What force is called electromotive force. EMF, power

This publication discusses the basic terms, laws and methods for calculating the emf of magnetic induction. Using the materials presented below, you can independently determine the current strength in interconnected circuits and the change in voltage in standard transformers. This information can be useful in solving various electrical problems.

Magnetic flux

It is known that passing current through a conductor is accompanied by the formation electro magnetic field. The operation of speakers, locking devices, relay drives, and other devices is based on this principle. By changing the parameters of the power source, the necessary force efforts are obtained to move (hold) the combined parts that have ferromagnetic properties.

However, the opposite is also true. If a frame of conductive material is moved between the poles of a permanent magnet along the corresponding closed circuit, the movement of charged particles will begin. By connecting appropriate devices, changes in current (voltage) can be recorded. In the course of an elementary experiment, you can find out the increase in the effect in the following situations:

• perpendicular arrangement of the conductor/power lines;
• acceleration of movements.

The picture above shows how to determine the direction of current in a conductor using a simple rule.

What is induced emf

The movement of charges noted above creates a potential difference if the circuit is open. The presented formula shows exactly how the EMF will depend on the main parameters:

• vector expression of magnetic flux (B);
• length (l) and speed of movement (v) of the control conductor;
• angle (α) between the motion/induction vectors.

A similar result can be obtained if the system is composed of a stationary conducting circuit that is affected by a moving magnetic field. By closing the circuit, they create suitable conditions for the movement of charges. If you use many conductors (coil) or move faster, the current will increase. The presented principles are successfully used to convert mechanical forces into electricity.

Designation and units of measurement

EMF in formulas is denoted by vector E. This refers to the tension created by external forces. Accordingly, this value can be estimated from the potential difference. According to current international standards (SI), the unit of measurement is one volt. Large and small values ​​are indicated using multiple prefixes: “micro”, “kilo”, etc.

Faraday and Lenz's laws

If electromagnetic induction is considered, the formulas of these scientists help clarify mutual influence significant system parameters. Faraday's definition makes it possible to clarify the dependence of the emf (E– average value) from changes in magnetic flux (ΔF) and time (Δt):

E = – ΔF/ Δt.

Interim conclusions:

• the current increases if per unit time the conductor crosses a larger number of magnetic force lines;
• “-” in the formula helps to take into account the mutual relationships between the polarity E, the speed of movement of the frame, and the direction of the induction vector.

Lenz substantiated the dependence of EMF on any changes in magnetic flux. When the coil circuit is closed, conditions are created for the movement of charges. In this embodiment, the design is converted into a typical solenoid. A corresponding electromagnetic field is formed next to it.

This scientist substantiated important feature induced emf. The field generated by the coil prevents changes in the external flow.

Movement of a wire in a magnetic field

As shown in the first formula (E = B * l * v * sinα), the amplitude electromotive force largely depends on the parameters of the conductor. More precisely, the influence is exerted by the number of power lines per unit length of the circuit’s working area. A similar conclusion can be drawn taking into account changes in movement speed. One should not forget about the relative position of the marked vector quantities (sinα).

Important! The movement of a conductor along the lines of force does not provoke the induction of an electromotive force.

Rotating reel

It is difficult to ensure optimal positioning of functional components while simultaneously moving them when using the straight wire shown in the example. However, by bending the frame, you can get a simple electricity generator. The maximum effect is provided by increasing the number of conductors per unit of working volume. The design corresponding to the noted parameters is a coil, a typical element of a modern alternating current generator.

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

F = B * S * cosα,

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

Explanation. With uniform rotation of the rotor, a corresponding cyclic sinusoidal change in the magnetic flux occurs. The amplitude of the output signal changes in a similar way. It is clear from the figure that the size of the gap between the main functional components of the structure has a certain significance.

Self-induced emf

If alternating current is passed through the coil, an electromagnetic field with similar (uniformly varying) power characteristics will be formed nearby. It creates an alternating sinusoidal magnetic flux, which, in turn, provokes the movement of charges and the formation of electromotive force. This process is called self-induction.

Taking into account the basic principles considered, it is not difficult to determine that F = L * l. The L value (in henry) determines the inductive characteristics of the coil. This parameter depends on the number of turns per unit length (l) and the cross-sectional area of ​​the conductor.

Mutual induction

If you assemble a module from two coils, under certain conditions you can observe the phenomenon of mutual induction. A basic measurement will show that as the distance between elements increases, the magnetic flux decreases. The opposite phenomenon is observed as the gap decreases.

To find suitable components when creating electrical circuits, you need to study thematic calculations:

• you can take as an example coils with different numbers of turns (n1 and n2);
• mutual induction (M2) when current passes through the first circuitI1 will be calculated as follows:

M2 = (n2 * F)/ I1

• after transforming this expression, determine the value of the magnetic flux:

F = (M2/ n2) *I1

• To calculate the emf of electromagnetic induction, the formula is suitable from the description of the basic principles:

E2 = – n2 * ΔF/ Δt = M 2 * ΔI1/ Δt

If necessary, you can use a similar algorithm to find the ratio for the first coil:

E1 = – n1 * ΔF/ Δt = M 1 * ΔI2/ Δt.

It should be noted that in this case it is the force (I2) in the second operating circuit that matters.

The joint influence (mutual induction - M) is calculated using the formula:

M = K * √(L1 * l2).

A special coefficient (K) takes into account the actual coupling force between the coils.

Where are different types of EMF used?

The movement of a conductor in a magnetic field is used to generate electricity. The rotation of the rotor is ensured by the difference in liquid levels (hydroelectric power station), wind energy, tides, and fuel engines.

Different numbers of turns (mutual inductance) are used to change the voltage in the secondary winding of the transformer as desired. In such designs, mutual coupling is increased using a ferromagnetic core. Magnetic induction is used to generate a powerful repulsive force when creating ultra-modern transport highways. The created levitation makes it possible to eliminate the force of friction and significantly increase the speed of the train.

Video

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 that show methods of connecting electrical devices in a circuit are called electrical diagrams. Devices on the diagrams are indicated by symbols.

As noted, to maintain the chain electric current it is necessary that at its ends (Fig. 2) there is a constant potential difference φ A- φ B. Let at the initial moment of time φ A> φ B , then carry positive charge q from point A exactly IN 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 V A. If in the direction A → IN charges move under the influence of electrostatic field forces, then in the direction IN → A the movement of charges occurs against the forces of the electrostatic field, i.e. under the influence of forces of a non-electrostatic nature, the so-called third-party forces. This condition is satisfied in the current source that supports the movement electric charges. In most current sources, only electrons move; in galvanic cells, ions of both signs move.

Sources of electric current may vary in design, but in any of them work is done to separate positively and negatively charged particles. The separation of charges occurs under the influence 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 electromotive force - EMF.

Electromotive force ε current source is a physical scalar quantity equal to the work done by external forces to move a single 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 process of separating charged particles, a transformation of mechanical, light, internal, etc. occurs. energy into electrical energy. The separated particles accumulate at the poles of the current source (the places to which consumers are connected using terminals or clips). 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 a current source are connected by a conductor, then an electric current arises in such an electrical circuit. 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 cross-section of the end of the metal wire connected to it in the form of a spherical conductor. The dotted line shows some lines of the field strength of the terminal 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. Under the influence of the Coulomb forces of the terminal field, electrons at various points in the cross section of the wire acquire motion not only along the axis of the wire. For example, an electron located at a point 1 , turns out to be involved in the “current” movement. But near the 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 the terminal of a current source will cause some electrons to move along the wire, and some electrons will accumulate on the surface. The uneven distribution of electrons on its surface ensures the non-equipotentiality of this surface, the presence of voltage components electric field, directed along the surface of the conductor. This field of redistributed electrons of the conductor itself ensures the ordered movement of other electrons. If the distribution of electrons over the surface of a conductor does not change over time, then such a field is called stationary electric field. Thus, the main role in creating a stationary electric field is played by the charges located at the poles of the current source. When an electrical circuit is closed, the interaction of precisely 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 tension lines are directed along the axis of the conductor (Fig. 4). The process of establishing an electric field along a conductor occurs at a speed c≈ 3·10 8 m/s.

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

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

2. the electrostatic field exists outside the conductor. The electrostatic field strength is always equal to 0 inside the volume of the conductor, and at every point on the outer surface of the conductor it is directed perpendicular to this surface. A stationary electric field exists both outside and inside a conductor. The strength of the stationary electric field is not 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. potentials different points conductor through which it passes D.C., different (the surface and volume of the conductor are not equipotential). The potentials of all points on the surface of a conductor located 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 school year Many scientists need the emf formula for various calculations. Experiments involving , also require information about the electromotive force. But for beginners it is not so easy to understand what it is.

Formula for finding emf

First of all, let's look at the definition. What does this abbreviation mean?

EMF or electromotive force is a parameter characterizing the work of any forces of a non-electrical nature operating in circuits where the current strength, both direct and alternating, is the same along the entire length. In an interconnected conductive circuit, the EMF is equal to the work of these forces to move a single plus (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.

Outside forces- these are the forces that separate charges in the source and ultimately form a potential difference at its poles.

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

Only when there is no current in the battery, the electromotive force will be equal to the voltage at the poles.

Induction emf:

Induction emf 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 electromotive force

Let's assume that our village has a water tower. It is completely filled with water. Let's assume that this is an ordinary battery. The tower is a battery!

All the water will put strong 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 less pressure of the stream. Having opened the tap, we will notice that every minute the range of the jet will decrease.

As a result:

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

Everything is the same with the battery.

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

If we take a water tower as an example, then the EMF is a pump constantly pumping 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 calculations 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 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 stretching or compressing a material. It is used to make quartz energy generators and various sensors.
2. The chemical is used in batteries.
3. Induction appears when a conductor crosses a magnetic field. Its properties are used in transformers, electric motors, and generators.
4. Thermoelectric is formed when contacts of different types of metals are heated. It has found its application in refrigeration units and thermocouples.
5. Photoelectric is used to produce photocells.

In the material we will understand the concept of induced emf in situations of its occurrence. We will also consider inductance as a key parameter for the occurrence of magnetic flux when an electric field appears in a conductor.

Electromagnetic induction is the generation of electric current by magnetic fields that change over time. Thanks to the discoveries of Faraday and Lenz, patterns were formulated into laws, which introduced symmetry into the understanding of electromagnetic flows. Maxwell's theory brought together knowledge about electric current and magnetic fluxes. Thanks to Hertz's discovery, humanity learned about telecommunications.

An electromagnetic field appears around a conductor carrying electric current, but in parallel, the opposite phenomenon also occurs - electromagnetic induction. Let's consider magnetic flux using an example: if a frame made of a conductor is placed in an electric field with induction and moved from top to bottom along magnetic lines of force or left and right perpendicular to them, then the magnetic flux passing through the frame will be a constant value.

When the frame rotates around its axis, then after some time the magnetic flux will change by a certain amount. As a result, an induced emf appears in the frame and an electric current appears, which is called induction.

induced emf

Let us understand in detail what the concept of induced emf is. When a conductor is placed in a magnetic field and moves with the intersection of field lines, an electromotive force called induced emf appears in the conductor. It also occurs if the conductor remains stationary, and the magnetic field moves and intersects the conductor with lines of force.

When the conductor where the EMF occurs is closed to the external circuit, due to the presence of this EMF, an induced current begins to flow through the circuit. Electromagnetic induction involves the phenomenon of inducing an EMF in a conductor at the moment it is crossed by magnetic field lines.

Electromagnetic induction is reverse process transformation of mechanical energy into electric current. This concept and its laws are widely used in electrical engineering; most electric machines are based on this phenomenon.

Faraday and Lenz's laws

Faraday's and Lenz's laws reflect the patterns of occurrence of electromagnetic induction.

Faraday discovered that magnetic effects arise as a result of changes in magnetic flux over time. At the moment the conductor crosses the variable magnetic current, an electromotive force arises in it, which leads to the generation of electric current. Both a permanent magnet and an electromagnet can generate current.

The scientist determined that the intensity of the current increases with a rapid change in the number of lines of force that intersect the circuit. That is, the EMF of electromagnetic induction is directly dependent on the speed of the magnetic flux.

According to Faraday's law, the induced emf formulas are defined as follows:

The minus sign indicates the relationship between the polarity of the induced emf, the direction of flow and the changing speed.

According to Lenz's law, electromotive force can be characterized depending on its direction. Any change in the magnetic flux in the coil leads to the appearance of an induced emf, and with a rapid change, an increasing emf is observed.

If a coil, where there is an induced emf, has a short circuit to an external circuit, then an induced current flows through it, as a result of which a magnetic field appears around the conductor and the coil acquires the properties of a solenoid. As a result, its own magnetic field is formed around the coil.

E.H. Lenz established a pattern according to which the direction of the induced current in the coil and the induced emf are determined. The law states that the induced emf in a coil, when the magnetic flux changes, forms a current in the coil in the direction in which a given magnetic flux of the coil makes it possible to avoid changes in extraneous magnetic flux.

Lenz's law applies to all situations of induction of electric current in conductors, regardless of their configuration and method of changing the external magnetic field.

Movement of a wire in a magnetic field

The value of the induced emf is determined depending on the length of the conductor crossed by the field lines. With a larger number of power lines, the magnitude of the induced emf increases. As the magnetic field and induction increase, a greater value of EMF occurs in the conductor. Thus, the value of the induced emf in a 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 = Blv, where E is the induced emf; B is the value of magnetic induction; I is the length of the conductor; v is the speed of its movement.

Note that in a conductor that moves in a magnetic field, induced emf appears only when it crosses the magnetic field lines. If the conductor moves along the lines of force, then no emf is induced. For this reason, the formula applies only in cases where the movement of the conductor is directed perpendicular to the lines of force.

The direction of the induced emf and electric current in the conductor is determined by the direction of movement of the conductor itself. To identify the direction, a right-hand rule has been developed. If you hold the palm of your right hand in such a way that the field lines enter in its direction, and thumb indicates the direction of movement of the conductor, then the other four fingers indicate the direction of the induced emf and the direction of the electric current in the conductor.

Rotating reel

The operation of an electric current generator is based on the rotation of a coil in a magnetic flux, where there is a certain number of turns. EMF is induced in an electric circuit whenever a magnetic flux crosses it, based on the magnetic flux formula Ф = B x S x cos α (magnetic induction multiplied by the surface area through which the magnetic flux passes and the cosine of the angle formed by the direction vector and perpendicular to the plane lines).

According to the formula, F is affected by changes in situations:

• when the magnetic flux changes, the direction vector changes;
• the area enclosed in the contour changes;
• the angle changes.

It is allowed to induce an EMF with a stationary magnet or a constant current, but simply by rotating the coil around its axis within the magnetic field. In this case, the magnetic flux changes when the angle value changes. During rotation, the coil crosses the magnetic flux lines, resulting in an emf. With uniform rotation, a periodic change in the magnetic flux occurs. Also, the number of field lines that intersect every second becomes equal to the values ​​at equal time intervals.

In practice, in alternating current generators, the coil remains stationary, and the electromagnet rotates around it.

Self-induced emf

When an alternating electric current passes through the coil, an alternating magnetic field is generated, which is characterized by a changing magnetic flux that induces an emf. This phenomenon is called self-induction.

Due to the fact that the magnetic flux is proportional to the intensity of the electric current, then the formula for the self-induction emf looks like this:

Ф = L x I, where L is inductance, which is measured in H. Its value is determined by the number of turns per unit length and the size of their cross section.

Mutual induction

When two coils are placed side by side, a mutual inductive emf is observed in them, which is determined by the configuration of the two circuits and their mutual orientation. As the separation of the circuits increases, the value of mutual inductance decreases, since there is a decrease in the total magnetic flux for the two coils.

Let us consider in detail the process of mutual induction. There are two coils, a current I1 flows along the wire of one with N1 turns, which creates a magnetic flux and goes through the second coil with N2 number of turns.

The mutual inductance value of the second coil in relation to the first:

M21 = (N2 x F21)/I1.

Magnetic flux value:

F21 = (M21/N2) x I1.

The induced emf is calculated by the formula:

E2 = - N2 x dФ21/dt = - M21x dI1/dt.

In the first coil the value of the induced emf is:

E1 = - M12 x dI2/dt.

It is important to note that the electromotive force generated by mutual induction in one of the coils is in any case directly proportional to the change in electric current in the other coil.

Then the mutual inductance is considered equal:

M12 = M21 = M.

As a consequence, E1 = - M x dI2/dt and E2 = M x dI1/dt. M = K √ (L1 x L2), where K is the coupling coefficient between two inductivity values.

Mutual induction is widely used in transformers, which make it possible to change the values ​​of alternating electric current. The device consists of a pair of coils that are wound on a common core. The current in the first coil forms a changing magnetic flux in the magnetic circuit and a current in the second coil. With fewer 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 decreases.

In addition to the generation and transformation of electrical energy, the phenomenon of magnetic induction is used in other devices. For example, in magnetic levitation trains moving without direct contact with the current in the rails, but a couple of centimeters higher due to electromagnetic repulsion.

What's happened EMF(electromotive force) in physics? Not everyone understands electric current. Like the cosmic distance, only right under your nose. In general, even scientists do not fully understand it. Suffice it to recall his famous experiments, centuries ahead of their time and even today remaining in an aura 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– electromotive force. Denoted by the letter E or the small Greek letter epsilon.

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

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

Voltage(between points A and B) – physical quantity, equal to work effective electric field, which occurs 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 example-analogy. Let's imagine that we have a water tower completely filled with water. Let's compare this tower with a battery.

Water exerts maximum pressure on the bottom of the tower when the tower is completely filled. 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, first under strong pressure, and then more and more slowly until the pressure weakens completely. Here, tension is the pressure that water exerts on the bottom. Let us take the very bottom of the tower as the zero voltage level.

It's the same with the battery. First, we connect our current source (battery) to the circuit, closing it. Let it be a watch or a flashlight. As long as 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 dry 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 represented as a pump that ensures the influx of new water into the tower.

Nature of EMF

The reason for the occurrence of EMF in different current sources is different. According to the nature of occurrence they distinguish following types:

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

Dear friends, today we looked at the topic “EMF for dummies”. As we can see, EMF – non-electrical force, which maintains the flow of electric current in the circuit. If you want to find out how problems with EMF are solved, we advise you to contact carefully selected and proven specialists who will quickly and clearly explain the process of solving any thematic problem. And by tradition, at the end we invite you to watch a training video. Enjoy watching and good luck with your studies!