit of the load of the load. This phenomenon did not correspond to the existing ideas about electricity because electrostatic charges should have been recombined in this case.
Volta introduced a new definition of force acting in the direction of separation of charges and maintain them in such a state. He called her electromotive. A similar explanation of the description of the battery did not fit into the theoretical foundations of physics of that time. In the Coulomb paradigm of the first third of the XIX century E. d. s. Volta was determined by the ability of one bodies to produce electricity in others.
The most important contribution to the explanation of the work of the electrical chains was made. The results of a number of experiments led it to the construction of the theory of electrical conductivity. He introduced the magnitude of the "tension" and determined it as a potential difference on contacts. Like Fourier, which in its theory differed the amount of heat and temperature in heat transfer, OM created a model by analogy that binds the amount of the charged charge, voltage and electrical conductivity. Ohm's law did not contradict the accumulated knowledge of electrostatic electricity.
Then, thanks to Maxwell and Faraday, the explanatory models of the current obtained a new field theory. This made it possible to develop a function-related energy concept for both static potentials and electromotive power. The main dates of the evolution of the concept of EDC:
- 1800 - the creation of a volta galvanic battery;
- 1826 - ohms formulates its law for the total chain;
- 1831 - detection of electromagnetic induction by Faraday.
Definition and physical meaning
The application of some potential difference between the two ends of the conductor will create the flow of electrons from one end to another. But this is not enough to maintain the stream of charges in the conductor. The electron drift leads to a reduction in the potential until its balancing (cessation of current). Thus, for the creation of a DC, mechanisms are needed, continuously returning the described system to the initial configuration, that is, that prevent charge aggregation as a result of their movement. For this purpose, special devices are used, called power supplies.
As an illustration of their work, it is convenient to consider a closed contour from the resistance and a galvanic power supply (battery). If we suggest that there is no current battery, then the described problem of union charge remains unresolved. But in the circuit with a real power source electrons move constantly. This occurs due to the fact that the flow of ions proceeds inside the battery from the negative electrode to the positive. The source of energy moving these charges in the battery is chemical reactions. Such energy is called the electromotive force.
EMF is a characteristic of any energy source capable of controlling the movement of electrical charges in the chain. In analogy with a closed hydraulic circuit, the operation of the source e. d. s. Corresponds to the operation of the pump to create water pressure. Therefore, the icon denoting these devices is indistinguishable on hydraulic and electrical circuits.
Despite the name, the electromotive force is not really strength and is measured in volts. Its numerical value is equal to work on the movement of the charge on the closed chain. EMF of the source is expressed by the formula E \u003d A / Q, wherein:
- E - electromotive strength in volts;
- A - the work of third-party charges for the movement of charge in Joules;
- q - displaced charge in the coulutes.
From this formula, the EDC follows that the electromotive force is not a chain or load property, but is the ability of the electricity generator to separated charges.
The electromotive power and the potential difference in the chain is very similar physical quantities, as both are measured in volts and are determined by the work on the movement of charge. One of the basic semantic differences is that E. d. s. (E) It is caused by transforming any energy into electrical, while the difference in potentials (U) implements electrical energy to other types. Other differences look like this:
- E transmits the energy of the entire chain. U is a measure of energy between two points in the diagram.
- E is the cause of U, but not the opposite.
- E is induced in an electric, magnetic and gravitational field.
- Concept e. d. s. Applicable only to the electric field, while the potential difference is applicable to magnetic, gravitational and electric fields.
Voltage on the power source terminals, as a rule, differs from the EMF of the source. This is due to the presence of internal resistance of the source (electrolyte and electrodes, generator windings). Binding the difference of potentials and the EMF of the current source of the formula looks like U \u003d E-IR. In this expression:
- U is the voltage on the terminals of the source;
- r is the internal resistance of the source;
- I - Current in the chain.
From this formula of the electromotive force it follows that e. d. s. equal to voltage when the current in the chain does not flow. The ideal source of EDS creates the difference in potentials, regardless of the load (current current) and does not have internal resistance.
In nature, there can be no source with infinite power when closed on terminals, as well as material with infinite conductivity. The perfect source is used as an abstract mathematical model.
The essence of the source of the EMF is to transform other types of energy into electric with third-party strength. From the point of view of physics to ensure er D. C differ the following two main types of sources:
- galvanic;
- electromagnetic.
The first are electrochemical sources based on the involvement in the chemical reaction of the electron transfer process. In normal conditions, chemical interactions are accompanied by the release or absorption of heat, but there are many reactions, as a result of which electrical energy is generated.
Electrochemical processes are in most cases reversible, since the electric current energy can be used to force the substances to be reacting. This feature allows you to create renewable galvanic sources - batteries.
In current generators d. s. Created in another way. The separation of charges occurs by the phenomenon of electromagnetic induction, which is that the change in the magnitude or direction of the magnetic field creates EMF. According to the Faraday law, finding e. d. s. Induction is possible from an E \u003d -DF / DT expression. In this formula:
- F - magnetic stream;
- t - Time.
EMF induction is also measured in volts. Depending on which changes in the magnetic flux are caused, distinguish:
- Dynamically induced. When the conductor moves in a stationary magnetic field. Characterized for generators.
- Statically induced. When flow changes occur due to changes in the magnetic field around the fixed conductor. So there are transformers.
There are also sources E. C, not based on electrochemistry or magnetic induction. These devices include semiconductor photocells, contact potentials and piezocrystals. The concept of EMF has practical application primarily as a parameter for selecting power sources for certain purposes. To get the maximum effect of the devices in the chain, you need to coordinate their capabilities and characteristics. First of all, the internal resistance of the source of EMF power with the characteristics of the connected load.
In the material we'll figure it out in the concept of EDC induction in situations of its occurrence. We will also consider the inductance as a key parameter for the occurrence of the magnetic flux when the electric field appears in the conductor.
Electromagnetic induction is the generation of electric current by magnetic fields that change over time. Thanks to the discovery of Faraday and Lenz, the patterns were formulated in laws, which introduced symmetry in understanding electromagnetic flows. Maxwell Theory gathered together knowledge about electric current and magnetic flux. Thanks to the discovery of Hertz, humanity learned about telecommunications.
An electromagnetic field appears around the conductor with electrotox, but in parallel the reverse phenomenon also occurs - electromagnetic induction. Consider the magnetic stream on the example: if the frame from the conductor is placed in an electrical field with induction and move it from top to bottom along the magnetic power lines or right-left perpendicular to them, then the magnetic flux passing through the frame will be constant.
When the frame is rotated around its axis, then after a while, the magnetic stream will change to a certain amount. As a result, the EDC induction occurs in the frame and an electric current appears, which is called induction.
EMF induction
We will understand in detail what the concept of EMF induction is. When placed in the magnetic field of the conductor and its movement with the intersection of power lines of the field, an electromotive force called EDC induction appears in the explorer. It also occurs if the conductor remains in a fixed state, and the magnetic field moves and intersects with the conductor with power lines.
When the conductor, where the occurrence of EMF occurs, closes on a widespread chain, due to the presence of this EMF on the chain begins to flow induction current. Electromagnetic induction involves the induce induction phenomenon in the conductor at the time of its intersection by the power lines of the magnetic field.
Electromagnetic induction is the reverse process of transformation of mechanical energy in electrical strokes. This concept and its patterns are widely used in electrical engineering, most of the electromasic are based on this phenomenon.
Faraday and Lenza laws
Faraday and Lenza laws reflect the patterns of occurrence of electromagnetic induction.
Faradays revealed that magnetic effects appear as a result of changing the magnetic flux in time. At the time of intersection of the conductor by alternating magnetic current, there is an electromotive force in it, which leads to the occurrence of electric current. Generate current can be both a permanent magnet and an electromagnet.
The scientist determined that the current intensity increases with a rapid change in the number of power lines that intersect the contour. That is, the emf of electromagnetic induction is directly dependent on the speed of the magnetic flux.
According to the Faraday law, the formula of EMF induction is defined as follows:
The "minus" sign indicates the relationship between the polarity of the induced EMF, the direction of flow and changing speed.
According to the law of Lenz, it is possible to characterize the electromotive force depending on its direction. Any change in the magnetic flux in the coil leads to the appearance of EMF induction, and with a rapid change, an increasing EMF is observed.
If the coil, where there is an EDC induction, has a closure on the outer chain, then the induction current flows through it, as a result of which the magnetic field appears around the conductor and the coil acquires the properties of the solenoid. As a result, around the coil is formed its magnetic field.
E.H. Lenz has established a pattern according to which the direction of the induction current in the coil and EMF induction is determined. The law says that the EMF induction in the coil with a change in the magnetic flux forms a direction in the coil at which this magnetic flux of the coil makes it possible to avoid changing the foreign magnetic flux.
Lenza law applies to all expanding situations in conductors, regardless of their configuration and method of changing the external magnetic field.
Wire Movement in Magnetic Field
The value of the induced EMF is determined depending on the length of the conductor intersectable by the power lines of the field. With more power lines, the value of the induced EMF increases. With an increase in the magnetic field and induction, the greater EMF value occurs in the conductor. Thus, the value of the EMF induction in the conductor moving in a magnetic field is directly dependent on the induction of the magnetic field, the length of the conductor and the speed of its movement.
This dependence is reflected in the formula E \u003d BLV, where E - EMF induction; In - the value of magnetic induction; I - Explorer length; V-Speed \u200b\u200bof its movement.
Note that in the conductor, which moves in the magnetic field, the EDC induction appears only when it crosses the power lines of the magnetic field. If the conductor moves according to the power lines, then the EMF is not induce. For this reason, the formula applies only in cases when the conductory movement is sent perpendicular to the power lines.
The direction of the induced EMF and the electric flow in the conductor is determined by the direction of movement of the conductor itself. To identify the direction, the rule of the right hand is developed. If you keep the palm of the right hand so that in its direction the power lines of the field, and the thumb indicates the direction of movement of the conductor, then the remaining four fingers show the direction of the induced EMF and the direction of the electric fiber in the explorer.
Rotating coil
The operation of the electrotock generator is based on the rotation of the coil in a magnetic stream, where there is a certain number of turns. EMF is always induced in an electrical circuit when it intersects it with a magnetic flux, based on the formula of the magnetic flux F \u003d B x S x Cos α (magnetic induction, multiplied by the surface area through which the magnetic flow passes, and the cosine of the angle, formed by the direction vector and perpendicular plane lines).
According to the formula, the changes in situations are influenced by:
- with a change in the magnetic flux, the direction vector changes;
- changes the area concluded in the contour;
- changes the angle.
EMF induction is allowed with a fixed magnet or constant current, and simply when the coil rotates around its axis within the magnetic field. In this case, the magnetic flow changes when the angle value is changed. The coil during the rotation process crosses the power lines of the magnetic flux, as a result, EMF appears. With uniform rotation, a periodic change of the magnetic flux occurs. Also, the number of power lines that intersect every second becomes equal to values \u200b\u200bat equal time intervals.
In practice, in the alternators of an alternating power, the coil remains in a fixed state, and the electromagnet performs rotation around it.
EMF self-induction
When passing through the coil of the variable electrotock, an alternating magnetic field is generated, which is characterized by a changing magnetic flow induced by EMF. This phenomenon is called self-induction.
Due to the fact that the magnetic flow is proportional to the intensity of the electric flow, then the emf of self-induction formula looks like this:
F \u003d L x i, where L is the inductance that is measured in GG. Its value is determined by the number of turns per unit length and the magnitude of their cross-section.
Constraction
When two coils are located nearby, there is an emf of mutual induction, which is determined by the configuration of two schemes and their mutual orientation. As the chain separation increases, the value of the inteducity is reduced, since there is a decrease in total for two magnetic flux coils.
Consider in detail the process of occurrence of mutual induction. There are two coils, on the wire of one with N1 turns flowing current I1, which creates a magnetic flux and goes through the second coil with N2 the number of turns.
The value of the interdigality of the second coil with respect to the first:
M21 \u003d (n2 x F21) / i1.
The value of the magnetic flux:
F21 \u003d (M21 / N2) x i1.
Induced EMF is calculated by the formula:
E2 \u003d - N2 X DF21 / DT \u003d - M21X DI1 / DT.
In the first coil, the value of the EDC induced:
E1 \u003d - M12 X DI2 / DT.
It is important to note that the electromotive force provoked by mutually induction in one of the coils in any case is directly proportional to the change in the electric current in another coil.
Then mutually induction is considered to be equal:
M12 \u003d M21 \u003d M.
As a result, E1 \u003d - M x Di2 / DT and E2 \u003d M x Di1 / DT. M \u003d K √ (L1 x L2), where K is a communication coefficient between the two values \u200b\u200bof the injectivity.
The mutually perduction is widely used in transformers that make it possible to change the values \u200b\u200bof the variable electrotock. The device is a pair of coils that are wound on the overall core. The current in the first coil forms a changing magnetic flux in the magnetic circuit and the current in the second coil. With a smaller number of turns in the first coil than in the second, the voltage increases, and, accordingly, with a larger number of turns in the first winding, the voltage is reduced.
In addition to generating and transformation of electrical energy, the phenomenon of magnetic induction is used in other devices. For example, in magnetic levitational trains moving without direct contact with current in rails, and for a couple of centimeters above the cause of electromagnetic repulsion.
