What EMF(electromotive force) in physics? Electric current is not understood by everyone. Like space distance, only under the very nose. In general, it is not fully understood by scientists either. It is enough to remember with his famous experiments, which were centuries ahead of their time and even today remain in a halo of mystery. Today we are not solving big mysteries, but we are trying to figure out what is emf in physics.

Definition of EMF in physics

EMF is the electromotive force. Denoted by letter E or the small Greek letter epsilon.

Electromotive force- scalar physical quantity characterizing the work of external forces ( forces of non-electric origin) operating in electrical circuits of alternating and direct current.

EMF, like voltage e, measured in volts. However, EMF and voltage are different phenomena.

Voltage(between points A and B) - a physical quantity equal to the work of the effective electric field performed when a unit test charge is transferred from one point to another.

We explain the essence of EMF "on the fingers"

To understand what is what, we can give an analogy example. Imagine that we have a water tower completely filled with water. Compare this tower with a battery.

Water exerts maximum pressure on the bottom of the tower when the tower is full. Accordingly, the less water in the tower, the weaker the pressure and pressure of the water flowing from the tap. If you open the tap, the water will gradually flow out at first under strong pressure, and then more and more slowly until the pressure weakens completely. Here stress is the pressure that the water exerts on the bottom. For the level of zero voltage, we will take the very bottom of the tower.

It's the same with the battery. First, we include our current source (battery) in the circuit, closing it. Let it be a clock or a flashlight. While the voltage level is sufficient and the battery is not discharged, the flashlight shines brightly, then gradually goes out until it goes out completely.

But how to make sure that the pressure does not run out? In other words, how to maintain a constant water level in the tower, and a constant potential difference at the poles of the current source. Following the example of the tower, the EMF is presented as a pump, which ensures the influx of new water into the tower.

The nature of the emf

The reason for the occurrence of EMF in different current sources is different. According to the nature of occurrence, the following types are distinguished:

• Chemical emf. Occurs in batteries and accumulators due to chemical reactions.
• Thermo EMF. Occurs when contacts of dissimilar conductors at different temperatures are connected.
• EMF induction. Occurs in a generator when a rotating conductor is placed in a magnetic field. EMF will be induced in the conductor when the conductor crosses DC lines of force magnetic field or when the magnetic field changes in magnitude.
• Photoelectric EMF. The occurrence of this EMF is facilitated by the phenomenon of an external or internal photoelectric effect.
• Piezoelectric emf. EMF occurs when a substance is stretched or compressed.

Dear friends, today we have considered the topic "EMF for Dummies". As you can see, the EMF force of non-electric origin, which maintains the flow of electric current in the circuit. If you want to know how to solve problems with EMF, we advise you to contact - carefully selected and proven specialists who will quickly and clearly explain the solution to any problem. thematic task. And by tradition, at the end we invite you to watch the training video. Happy viewing and good luck with your studies!

« Physics - Grade 10 "

Any current source is characterized by an electromotive force, or EMF for short. So, on a round battery for flashlight written: 1.5 V.
What does it mean?

If you connect two oppositely charged balls with a conductor, then the charges quickly neutralize each other, the potentials of the balls will become the same, and the electric field will disappear (Fig. 15.9, a).

Third party forces.

In order for the current to be constant, it is necessary to maintain a constant voltage between the balls. This requires 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 for charges other than electrical forces, forces of non-electrostatic origin must act (Fig. 15.9, b). Only one electric field of charged particles ( 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), are called 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 of this field when moving charged particles in it along a closed electric 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 necessarily act. The work of these forces along a closed contour 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 electric circuit.

External forces set in motion charged particles inside all current sources: in generators at power plants, in galvanic cells, batteries, etc.

When the circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, the charges move under the influence of external forces vs. Coulomb forces(electrons from a positively charged electrode to a negative one), and in the external circuit they are set in motion by an electric field (see Fig. 15.9, b).

The nature of extraneous forces.

The nature of outside forces can be varied. In power plant generators, external forces are forces acting from the magnetic field on electrons in a moving conductor.

In a galvanic cell, for example, in the Volta cell, chemical forces act.

The Volta element consists of zinc and copper electrodes placed in a solution of sulfuric acid. 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 the charge in a closed loop to absolute value 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 the electromotive force in any part of the circuit. This is the specific work of external forces (the work of moving a unit charge) not in the entire circuit, but only in this area.

The electromotive force of a galvanic cell is a value numerically equal to the work of external forces when moving a single positive charge inside the cell from one pole to another.

The work of external forces cannot be expressed in terms of the potential difference, since external forces are non-potential and their work depends on the shape of the charge trajectory.

To maintain a given value of electric current in the conductor, some external source energy, which all the time would provide the necessary potential difference at the ends of this conductor. Such sources of energy are the so-called sources of electric current, which have some given electromotive force, which is able to create and maintain a potential difference for a long time.

The electromotive force or abbreviated EMF is indicated by the Latin letter E. Unit of measurement is an volt. Thus, in order to obtain a continuous movement of electric current in a conductor, an electromotive force is needed, that is, a source of electric current is required.

History reference. The first such current source in electrical engineering was the "voltaic column", which was made of several copper and zinc circles, lined with cowhide dipped in a weak acid solution. Thus, the most in a simple way obtaining an electromotive force is considered chemical interaction a number of substances and materials, as a result of which chemical energy is converted into electrical energy. Power sources in which the electromotive force of the EMF is generated by a similar method are called chemical current sources.

Today, chemical power sources - batteries and all possible types of batteries - are widely used in electronics and electrical engineering, as well as in the electric power industry.

Various types of generators are also common, which, as the only source, are capable of supplying industrial enterprises with electric energy, providing lighting to cities, for the operation of systems railways, trams and metro.

EMF acts in exactly the same way both on chemical sources and on generators. Its action is to create a potential difference at each of the power supply terminals and maintain it for the entire necessary time. The terminals of the power supply are called poles. At one of the poles, a shortage of electrons is always created, i.e. such a pole has a positive charge and is marked " + ”, and on the other hand, on the contrary, an increased concentration of free electrons is created, i.e. this pole has a negative charge and is marked with the sign " - ».

EMF sources are used to connect various devices and devices that are consumers of electrical energy. With the help of wires, consumers are connected to the poles of current sources, so that a closed electrical circuit is obtained. The potential difference that has arisen in a closed electrical circuit has received a name and is denoted by the Latin letter "U". Voltage unit one volt. For example, the entry U=12 V indicates that the voltage of the EMF source is 12 V.

In order to measure voltage or emf, a special measuring device - .

If it is necessary to make correct measurements of EMF or power supply voltage, the voltmeter is connected directly to the poles. With an open electrical circuit, the voltmeter will show the EMF. When the circuit is closed, the voltmeter will display the voltage value at each terminal of the power supply. PS: The current source always develops more EMF than the voltage at the terminals.

Video lesson: EMF

Video lesson: Electromotive force from a physics teacher

The voltage at each of the terminals of the current source is less than the electromotive force by the value of the voltage drop that occurs on the internal resistance of the power source:

Ideal Source

For ideal sources, the voltage at the terminals does not depend on the amount of current drawn.

All sources of electromotive force have parameters that characterize them: open circuit voltage U xx, short circuit current I kz and internal resistance (for a DC source R ext). U xx is the voltage when the source current is zero. At an ideal source at any current U xx \u003d 0. I kz is the current at zero voltage. For an ideal voltage source, it is infinite I kz = ∞. Internal resistance is determined from the ratios . Since the voltage at an ideal voltage source is constant at any current ∆U = 0, then its internal resistance also has zero values.

R ext \u003d ΔU / ΔI \u003d 0;

With a positive voltage and current, the source sends its electrical energy to the circuit and operates in the generator mode. With the opposite current flow, the source receives electrical energy from the circuit and operates in the receiver mode.

In the case of an ideal current source, its value does not depend on the magnitude of the voltage at its terminals: i = const.

Since the current from an ideal current source is unchanged ∆I = 0, then it has an internal resistance equal to infinity.

R ext \u003d ΔU / ΔI \u003d ∞

With a positive voltage and current, the source sends energy into the circuit and operates in generator mode. In the opposite direction, it works in receiver mode.

Real source of electromotive force

With a real source of electromotive force, the voltage across the terminals decreases as the current increases. Such a CVC corresponds to an equation for determining the voltage at any current value.

U \u003d U xx - R ext × I,

Where , is calculated by the formula

R ext \u003d ΔU / Δ I≠ 0

It can also be calculated via U xx and I kz

R vn \u003d U xx / II kz

Self-induction. EMF self-induction

When a current source is connected to any closed circuit, the area bounded by this circuit begins to be pierced by external magnetic lines of force. Each field line, from the outside, crossing the conductor, inducing an EMF of self-induction in it.

Electrical circuit consists of a current source, electricity consumers, connecting wires and a key that serves to open and close the circuit and other elements (Fig. 1).

Drawings showing connection methods electrical appliances in a chain are called electrical diagrams. Devices on the diagrams are indicated by conventional signs.

As noted, in order to maintain an electric current in the circuit, it is necessary that at its ends (Fig. 2) there is a constant potential difference φ A- φ b. Let at the initial time φ A > φ B , then the positive charge transfer q from a point BUT exactly AT will lead to a decrease in the potential difference between them. To maintain a constant potential difference, it is necessary to transfer exactly the same charge from B in A. If in the direction BUT → AT charges move under the action of the forces of an electrostatic field, then in the direction AT → BUT the movement of charges occurs against the forces of the electrostatic field, i.e. under the action of forces of a non-electrostatic nature, the so-called third-party forces. This condition is met in a current source that supports the movement electric charges. In most current sources, only electrons move, in galvanic cells - ions of both signs.

Sources of electric current may be different in their design, but in any of them work is done to separate positively and negatively charged particles. The separation of charges occurs under the action outside forces. Third-party forces act only inside the current source and can be caused by chemical processes (batteries, galvanic cells), the action of light (photocells), changing magnetic fields (generators), etc.

Any current source is characterized by an electromotive force - EMF.

electromotive force ε current source is called a physical scalar quantity, equal to work external forces on the movement of a unit positive charge along a closed circuit

The SI unit of electromotive force is the volt (V).

EMF is an energy characteristic of a current source.

In the current source, in the course of work on the separation of charged particles, a transformation of mechanical, light, internal, etc. occurs. energy into electricity. Separated particles accumulate at the poles of the current source (the places to which consumers are connected using terminals or clamps). One pole of the current source is charged positively, the other negatively. An electrostatic field is created between the poles of the current source. If the poles of the current source are connected by a conductor, then in such an electrical circuit a electricity. In this case, the nature of the field changes, it ceases to be electrostatic.

Figure 3 schematically shows the negative terminal of the current source and the section of the end of the metal wire attached to it in the form of a spherical conductor. The dotted line shows some lines of the terminal field strength before the wire is inserted into it, and the arrows show the forces acting on the free electrons of the wire located at the points marked with numbers. Electrons at different points of the cross section of the wire under the action of the Coulomb forces of the terminal field acquire movement not only along the axis of the wire. For example, an electron located at a point 1 , is involved in the "current" movement. But near points 2, 3, 4, 5 electrons have the ability to accumulate on the surface of the wire. Moreover, the surface distribution of electrons along the length of the wire will not be uniform. Therefore, connecting a wire to a current source terminal will cause some electrons to move along the wire, and some electrons to accumulate on the surface. The uneven distribution of electrons on its surface ensures the non-equipotentiality of this surface, the presence of components of the electric field strength directed along the surface of the conductor. This is the field of redistributed electrons of the conductor itself and ensures the ordered movement of other electrons. If the distribution of electrons over the surface of the conductor does not change over time, then such a field is called stationary electric field. Thus, leading role in the creation of a stationary electric field, the charges located at the poles of the current source play. When the electrical circuit is closed, the interaction of these charges with the free charges of the conductor leads to the appearance of uncompensated surface charges on the entire surface of the conductor. It is these charges that create a stationary electric field inside the conductor along its entire length. This field inside the conductor is uniform, and the lines of tension are directed along the axis of the conductor (Fig. 4). The process of establishing an electric field along the conductor occurs at a speed c≈ 3 10 8 m/s.

Like an electrostatic field, it is potential. But there are significant differences between these fields:

1. electrostatic field - the field of fixed charges. The source of a stationary electric field are moving charges, and total number charges and the pattern of their distribution in a given space do not change over time;

2. An electrostatic field exists outside the conductor. The strength of the electrostatic field is always equal to 0 inside the volume of the conductor, and at each point of the outer surface of the conductor is directed perpendicular to this surface. A stationary electric field exists both outside and inside the conductor. The intensity of a stationary electric field is not equal to zero inside the volume of the conductor, and on the surface and inside the volume there are components of the intensity that are not perpendicular to the surface of the conductor;

3. the potentials of different points of the conductor through which the direct current passes are different (the surface and volume of the conductor are not equipotential). The potentials of all points on the surface of a conductor in an electrostatic field are the same (the surface and volume of the conductor are equipotential);

4. An electrostatic field is not accompanied by the appearance of a magnetic field, but a stationary electric field is accompanied by its appearance and is inextricably linked with it.

In the midst of school year many scientists require an emf formula for various calculations. Experiments related to also need information about the electromotive force. But for beginners, it is not so easy to understand what it is.

The formula for finding emf

Let's deal with the definition first. What does this abbreviation mean?

EMF or electromotive force is a parameter characterizing the work of any forces of a non-electric nature operating in circuits where the current strength, both direct and alternating, is the same along the entire length. In a coupled conductive circuit, the EMF is equated to the work of these forces in moving a single positive (positive) charge along the entire circuit.

The figure below shows the emf formula.

Ast - means the work of external forces in joules.

q is the transferred charge in coulombs.

Third party forces- these are the forces that carry out the separation of charges in the source and, as a result, form a potential difference at its poles.

For this force, the unit of measure is volt. It is denoted in the formulas by the letter « E".

Only at the moment of the absence of current in the battery, the electromotive si-a will be equal to the voltage at the poles.

EMF induction:

EMF of induction in a circuit havingNturns:

When driving:

Electromotive force induction in a circuit rotating in a magnetic field at a speedw:

Table of values

A simple explanation of the electromotive force

Suppose there is a water tower in our village. It is completely filled with water. Let's think that this is an ordinary battery. The tower is a battery!

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

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

As a result:

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

The battery is the same.

First of all, we connect a source of energy to the circuit. And we close it accordingly. For example, insert a battery into a flashlight and turn it on. Initially, note that the device is lit brightly. After a while, its brightness will noticeably decrease. That is, the electromotive force has decreased (leaked when compared with water in the tower).

If we take a water tower as an example, then the EMF is a pump that constantly pumps water into the tower. And it never ends there.

EMF of a galvanic cell - formula

The electromotive force of a battery can be calculated in two ways:

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

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

Where are different types of EMF used?

1. Piezoelectric is used when a material is stretched or compressed. With the help of it, quartz energy generators and various sensors are made.
2. Chemical is used in and batteries.
3. Induction appears at the moment the conductor crosses the magnetic field. Its properties are used in transformers, electric motors, generators.
4. Thermoelectric is formed at the moment of heating contacts of different types of metals. It has found its application in refrigeration units and thermocouples.
5. Photo electric is used to produce photovoltaic cells.