In electrical engineering, power sources for electrical circuits are characterized by electromotive force (EMF).
In the external circuit of the electrical circuit, electric charges move from the plus of the source to the minus and create an electric current. To maintain its continuity in the circuit, the source must have a force that can move charges from lower to higher potential. This force of non-electrical origin is the emf of the source. For example, the emf of a galvanic cell.
According to this, EMF (E) can be calculated as:
E=A/q, Where:
The magnitude of EMF in the SI system is measured in volts (V).
EMF is the work done by external forces to move a unit charge along an electrical circuit
The circuit of a closed electrical circuit includes an external part, characterized by resistance R, and an internal part with source resistance Rin. A continuous current (In) in the circuit will flow as a result of the action of the EMF, which overcomes both the external and internal resistance of the circuit.
The current in the circuit is determined by the formula (Ohm's law):
In = E/(R+Rin).
In this case, the voltage at the source terminals (U 12) will differ from the EMF by the amount of the voltage drop across the internal resistance of the source.
U 12 = E - In*Rin.
If the circuit is open and the current in it is 0, then the emf of the source will be equal to the voltage U 12.
Developers of power supplies try to reduce the internal resistance Rin, as this may allow more current to be received from the source.
Various types of EMF are used in technology:
Thus, EMF is necessary to maintain constant current and is used in various types of technology.
« Physics - 10th grade"
Any current source is characterized by electromotive force, or abbreviated EMF. So, on a round flashlight battery it says: 1.5 V.
What does it mean?
If you connect two oppositely charged balls with a conductor, the charges will quickly neutralize each other, the potentials of the balls will become the same, and the electric field will disappear (Fig. 15.9, a).
Outside forces.
In order for the current to be constant, it is necessary to maintain a constant voltage between the balls. To do this, you need a device (current source) that would move charges from one ball to another in the direction opposite to the direction of the forces acting on these charges from the electric field of the balls. In such a device, in addition to electrical forces, charges must be acted upon by forces of non-electrostatic origin (Fig. 15.9, b). The electric field of charged particles alone ( Coulomb field) is not capable of maintaining a constant current in the circuit.
Any forces acting on electrically charged particles, with the exception of forces of electrostatic origin (i.e., Coulomb forces), are called outside forces.
The conclusion about the need for external forces to maintain constant current in the circuit will become even more obvious if we turn to the law of conservation of energy.
The electrostatic field is potential. The work done by this field when charged particles move in it along a closed electrical circuit is zero. The passage of current through the conductors is accompanied by the release of energy - the conductor heats up. Therefore, there must be some source of energy in the circuit that supplies it to the circuit. In it, in addition to the Coulomb forces, third-party, non-potential forces must act. The work of these forces along a closed loop must be different from zero.
It is in the process of doing work by these forces that charged particles acquire energy inside the current source and then give it to the conductors of the electrical circuit.
Third-party forces set in motion charged particles inside all current sources: in generators at power plants, in galvanic cells, batteries, etc.
When a circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, charges move under the influence of external forces against Coulomb forces(electrons from a positively charged electrode to a negative one), and in an external circuit they are driven by an electric field (see Fig. 15.9, b).
The nature of external forces.
The nature of external forces can be varied. In power plant generators, extraneous forces are forces acting from a magnetic field on electrons in a moving conductor.
In a galvanic cell, such as a Volta cell, chemical forces act.
The Volta cell consists of zinc and copper electrodes placed in a sulfuric acid solution. Chemical forces cause the zinc to dissolve in the acid. Positively charged zinc ions pass into the solution, and the zinc electrode itself becomes negatively charged. (Copper dissolves very little in sulfuric acid.) A potential difference appears between the zinc and copper electrodes, which determines the current in the external electrical circuit.
The action of external forces is characterized by an important physical quantity called electromotive force(abbreviated EMF).
Electromotive force current source is equal to the ratio of the work of external forces when moving a charge along a closed circuit to the absolute value of this charge:
Electromotive force, like voltage, is expressed in volts.
The potential difference across the battery terminals when the circuit is open is equal to the electromotive force. The emf of one battery cell is usually 1-2 V.
We can also talk about electromotive force in any part of the circuit. This is the specific work of external forces (work to move a single charge) not throughout the entire circuit, but only in a given area.
The electromotive force of a galvanic cell is a quantity numerically equal to the work of external forces when moving a single positive charge inside the element from one pole to another.
The work of external forces cannot be expressed through a potential difference, since external forces are non-potential and their work depends on the shape of the trajectory of the charges.
Until now, when studying electric current, we have considered the directional movement of free charges in external circuit, that is, in the conductors connected to the terminals of the current source.
As we know, positive charge:
It goes into the external circuit from the positive terminal of the source;
Moves in an external circuit under the influence of a stationary electric field created by other moving charges;
It arrives at the negative terminal of the source, completing its path in the external circuit.
Now our positive charge needs to close its path 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 about it: he doesn’t want to go there at all! The negative terminal attracts it towards itself, the positive terminal repels it from itself, and as a result, our charge inside the source is acted upon by an electric force directed against movement of the charge (i.e. against the direction of the current).
Nevertheless, current flows through the circuit; therefore, there is a force that “pulls” the charge through the source despite the resistance of the electric field of the terminals (Fig. 1).
Rice. 1. Third party force
This force is called outside force; It is thanks to it that the current source functions. The external force has nothing to do with the stationary electric field - it is said to have non-electric origin; in batteries, for example, it arises due to the occurrence of appropriate chemical reactions.
Let us denote by the work of an external force to move a positive charge q inside the current source from the negative terminal to the positive. This work is positive, since the direction of the external force coincides with the direction of charge movement. The work of an external force is also called operation of the current source.
There is no external force in the external circuit, so the work done by the external force to move the charge in the external circuit is zero. Therefore, the work of an external force to move a charge around the entire circuit is reduced to the work of moving this charge only inside the current source. Thus, this is also the work of an external force to move the charge throughout the chain.
We see that the external force is non-potential - its work when moving a charge along a closed path is not zero. It is this non-potentiality that ensures the circulation of electric current; a potential electric field, as we said earlier, cannot support a constant current.
Experience shows that work is directly proportional to the charge being moved. Therefore, the ratio no longer depends on the charge and is a quantitative characteristic of the current source. This relationship is denoted by:
(1)
This quantity is called electromotive force(EMF) of the current source. As you can see, EMF is measured in volts (V), so the name “electromotive force” is extremely unfortunate. But it has long been ingrained, so you have to come to terms with it.
When you see the inscription on the battery: “1.5 V”, then know that this is exactly the EMF. Is this value equal to the voltage created by the battery in the external circuit? It turns out not! Now we will understand why.
Any current source has its own resistance, which is called internal resistance this source. Thus, the current source has two important characteristics: emf and internal resistance.
Let a current source with an emf equal to and internal resistance be connected to a resistor (which in this case is called external resistor, or external load, or payload). All this together is called full chain(Fig. 2).
Rice. 2. Complete circuit
Our task is to find the current in the circuit and the voltage across the resistor.
Over time, a charge passes through the circuit. According to formula (1), the current source does the following work:
(2)
Since the current strength is constant, the work of the source is entirely converted into heat, which is released at the resistances and. This amount of heat is determined by the Joule–Lenz law:
(3)
So, , and we equate the right-hand sides of formulas (2) and (3):
After reducing by we get:
So we found the current in the circuit:
(4)
Formula (4) is called Ohm's law for a complete circuit.
If you connect the terminals of the source with a wire of negligible resistance, you will get short circuit. In this case, the maximum current will flow through the source - short circuit current:
Due to the small internal resistance, the short circuit current can be quite large. For example, a AA battery gets so hot that it burns your hands.
Knowing the current strength (formula (4)), we can find the voltage across the resistor using Ohm’s law for a section of the circuit:
(5)
This voltage is the potential difference between points and (Fig. 2). The potential of the point is equal to the potential of the positive terminal of the source; the potential of the point is equal to the potential of the negative terminal. Therefore, voltage (5) is also called voltage at the source terminals.
We see from formula (5) what will happen in a real circuit - after all, it is multiplied by a fraction less than one. But there are two cases when .
1. Ideal current source. This is the name of a source with zero internal resistance. When formula (5) gives .
2. Open circuit. Let's consider the current source by itself, outside the electrical circuit. In this case, we can assume that the external resistance is infinitely large: . Then the quantity is indistinguishable from , and formula (5) again gives us .
The meaning of this result is simple: if the source is not connected to the circuit, then a voltmeter connected to the poles of the source will show its emf.
It's not hard to see why a resistor is called a payload. Imagine it's a light bulb. The heat generated by a light bulb is useful, since thanks to this warmth the light bulb fulfills its purpose - giving light.
Let us denote the amount of heat released by the payload during time .
If the current in the circuit is equal to , then
A certain amount of heat is also released at the current source:
The total amount of heat released in the circuit is equal to:
Electrical circuit efficiency is the ratio of useful heat to total heat:
The efficiency of the circuit is equal to unity only if the current source is ideal.
Ohm's simple law is valid for the so-called homogeneous section of the circuit - that is, the section in which there are no current sources. Now we will obtain more general relations, from which both Ohm’s law for a homogeneous section and Ohm’s law obtained above for the complete chain follow.
The section of the chain is called heterogeneous, if there is a current source on it. In other words, an inhomogeneous area is an area with an EMF.
In Fig. Figure 3 shows a non-uniform section containing a resistor and a current source. The emf of the source is equal to , its internal resistance is considered equal to zero (if the internal resistance of the source is equal to , you can simply replace the resistor with a resistor ).
Rice. 3. EMF “helps” the current:
The current strength in the area is equal to , the current flows from point to point. This current is not necessarily caused by a single source. The section under consideration, as a rule, is part of a certain circuit (not shown in the figure), and other current sources may be present in this circuit. Therefore, the current is the result of the combined action everyone sources available in the circuit.
Let the potentials of points and be equal to and respectively. Let us emphasize once again that we are talking about the potential of a stationary electric field generated by the action of all sources of the circuit - not only the source belonging to this section, but also, possibly, those located outside this section.
The voltage in our area is equal to: . Over time, a charge passes through the area, while a stationary electric field does work:
In addition, positive work is performed by the current source (after all, the charge passed through it!):
The current strength is constant, therefore the total work on advancing the charge, performed in the area by the stationary electric field and external forces of the source, is entirely converted into heat: .
We substitute here expressions for , and the Joule–Lenz law:
Reducing by , we get Ohm's law for a non-uniform section of a circuit:
(6)
or, which is the same:
(7)
Please note: there is a plus sign in front of it. We have already indicated the reason for this - the current source in this case performs positive work, “dragging” a charge inside itself from the negative terminal to the positive one. Simply put, a source "helps" current flow from point to point.
Let us note two consequences of the derived formulas (6) and (7).
1. If the area is homogeneous, then . Then from formula (6) we obtain Ohm’s law for a homogeneous section of the chain.
2. Let us assume that the current source has internal resistance. This, as we already mentioned, is equivalent to replacing it with:
Now let’s close our section by connecting the points and . We obtain the complete circuit discussed above. In this case, it turns out that the previous formula will turn into Ohm’s law for the complete chain:
Thus, Ohm's law for a homogeneous section and Ohm's law for a complete chain both follow from Ohm's law for a non-uniform section.
There may be another case of connection, when the source “prevents” the current from flowing through the area. This situation is shown in Fig. 4. Here the current coming from to is directed against the action of external forces of the source.
Rice. 4. EMF “interferes” with the current:
How is this possible? It’s very simple: other sources present in the circuit outside the section under consideration “overpower” the source in the section and force the current to flow against. This is exactly what happens when you put your phone on charge: the adapter connected to the socket causes charges to move against the action of external forces in the phone's battery, and the battery is thereby charged!
What will change now in the derivation of our formulas? There is only one thing - the work of external forces will become negative:
Then Ohm's law for a non-uniform area will take the form:
(8)
where is still the tension in the area.
Let's put together formulas (7) and (8) and write Ohm's law for the section with EMF as follows:
The current flows from point to point. If the direction of the current coincides with the direction of external forces, then a “plus” is placed in front of it; if these directions are opposite, then a “minus” is given.
>>Physics: Electromotive force
Any current source is characterized by electromotive force, or, in short, EMF. So, on a round flashlight battery it says: 1.5 V. What does this mean?
Connect two metal balls carrying charges of opposite signs with a conductor. Under the influence of the electric field of these charges, an electric current arises in the conductor ( Fig.15.7). But this current will be very short-lived. The charges quickly neutralize each other, the potentials of the balls will become the same, and the electric field will disappear.
Outside forces. In order for the current to be constant, it is necessary to maintain a constant voltage between the balls. For this you need a device ( current source), which would move charges from one ball to another in the direction opposite to the direction of the forces acting on these charges from the electric field of the balls. In such a device, in addition to electrical forces, charges must be acted upon by forces of non-electrostatic origin ( Fig.15.8). The electric field of charged particles alone ( Coulomb field) is not capable of maintaining a constant current in the circuit.
Any forces acting on electrically charged particles, with the exception of forces of electrostatic origin (i.e., Coulomb forces), are called by outside forces.
The conclusion about the need for external forces to maintain a constant current in the circuit will become even more obvious if we turn to the law of conservation of energy. The electrostatic field is potential. The work done by this field when charged particles move in it along a closed electrical circuit is zero. The passage of current through the conductors is accompanied by the release of energy - the conductor heats up. Therefore, there must be some source of energy in the circuit supplying it to the circuit. In addition to Coulomb forces, third-party, non-potential forces must act in it. The work of these forces along a closed loop must be different from zero. It is in the process of doing work by these forces that charged particles acquire energy inside the current source and then give it to the conductors of the electrical circuit.
Third-party forces set in motion charged particles inside all current sources: in generators at power plants, in galvanic cells, batteries, etc.
When a circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, charges move under the influence of external forces against Coulomb forces(electrons from a positively charged electrode to a negative one), and in an external circuit they are driven by an electric field (see. Fig.15.8).
The nature of external forces. The nature of external forces can be varied. In power plant generators, external forces are forces acting from a magnetic field on electrons in a moving conductor.
In a galvanic cell, such as a Volta cell, chemical forces operate. The Volta cell consists of zinc and copper electrodes placed in a sulfuric acid solution. Chemical forces cause the zinc to dissolve in the acid. Positively charged zinc ions pass into the solution, and the zinc electrode itself becomes negatively charged. (Copper dissolves very little in sulfuric acid.) A potential difference appears between the zinc and copper electrodes, which determines the current in a closed electrical circuit.
The action of external forces is characterized by an important physical quantity called electromotive force(abbreviated EMF).
The electromotive force of a current source is equal to the ratio of the work done by external forces when moving a charge along a closed circuit to the magnitude of this charge:
Electromotive force, like voltage, is expressed in volts.
We can also talk about electromotive force in any part of the circuit. This is the specific work of external forces (work to move a single charge) not throughout the entire circuit, but only in a given area. Electromotive force of a galvanic cell is a quantity numerically equal to the work of external forces when moving a single positive charge inside an element from one pole to another. The work of external forces cannot be expressed through a potential difference, since external forces are non-potential and their work depends on the shape of the trajectory of the charges. So, for example, the work of external forces when moving a charge between the terminals of a current source outside the source itself is zero.
Now you know what EMF is. If the battery says 1.5 V, this means that external forces (chemical in this case) do 1.5 J of work when moving a charge of 1 C from one pole of the battery to the other. Direct current cannot exist in a closed circuit if no external forces act in it, i.e. there is no EMF.
???
1. Why is the electric field of charged particles (Coulomb field) not capable of maintaining a constant electric current in a circuit?
2. What forces are usually called third-party?
3. What is called electromotive force?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade
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In this lesson we will take a closer look at the mechanism for providing long-term electric current. Let us introduce the concepts of “power source”, “external forces”, describe the principle of their operation, and also introduce the concept of electromotive force.
Topic: Laws of direct current
Lesson: Electromotive force
In one of the previous topics (conditions for the existence of electric current), the issue of the need for a power source to maintain the existence of electric current for a long time was already touched upon. The current itself, of course, can be obtained without such power sources. For example, a capacitor discharges when the camera flashes. But such a current will be too fleeting (Fig. 1).
Rice. 1. Short-term current during mutual discharge of two oppositely charged electroscopes ()
Coulomb forces always strive to bring opposite charges together, thereby equalizing the potentials throughout the entire circuit. And, as you know, for the presence of a field and a current, a potential difference is necessary. Therefore, it is impossible to do without any other forces that separate the charges and maintain the potential difference.
Definition. Third-party forces are forces of non-electrical origin aimed at diluting charges.
These forces can be of different nature depending on the type of source. In batteries they are of chemical origin, in electric generators they are of magnetic origin. They ensure the existence of current, since the work of electrical forces in a closed circuit is always zero.
The second task of energy sources, in addition to maintaining the potential difference, is to replenish energy losses due to collisions of electrons with other particles, as a result of which the former lose kinetic energy, and the internal energy of the conductor increases.
Extraneous forces inside the source do work against the electrical forces, spreading the charges in directions opposite to their natural course (as they move in the external circuit) (Fig. 2).
Rice. 2. Scheme of action of third-party forces
An analogue of the action of a power source can be considered a water pump, which releases water against its natural flow (from bottom to top, into apartments). The water naturally flows back down under the influence of gravity, but for continuous operation of the water supply to the apartment, continuous operation of the pump is necessary.
Definition. Electromotive force is the ratio of the work of external forces to move a charge to the magnitude of this charge. Designation - :
Unit:
Consider the following circuit (Fig. 3):
Rice. 3.
With the switch open and an ideal voltmeter (resistance is infinitely high), there will be no current in the circuit, and only work on separating charges will be done inside the galvanic cell. In this case, the voltmeter will show the EMF value.
When the key is closed, current will flow through the circuit, and the voltmeter will no longer show the EMF value, it will show the voltage value, the same as at the ends of the resistor. With a closed loop:
Here: - voltage on the external circuit (on the load and supply wires); - voltage inside the galvanic cell.
In the next lesson we will study Ohm's law for a complete circuit.
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