Electromotive force. Internal resistance of the current source. What is induced emf and when does it occur? What is emf in physics

In this lesson we will take a closer look at the mechanism for ensuring 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 internal energy conductor rises.

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). On the contrary, the water naturally flows down under the influence of gravity, but for the continuous operation of the apartment’s water supply it is necessary continuous operation pump

Definition. Electromotive force is the ratio of the work of external forces to move a charge to the magnitude of this charge. Designation - :

Unit:

Insert. EMF of open and closed circuit

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.

References

Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemosyne, 2012.
Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Ilexa, 2005.
Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.

ens.tpu.ru ().
physbook.ru ().
electrodynamics.narod.ru ().

Homework

What are external forces, what is their nature?
How is the voltage at the open poles of a current source related to its EMF?
How is energy converted and transferred in a closed circuit?
*The emf of a flashlight battery is 4.5 V. Will a light bulb designed for 4.5 V burn at full intensity from this battery? Why?

Content:

When the concept of “electron” was born, people immediately associated it with a specific job. Electron is the Greek word for amber. The fact that the Greeks, in order to find this useless, in general, magical stone, had to travel quite far to the north - such efforts, in general, do not count here. But it was worth doing some work - rubbing the pebble with your hands on a dry woolen cloth - and it acquired new properties. Everyone knew this. Rub it just like that, for the sake of purely disinterested interest, to observe how small debris now begins to be attracted to the “electron”: specks of dust, hairs, threads, feathers. Later, when a whole class of phenomena appeared, later united under the concept of “electricity,” the work that must necessarily be expended did not give people peace. Since you need to spend money to get a trick with specks of dust, it means it would be good to somehow save this work, accumulate it, and then get it back.

Thus, from increasingly complex tricks with different materials and philosophical reasoning, we learned this magical power collect in a jar. And then make it so that it is gradually released from the jar, causing actions that can already be felt, and very soon measured. And they measured it so ingeniously, having only a couple of silk balls or sticks and spring torsion balances, that even now we quite seriously use the same formulas for calculating electrical circuits that have now permeated the entire planet, infinitely complex compared to those first devices .

And the name of this mighty genie sitting in a jar still contains the delight of long-time discoverers: “Electromotive force.” But this force is not electrical at all. On the contrary, an extraneous terrible force forcing electric charges move “against your will,” that is, overcoming mutual repulsion, and gather somewhere on one side. This results in a potential difference. It can be used by releasing charges in a different way. Where they are not “guarded” by this terrible EMF. And thus force you to do some work.

Operating principle

EMF is a force of a very different nature, although it is measured in volts:

Chemical. Occurs from the processes of chemical replacement of ions of some metals with ions of others (more active). As a result, extra electrons are formed, trying to “escape” at the edge of the nearest conductor. This process can be reversible or irreversible. Reversible - in batteries. They can be charged by returning charged ions back into the solution, causing it to become more acidic, for example (in acid batteries). The acidity of the electrolyte is the cause of the battery's emf; it works continuously until the solution becomes completely chemically neutral.

Magnetodynamic. Occurs when a conductor, oriented in some way in space, is exposed to a changing magnetic field. Either from a magnet moving relative to a conductor, or from the movement of a conductor relative to a magnetic field. In this case, electrons also tend to move in the conductor, which allows them to be captured and placed on the output contacts of the device, creating a potential difference.

Electromagnetic. An alternating magnetic field is created in a magnetic material by an alternating electrical voltage in the primary winding. In the secondary winding, electron movement occurs, and hence a voltage proportional to the voltage in the primary winding. Transformers can be designated by the EMF symbol in equivalent equivalent circuits.

Photovoltaic. Light hitting some conductive materials can knock out electrons, that is, make them free. An excess of these particles is created, causing the excess ones to be pushed towards one of the electrodes (anode). Voltage arises, which can generate electric current. Such devices are called photocells. Initially, vacuum photocells were invented, in which the electrodes were installed in a flask with a vacuum. In this case, the electrons were pushed outside the metal plate (cathode) and were captured by another electrode (anode). Such photocells have found application in light sensors. With the invention of more practical semiconductor photocells, it became possible to create powerful batteries from them in order to generate a significant voltage by summing the electromotive force of each of them.

Thermoelectric. If two different metals or semiconductors are soldered at one point, and then heat is delivered to this point, for example, a candle, then at the opposite ends of the pair of metals (thermocouples) a difference in the densities of the electron gas appears. This difference can accumulate if thermocouples are connected in series, similar to the connection of galvanic cells in a battery or individual photovoltaic cells in solar battery. ThermoEMF is used in very accurate temperature sensors. Several effects are associated with this phenomenon (Peltier, Thomson, Seebeck), which are being successfully studied. It is a fact that heat can be directly converted into electromotive force, that is, voltage.

Electrostatic. Such sources of EMF were invented almost simultaneously with galvanic elements or even earlier (if we consider rubbing amber with silk as a normal production of EMF). They are also called electrophoric machines, or, after the name of the inventor, Wimshurst generators. Although Wimshurst created a clear technical solution, allowing the removed potential to be accumulated in a Leyden jar - the first capacitor (and of good capacity). The first electrophore machine can be considered a huge ball of sulfur mounted on an axis - the apparatus of the Magdeburg burgomaster Otto von Guericke in the middle of the 17th century. The principle of operation is rubbing materials that are easily electrified by friction. True, von Guericke’s progress can be called, as the saying goes, driven by laziness, when there is no desire to rub amber or anything else by hand. Although, of course, this inquisitive politician had plenty of imagination and activity. Let us at least recall his well-known experiment with two strings of donkeys (or mules) tearing a ball without air by chains into two hemispheres.

Electrification, as originally assumed, occurs precisely from “friction,” that is, by rubbing amber with a rag, we “tear” electrons from its surface. However, research has shown that it is not so simple. It turns out that there are always charge irregularities on the surface of dielectrics, and ions from the air are attracted to these irregularities. An air-ionic coat is formed, which we damage by rubbing the surface.

Thermionic. When metals are heated, electrons are stripped from their surface. In a vacuum, they reach another electrode and induce a negative potential there. This is a very promising direction right now. The figure shows a scheme for protecting a hypersonic aircraft from overheating of parts of the body by a counter flow of air, and thermionic electrons emitted by the cathode (which is cooled at the same time - the simultaneous action of the Peltier and/or Thomson effects) reach the anode, inducing a charge on it. The charge, or rather the voltage, which is equal to the resulting EMF, can be used in the consumption circuit inside the device.

1 - cathode, 2 - anode, 3, 4 - cathode and anode taps, 5 - consumer

Piezoelectric. Many crystalline dielectrics, when they experience mechanical pressure on themselves in any direction, react to it by inducing a potential difference between their surfaces. This difference depends on the applied pressure, so it is already used in pressure sensors. Piezoelectric lighters for gas stoves do not require any other source of energy - just pressing a button with your finger. There are known attempts to create a piezoelectric ignition system in cars based on piezoceramics, which receives pressure from a system of cams connected to the main shaft of the engine. “Good” piezoelectrics - in which the proportionality of EMF to pressure is highly accurate - are very hard (for example, quartz), and are almost not deformed under mechanical pressure.

However, prolonged exposure to pressure causes their destruction. In nature, thick layers of rocks are also piezoelectric; the pressures of the earth’s strata induce enormous charges on their surfaces, which gives rise to titanic storms and thunderstorms in the depths of the earth. However, not everything is so scary. Elastic piezoelectrics have already been developed, and even the production of products based on them (and on the basis of nanotechnology) for sale has already begun.

It is clear that the unit of measurement of EMF is the unit of electrical voltage. Since the most diverse mechanisms that create the electromotive force of a current source all convert their types of energy into the movement and accumulation of electrons, and this ultimately leads to the appearance of such voltage.

Current arising from EMF

The electromotive force of a current source is a driving force because the electrons from it begin to move if the electrical circuit is closed. They are forced to do this by the EMF, using its non-electric “half” of nature, which, after all, does not depend on the half associated with electrons. Since it is believed that the current in the circuit flows from plus to minus (this determination of direction was made before everyone knew that the electron is a negative particle), then inside the device with EMF the current makes a final movement - from minus to plus. And they always draw at the EMF sign, where the arrow is directed – +. Only in both cases - both inside the EMF of the current source and outside, that is, in the consuming circuit - we are dealing with electric current with all its mandatory properties. In conductors, current encounters resistance. And here, in the first half of the cycle, we have the load resistance, in the second, internal, we have the source resistance or internal resistance.

The internal process does not work instantly (although very quickly), but with a certain intensity. It does work to deliver charges from minus to plus, and this also encounters resistance...

Resistance is of two kinds.

Internal resistance works against the forces separating charges, it has a nature “close” to these separating forces. At least it works with them in a single mechanism. For example, an acid that takes oxygen from lead dioxide and replaces it with SO 4 - ions definitely experiences some chemical resistance. And this is precisely what manifests itself as the work of the internal resistance of the battery.
When the outer (output) half of the circuit is not closed, the appearance of more and more electrons at one of the poles (and their decrease from the other pole) causes an increase in the electrostatic field strength at the battery poles and an increase in repulsion between electrons. This allows the system to “not go crazy” and stop at a certain state of saturation. No more electrons are accepted from the battery to the outside. And this outwardly looks like the presence of a constant electrical voltage between the battery terminals, which is called U xx, the open circuit voltage. And it is numerically equal to EMF - electromotive force. Therefore, the unit of measurement for EMF is the volt (in the SI system).

But if you just connect a load of conductors with non-zero resistance to the battery, then a current will immediately flow, the strength of which is determined by Ohm’s law.

Measure internal resistance EMF source It would seem possible. It is worth connecting an ammeter to the circuit and shunting (shorting) the external resistance. However, the internal resistance is so low that the battery will begin to discharge catastrophically, generating enormous amounts of heat, both on the external short-circuited conductors and in the internal space of the source.

However, you can do it differently:

Measure E (remember, open circuit voltage, unit of measurement - volt).
Connect a resistor as a load and measure the voltage drop across it. Calculate the current I 1.
You can calculate the value of the internal resistance of the EMF source using the expression for r

Typically, a battery's ability to produce electricity is measured by its energy "capacity" in ampere hours. But it would be interesting to see what maximum current it can produce. Despite the fact that, perhaps, the electromotive force of the current source will cause it to explode. Since the idea is to arrange on it short circuit did not seem very tempting, this value can be calculated purely theoretically. The emf is equal to U xx. You just need to draw a graph of the voltage drop across the resistor versus the current (and therefore the load resistance) to the point at which the load resistance is zero. This is the point Ishort circuit, intersection of the red line with the coordinate line I , in which the voltage U has become zero, and the entire voltage E of the source will drop across the internal resistance.

Often seemingly simple basic concepts cannot always be understood without the use of examples and analogies. What electromotive force is and how it works can only be imagined by considering its many manifestations. But it’s worth considering the definition of EMF, as it is given by reputable sources using smart academic words - and start all over again: the electromotive force of a current source. Or just write it on the wall in gold letters:

To maintain electric current in a conductor long time, it is necessary that the charges delivered by the current are constantly removed from the end of the conductor, which has a lower potential (we take into account that current carriers are assumed to be positive charges), while charges are constantly supplied to the end with a higher potential. That is, it is necessary to ensure the circulation of charges. In this cycle, charges must move along a closed path. The movement of current carriers is realized using forces of non-electrostatic origin. Such forces are called third parties. It turns out that to maintain the current, external forces are needed that act along the entire length of the circuit or in individual sections of the circuit.

Definition and formula of EMF

Definition

A scalar physical quantity that is equal to the work of external forces to move a unit positive charge is called electromotive force (EMF), acting in a circuit or section of a circuit. EMF is indicated. Mathematically, we write the definition of EMF as:

where A is the work done by external forces, q is the charge on which the work is performed.

The electromotive force of the source is numerically equal to the potential difference at the ends of the element if it is open, which makes it possible to measure the EMF by voltage.

The EMF that acts in a closed circuit can be defined as the circulation of the tension vector of external forces:

where is the field strength of external forces. If the field strength of external forces is not zero only in part of the circuit, for example, on segment 1-2, then integration in expression (2) can be carried out only over this section. Accordingly, the EMF acting on circuit section 1-2 is defined as:

Formula (2) gives the most general definition of EMF, which can be used for any cases.

Ohm's law for an arbitrary section of a circuit

The section of the chain on which external forces act is called heterogeneous. It satisfies the following equality:

where U 12 =IR 21 – voltage drop (or voltage) in circuit section 1-2 (I-current); – potential difference between the ends of the section; – electromotive force contained in a section of the circuit. equal to the algebraic sum of the emf of all sources that are located in a given area.

It should be taken into account that EMF can be positive and negative. The EMF is called positive if it increases the potential in the direction of the current (the current flows from the minus to the plus of the source).

Units of measurement

The dimension of the EMF coincides with the dimension of the potential. The basic unit of measurement of EMF in the SI system is: =V

Examples of problem solving

Example

Exercise. The electromotive force of the element is 10 V. It creates a current in the circuit equal to 0.4 A. What is the work done by external forces in 1 minute?

Solution. As a basis for solving the problem, we use the formula for calculating the EMF:

The charge that passes through the circuit in question in 1 minute. can be found as:

We express the work from (1.1), use (1.2) to calculate the charge, we get:

Let's convert the time given in the conditions of the problem into seconds (min=60 s), and carry out the calculations:

Answer. A=240 J

Example

Exercise. A metal disk with a radius a rotates with an angular velocity and is connected to an electrical circuit using sliding contacts that touch the axis of the disk and its circumference (Fig. 1). What will be the emf that appears between the axis of the disk and its outer edge?

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. Enough to remember Nikola Tesla with 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 AC and DC.

EMF, as well as 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 transferring a unit test charge 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. Based on 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 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 know how problems with EMF are solved, we recommend that you contact to our authors– carefully selected and verified 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!

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.