Sections: Physics

Lesson goals.

Tutorial:

the formation of students' knowledge about the conditions for the occurrence and existence of electric current.

Developing:

development logical thinking attention, skills to use the acquired knowledge in practice.

Educational:

creating conditions for the manifestation of independence, attentiveness and self-esteem.

Equipment.

1. Galvanic cells, battery, generator, compass.
2. Cards (attached).
3. Demonstration material (portraits of outstanding physicists Ampère, Volta; posters "Electricity", "Electric charges").

Demos:

1. Action electric current in the conductor to the magnetic needle.
2. Current sources: galvanic cells, battery, generator.

Lesson Plan

1. Organizational moment.

2. Introductory speech of the teacher.

3. Preparation for the perception of new material.

4. Learning new material.

a) current sources;

b) the action of electric current;

c) physical operetta “Queen of Electricity”;

d) filling in the table “Electric current”;

e) safety measures when working with electrical appliances.

5. Summing up the lesson.

6. Reflection.

7. Homework:

a) Based on the knowledge gained in the lessons of life safety, special technologies, prepare and write down in a notebook a memo “Safety measures when working with electrical appliances”

b) Individual task: Prepare a report on the use of a power source in everyday life and technology.

Lesson summary

1. Organizational moment

Mark the presence of students, name the topic of the lesson, the goal.

2. Introductory speech of the teacher

With the words electricity, electric current, we are familiar from early childhood. Electric current is used in our homes, in transport, in production, in the lighting network.

But what is an electric current, what is its nature, is not easy to understand.

The word electricity comes from the word electron, which is translated from Greek as amber. Amber is the fossilized resin of ancient coniferous trees. The word current means the flow or movement of something.

3. Preparation for the perception of new material

Questions of the introductory conversation.

What are the two types of charges that exist in nature? How do they interact?

Answer: There are two types of charges in nature: positive and negative.

Positive charge carriers are protons, negative charge carriers are electrons. Like-charged particles repel each other, opposite-charged particles attract.

Is there an electric field around an electron?

Answer: Yes, there is an electric field around an electron.

What are free electrons?

Answer: These are the electrons most distant from the nucleus, they can freely move between atoms.

4. Learning new material

a) Current sources.

There are special devices on the table. What are their names? What are they needed for?

Answer: These are galvanic cells, a battery, a generator - the common name for current sources. They are necessary to supply electrical energy, create an electric field in the conductor.

We know that there are charged particles, electrons and protons, we know that there are devices called current sources.

b) Actions of electric current.

Tell me, how can we understand that there is an electric current in the circuit, by what actions?

Answer: Electric current has different types of action:

• Thermal - the conductor through which the electric current flows is heated (electric stove, iron, incandescent lamp, soldering iron).
• The chemical effect of the current can be observed when passing an electric current through a solution of copper sulphate - the release of copper from a solution of vitriol, chromium plating, nickel plating.
• Physiological - contraction of the muscles of humans and animals through which an electric current has passed.
• Magnetic - when an electric current passes through a conductor, if a magnetic needle is placed nearby, it can deviate. This action is the main one. Demonstration of experience: battery, incandescent lamp, connecting wires, compass.

c) Physical operetta “Queen Electricity”. (Appendix No. 1)

Now senior girls will present to your attention the operetta "Queen of Electricity". Do not forget the Russian folk proverb “The fairy tale is a lie, but there is a hint in it, a lesson for good fellows.” That is, you not only listen and watch, but also take certain information from it. Your task is to write down as many physical terms that occur in the view.

d) Filling in the table “Electric current”. (Appendix No. 2)

Tell me, what one concept unites all the terms that you wrote down?

Answer: electric current.

Let's start filling out the table "Electric current".

Filling in the table, let's summarize the knowledge gained in the lesson and get new information.

In the process of filling out the table, we conclude what conditions are necessary to create an electric current.

• The first condition is the presence of free charged particles.
• The second condition is the presence of an electric field inside the conductor.

e) Safety measures when working with electrical appliances.

Where on industrial practice, you are faced with the application of electric current? Student responses.

Answer: When working with electrical appliances.

Forbidden.

• Walk on the ground, holding electrical appliances plugged into the network. It is especially dangerous to walk barefoot on wet soil.
• Enter electrical and other electrical rooms.
• Take on broken, bare, hanging and lying on the ground wires.
• Drive nails into the wall in a place where hidden wiring can be located. It is deadly dangerous at this moment to ground on central heating batteries, water supply.
• Drilling walls in places of possible electrical wiring.
• Paint, whitewash, wash walls with external or hidden live wiring.
• Work with switched on electrical appliances near batteries or water pipes.
• Work with electrical appliances, change light bulbs, standing on the bathroom.
• Work with faulty electrical appliances.
• Repair broken electrical appliances.

5. Summing up the lesson

Following the laws of physics, time moves inexorably forward, and our lesson has come to its logical conclusion.

Let's summarize our lesson.

What do you think electric current is?

Answer: Electric current is the directed movement of charged particles.

What conditions are necessary to create an electric current?

Answer: The first condition is the presence of free charged particles.

The second condition is the presence of an electric field inside the conductor.

6. Reflection

7. Homework

a) Based on the knowledge gained in the lessons of life safety, special technologies, prepare and write down in a notebook a memo “Safety measures when working with electrical appliances”.

b) Individual task: Prepare a report on the use of a power source in everyday life and technology. (

And again, good day to you, dear. Without further ado, let's start our conversation today. It would seem that we have long figured out the causes of the current in the conductor. We placed a conductor in a field - electrons ran, a current arose. What else does. But it turns out that for this current to exist in the conductor constantly, it is necessary to observe certain conditions. For a clearer understanding of the physics of the process of the flow of electric current in a conductor, consider an example.

Suppose we have some conductor that we will place in an electric field as shown in figure 4.1.

Figure 4.1 - Conductor in an electric field

Let's conventionally denote the magnitude of the tension at the ends of the conductor as E 1 and E 2, and E 1 >E 2. As we found out earlier, free electrons in the conductor will begin to move towards a greater field strength, that is, to point A. However, over time, the potential formed by the accumulation of electrons at point A will become such that its own electromagnetic field E 0 created by it will be equal in absolute value to the external field, and the directions of the fields will be opposite, since the potential of point B is more positive (lack of electrons caused by the action of an external field).

Since the resulting action of two identical opposite forces is equal to zero: |E|+|(E 0)|=0, the electrons stop their ordered movement, the electric current stops. In order for the electron flow to be continuous, it is necessary: ​​firstly, to apply an additional force of a non-potential nature, which would compensate for the influence of its own electric field conductor and, secondly, to create a closed circuit, since the movement of electrons can occur only in conductors (we previously indicated that dielectrics, although they have some electrical conductivity, do not pass electric current) and to ensure the constancy of the compensating force, the constancy of the fields is necessary: ​​as an external as well as own.

Let's start with the second point. We will consider a conductor placed in a field, as shown in Figure 4.2. Let us assume that after the interaction of the external and intrinsic electromagnetic fields has been compensated, we have applied in addition to the external field one more of the same field. The total action of the external field will be 2 |E|. The current in the conductor will continue to flow in the same direction, but exactly until 2 |E|>|E 0 |, after which the electric current will stop again. That is external influence must increase continuously to allow current to flow in an open conductor, which is impossible.
If we close the conductor so that one part of it lies outside the field, then due to the work of an additional force in addition to the external field (this force in this case should not be potential, since the work of the potential force in closed circuit is zero and does not depend on the shape of the trajectory), then an electric current will appear in the conductor, due to the influence of only the external field, since the actual field of the conductor will be completely compensated. That is why any electrical circuit must always be closed.

You can try to explain the need to introduce additional force from the following consideration: if we could partially transfer charges from end B of the conductor to end A of the conductor, the electric current would also not stop. However, such "landing" also requires energy. Hence, the introduction of additional force is still necessary. Non-potential forces are also called external forces. And their sources are current sources or generators.

Figure 4.2 - The emergence of its own electromagnetic field in the conductor

So where can we get additional force, which, moreover, should not be created by the field, because without it we will not get current? It turns out that during the course of a chemical reduction-oxidation reaction, for example, the interaction of lead oxide and dilute sulfuric acid, free electrons are released:

In order to “attract” all the electrons released during the reaction to one point in space, several lead grids, called electrodes, are placed in a solution of sulfuric acid. One part of the electrodes is made of lead and is called the cathode, the other - the anode - is made of lead dioxide. The cathode is the source of free electrodes for the external circuit, and the anode is the receiver.

The above example corresponds to a device known to all motorists (and not only) - a lead-acid battery. Of course, the above example does not coincide much with what is happening inside the battery in reality, however, the essence of the appearance of the current reflects well. Thus, between the positive anode (few electrons) and the negative cathode (many electrons), an electric field arises, which forms external forces and creates a current in the conductor. This force depends only on the course of a chemical reaction, it is practically constant until the elements of this reaction exist - acid and lead oxide. Therefore, if we remove the electric field and connect the conductor to the anode and cathode, the electric current will still flow due to the fact that the battery creates an external force. The conductor will have its own electric field around it, which the battery needs to overcome in order to transfer the electron from the cathode to the anode. This is the essence of outside power.

Now consider the situation with the battery and the conductor connected to it. The electric field does positive work to move a positive charge (we are talking about positive charges, since the direction of their movement corresponds to the direction of the current) in the direction of decreasing the field potential. The current source carries out the separation of electric charges - positive charges accumulate on one pole, negative charges on the other. The strength of the electric field in the source is directed from the positive pole to the negative, so the work of the electric field to move the positive charge will be positive when it moves from "plus" to "minus". The work of external forces, on the contrary, is positive if the positive charges move from the negative pole to the positive, that is, from “minus” to “plus”. This is the fundamental difference between the concepts of potential difference and EMF, which must always be remembered.

Figure 4.3 shows the direction of current flow I in the conductor connected to the battery - from the positive anode to the negative cathode, however, inside the battery, third-party chemical reaction forces “drop” the electrons that came from the external circuit from the anode to the cathode and positive ions from the cathode to the anode, that is, they act against the direction of current flow and the direction of the field.

Figure 4.3 - Demonstration of external forces in the event of an electric current

From the above considerations, one can make following output: the forces acting on the charge inside the current source are different from the forces acting inside the conductor. Accordingly, it is necessary to distinguish these forces from each other. To characterize external forces, the magnitude of the electromotive force (EMF) was introduced - the work performed by external forces to move a single positive charge. It is denoted by the Latin letter ε (“epsilon”) and is measured in the same way as the potential difference - in volts.

Since the potential difference and EMF are forces of different types, we can say that the EMF outside the source leads is zero. Although in ordinary life these subtleties are neglected and they say: “The voltage on the battery is 1.5V”, although strictly speaking the voltage in the circuit section is the total work of electrostatic and third-party forces to move a single positive charge. In the future, we will still encounter these concepts and they will be useful to us when calculating complex electrical circuits.

This, perhaps, is all, because the lesson turned out to be too loaded ... But the concepts of voltage and EMF must be able to distinguish.

• For the existence of an electric current, two conditions are necessary:
  1) a closed electrical circuit;
  2) the presence of a source of third-party non-potential forces.
• Electromotive force (EMF) is the work done by external forces to move a single positive charge.
• Sources of extraneous forces in an electrical circuit are also called current sources.
• The positive terminal of the battery is called the anode, the negative terminal is called the cathode.

There will be no tasks this time, it is better to repeat this lesson in order to understand the whole physics of current flow in a conductor. As always, you can leave any questions, suggestions and wishes in the comments below! See you soon!

Without electricity it is impossible to imagine life modern man. Volts, Amps, Watts - these words are heard in a conversation about devices that run on electricity. But what is this electric current and what are the conditions for its existence? We will talk about this further, providing a brief explanation for beginner electricians.

Definition

An electric current is a directed movement of charge carriers - this is a standard formulation from a physics textbook. In turn, certain particles of matter are called charge carriers. They may be:

• Electrons are negative charge carriers.
• Ions are positive charge carriers.

But where do charge carriers come from? To answer this question, you need to remember the basic knowledge about the structure of matter. Everything that surrounds us is matter, it consists of molecules, its smallest particles. Molecules are made up of atoms. An atom consists of a nucleus around which electrons move in given orbits. Molecules also move randomly. The movement and structure of each of these particles depend on the substance itself and the influence on it. environment such as temperature, voltage, etc.

An ion is an atom in which the ratio of electrons and protons has changed. If the atom is initially neutral, then the ions, in turn, are divided into:

• Anions are the positive ion of an atom that has lost electrons.
• Cations are an atom with "extra" electrons attached to the atom.

The unit of current is Ampere, according to it is calculated by the formula:

where U is voltage [V] and R is resistance [Ohm].

Or directly proportional to the amount of charge transferred per unit of time:

where Q is the charge, [C], t is the time, [s].

Conditions for the existence of an electric current

We figured out what electric current is, now let's talk about how to ensure its flow. For electric current to flow, two conditions must be met:

1. The presence of free charge carriers.
2. Electric field.

The first condition for the existence and flow of electricity depends on the substance in which the current flows (or does not flow), as well as its state. The second condition is also feasible: for the existence of an electric field, the presence of different potentials is necessary, between which there is a medium in which charge carriers will flow.

Recall: Voltage, EMF is a potential difference. It follows that in order to fulfill the conditions for the existence of current - the presence of an electric field and an electric current, voltage is needed. These can be plates of a charged capacitor, a galvanic cell, an EMF that has arisen under the influence of a magnetic field (generator).

We figured out how it arises, let's talk about where it is directed. The current, basically, in our usual use, moves in conductors (electrical wiring in an apartment, incandescent bulbs) or in semiconductors (LEDs, your smartphone's processor and other electronics), less often in gases (fluorescent lamps).

So, in most cases, the main charge carriers are electrons, they move from minus (a point with a negative potential) to a plus (a point with a positive potential, you will learn more about this below).

But an interesting fact is that the direction of current movement was taken to be the movement of positive charges - from plus to minus. Although in fact the opposite is happening. The fact is that the decision on the direction of the current was made before studying its nature, and also before it was determined due to which the current flows and exists.

Electric current in different environments

We have already mentioned that in different media the electric current can differ in the type of charge carriers. Media can be divided according to the nature of conductivity (in descending order of conductivity):

1. Conductor (metals).
2. Semiconductor (silicon, germanium, gallium arsenide, etc.).
3. Dielectric (vacuum, air, distilled water).

in metals

Metals contain free charge carriers and are sometimes referred to as "electric gas". Where do free charge carriers come from? The fact is that metal, like any substance, consists of atoms. Atoms somehow move or oscillate. The higher the temperature of the metal, the stronger this movement. At the same time, the atoms themselves general view remain in their places, actually forming the structure of the metal.

In the electron shells of an atom, there are usually several electrons that have a rather weak bond with the nucleus. Under the influence of temperatures chemical reactions and the interaction of impurities, which in any case are in the metal, electrons are detached from their atoms, positively charged ions are formed. The detached electrons are called free and move randomly.

If they are affected by an electric field, for example, if you connect a battery to a piece of metal - chaotic movement electrons will become ordered. Electrons from a point to which a negative potential is connected (the cathode of a galvanic cell, for example) will begin to move towards a point with a positive potential.

in semiconductors

Semiconductors are materials in which there are no free charge carriers in the normal state. They are in the so-called forbidden zone. But if you apply external forces, such as an electric field, heat, various radiations (light, radiation, etc.), they overcome the band gap and pass into the free band or the conduction band. Electrons break away from their atoms and become free, forming ions - positive charge carriers.

Positive carriers in semiconductors are called holes.

If you simply transfer energy to a semiconductor, for example, heat it, a chaotic movement of charge carriers will begin. But if we are talking about semiconductor elements, such as a diode or a transistor, then at the opposite ends of the crystal (a metallized layer is applied to them and the leads are soldered), an EMF will appear, but this does not apply to the topic of today's article.

If you apply an EMF source to a semiconductor, then the charge carriers will also move into the conduction band, and their directed movement will also begin - holes will go to the side with a lower electric potential, and electrons - to the side with a larger one.

In vacuum and gas

A vacuum is a medium with a complete (ideal case) absence of gases or a minimized (in reality) its amount. Since there is no matter in vacuum, there is no source for charge carriers. However, the flow of current in a vacuum marked the beginning of electronics and an entire era electronic elements- vacuum lamps. They were used in the first half of the last century, and in the 50s they began to gradually give way to transistors (depending on the specific field of electronics).

Let's assume that we have a vessel from which all the gas has been pumped out, i.e. it is a complete vacuum. Two electrodes are placed in the vessel, let's call them an anode and a cathode. If we connect the negative potential of the EMF source to the cathode, and positive to the anode, nothing will happen and no current will flow. But if we start heating the cathode, the current will start to flow. This process is called thermionic emission - the emission of electrons from a heated surface of an electron.

The figure shows the process of current flow in a vacuum lamp. In vacuum tubes, the cathode is heated by a nearby filament in Fig. (H), such as that found in a lighting lamp.

At the same time, if you change the polarity of the supply - apply a minus to the anode, and apply a plus to the cathode - the current will not flow. This will prove that the current in vacuum flows due to the movement of electrons from the CATHODE to the ANODE.

A gas, like any substance, consists of molecules and atoms, which means that if the gas is under the influence of an electric field, then at a certain strength (ionization voltage), the electrons will break away from the atom, then both conditions for the flow of electric current will be fulfilled - the field and free media.

As already mentioned, this process is called ionization. It can occur not only from the applied voltage, but also when the gas is heated, x-rays, under the influence of ultraviolet radiation and others.

Current will flow through the air, even if a burner is installed between the electrodes.

The flow of current in inert gases is accompanied by gas luminescence; this phenomenon is actively used in fluorescent lamps. The flow of electric current in a gaseous medium is called a gas discharge.

in liquid

Let's say that we have a vessel with water in which two electrodes are placed, to which a power source is connected. If the water is distilled, that is, pure and does not contain impurities, then it is a dielectric. But if we add a little salt, sulfuric acid, or any other substance to the water, an electrolyte is formed and a current begins to flow through it.

An electrolyte is a substance that conducts electricity by dissociating into ions.

If copper sulfate is added to water, then a layer of copper will settle on one of the electrodes (cathode) - this is called electrolysis, which proves that the electric current in the liquid is carried out due to the movement of ions - positive and negative charge carriers.

Electrolysis is a physical and chemical process, which consists in the separation of the components that make up the electrolyte on the electrodes.

Thus, copper plating, gilding and coating with other metals occur.

Conclusion

To summarize, for the flow of electric current, free charge carriers are needed:

• electrons in conductors (metals) and vacuum;
• electrons and holes in semiconductors;
• ions (anions and cations) in liquids and gases.

In order for the movement of these carriers to become ordered, an electric field is needed. In simple words- apply voltage at the ends of the body or install two electrodes in an environment where electric current is expected to flow.

It is also worth noting that the current in a certain way affects the substance, there are three types of exposure:

• thermal;
• chemical;
• physical.

Useful

Ohm's law for a circuit section states that current is directly proportional to voltage and inversely proportional to resistance.

If the voltage acting in an electrical circuit is increased several times, then the current in this circuit will increase by the same amount. And if you increase the resistance of the circuit several times, then the current will decrease by the same amount. Likewise, the flow of water in a pipe is greater, the greater the pressure and the less resistance the pipe exerts to the movement of water.

Electrical resistance - physical quantity characterizing the properties of the conductor to prevent the passage of electric current and equal to the ratio voltage at the ends of the conductor to the strength of the current flowing through it.

Any body through which an electric current flows, has a certain resistance to it.

Electronic theory this explains the essence of the electrical resistance of metallic conductors. When moving along a conductor, free electrons encounter atoms and other electrons countless times on their way and, interacting with them, inevitably lose part of their energy. The electrons experience, as it were, resistance to their movement. Various metal conductors having different atomic structure, have different resistance to electric current.

The resistance of the conductor does not depend on the current strength in the circuit and voltage, but is determined only by the shape, size and material of the conductor.

The greater the resistance of the conductor, the worse it conducts electric current, and, conversely, the lower the resistance of the conductor, the easier it is for the electric current to pass through this conductor.

2 question. Visible movements of celestial bodies. The laws of planetary motion.

A) On a dark night, we can see about 2500 stars in the sky (taking into account the invisible hemisphere 5000), which differ in brightness and color. It seems that they are attached to the celestial sphere and, together with it, revolve around the Earth. To navigate among them, the sky was divided into 88 constellations. A special place among the constellations was occupied by 12 zodiac constellations through which the annual path of the Sun passes - the ecliptic. astronomers use different systems of celestial coordinates to navigate among the stars. One of them is the equatorial coordinate system (Fig. 15.1). It is based on the celestial equator - the projection of the earth's equator onto the celestial sphere. The ecliptic and the equator intersect at two points: the spring and autumn equinoxes. Any star has two coordinates: α - right ascension (measured in hours), b - deviation (measured in degrees). The star Altair has the following coordinates: α = 19 h 48 m 18 s; b = +8° 44 ‘. The measured coordinates of the stars are stored in catalogs, they are used to build star charts, which are used by astronomers when searching for the right stars. The mutual arrangement of stars in the sky does not change, they make a daily rotation along with the celestial sphere. The planets, along with the daily rotation, move slowly among the stars, and are called a wandering star.

The apparent movement of the planets and the Sun was described by Nicolaus Copernicus, using the geocentric system of the world.

B) The movement of planets and other celestial bodies around the Sun occurs according to Kepler's three laws:

Kepler's first law- under the influence of the force of attraction, one celestial body moves in the gravitational field of another celestial body according to one of the conic sections - a circle, an ellipse, a parabola or a hyperbola.

Kepler's second law- each planet moves in such a way that the radius vector of the planet covers equal areas in equal time intervals.

Kepler's third law- the cube of the semi-major axis of the body's orbit, divided by the square of the period of its revolution and the sum of the masses of the bodies, is a constant value.

and 3 / [T 2 * (M 1+ M 2)] = G / 4P 2 G is the gravitational constant.

Moon moving around Earth in an elliptical orbit. The change of lunar phases is determined by the change in the type of illumination of the side of the moon. The movement of the Moon around the Earth is explained by lunar and solar eclipses. The phenomena of ebbs and flows are due to the attraction of the Moon and the large size of the Earth.

Electricity. Ohm's law

If an insulated conductor is placed in an electric field, then on free charges q a force will act in the conductor. As a result, a short-term movement of free charges occurs in the conductor. This process will end when the own electric field of the charges that have arisen on the surface of the conductor completely compensates for the external field. The resulting electrostatic field inside the conductor will be zero (see § 1.5).

However, in conductors, under certain conditions, a continuous ordered movement of free electric charge carriers can occur. Such a movement is called electric shock . The direction of movement of positive free charges is taken as the direction of the electric current. For the existence of an electric current in a conductor, it is necessary to create an electric field in it.

The quantitative measure of electric current is current strength I – scalar physical quantity equal to the charge ratio Δ q, transferred through the cross section of the conductor (Fig. 1.8.1) for the time interval Δ t, to this time interval:

In the International System of Units SI, current is measured in amperes (A). The current unit 1 A is established by the magnetic interaction of two parallel conductors with current (see § 1.16).

A constant electric current can only be generated in closed circuit , in which free charge carriers circulate along closed paths. The electric field at different points in such a circuit is constant over time. Consequently, the electric field in the DC circuit has the character of a frozen electrostatic field. But when moving an electric charge in an electrostatic field along a closed path, the work of electric forces is zero (see § 1.4). Therefore, for the existence of direct current, it is necessary to have a device in the electrical circuit that can create and maintain potential differences in sections of the circuit due to the work of forces non-electrostatic origin. Such devices are called direct current sources . Forces of non-electrostatic origin acting on free charge carriers from current sources are called outside forces .

The nature of outside forces can be different. IN galvanic cells or batteries, they arise as a result of electrochemical processes, in DC generators, third-party forces arise when conductors move in a magnetic field. The current source in the electrical circuit plays the same role as the pump, which is necessary for pumping fluid in a closed hydraulic system. Under the influence of external forces, electric charges move inside the current source against forces of an electrostatic field, due to which a constant electric current can be maintained in a closed circuit.

When electric charges move along a DC circuit, external forces acting inside current sources do work.

Physical quantity equal to the ratio of work A st external forces when moving charge q from the negative pole of the current source to the positive to the value of this charge, is called source electromotive force(EMF):

Thus, the EMF is determined by the work done by external forces when moving a single positive charge. The electromotive force, like the potential difference, is measured in volts (V).

When a single positive charge moves along a closed DC circuit, the work of external forces is equal to the sum of the EMF acting in this circuit, and the work of the electrostatic field is zero.

The DC circuit can be divided into separate sections. Those sections on which external forces do not act (i.e., sections that do not contain current sources) are called homogeneous . Areas that include current sources are called heterogeneous .

When a unit positive charge moves along a certain section of the circuit, both electrostatic (Coulomb) and external forces do work. The work of electrostatic forces is equal to the potential difference Δφ 12 \u003d φ 1 - φ 2 between the initial (1) and final (2) points of the inhomogeneous section. The work of external forces is, by definition, the electromotive force 12 acting in this area. So the total work is

The German physicist G. Ohm in 1826 experimentally established that the current strength I, flowing through a homogeneous metal conductor (i.e., a conductor in which no external forces act), is proportional to the voltage U at the ends of the conductor:

Where R= const.

the value R called electrical resistance . A conductor with electrical resistance is called resistor . This ratio expresses Ohm's law for a homogeneous section of the circuit: The current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.

In SI, the unit of electrical resistance of conductors is ohm (Ohm). A resistance of 1 ohm has a section of the circuit in which, at a voltage of 1 V, a current of 1 A occurs.

Conductors that obey Ohm's law are called linear . Graphic dependence of current strength I from voltage U(such charts are called volt-ampere characteristics , abbreviated VAC) is represented by a straight line passing through the origin. It should be noted that there are many materials and devices that do not obey Ohm's law, such as a semiconductor diode or a gas discharge lamp. Even for metal conductors at currents of sufficiently large strength, a deviation from Ohm's linear law is observed, since the electrical resistance of metal conductors increases with increasing temperature.

For a circuit section containing EMF, Ohm's law is written in the following form:

Ohm's law

Adding both equalities, we get:

I (R + r) = Δφ cd + Δφ ab + .

But Δφ cd = Δφ ba = – Δφ ab. That's why

This formula will express Ohm's law for a complete circuit : the current strength in a complete circuit is equal to the electromotive force of the source, divided by the sum of the resistances of the homogeneous and inhomogeneous sections of the circuit.

Resistance r heterogeneous area in Fig. 1.8.2 can be seen as current source internal resistance . In this case, the plot ( ab) in fig. 1.8.2 is the internal section of the source. If the points a And b close with a conductor whose resistance is small compared to the internal resistance of the source ( R << r), then the circuit will flow short circuit current

Short circuit current - the maximum current that can be obtained from a given source with an electromotive force and internal resistance r. For sources with low internal resistance, the short-circuit current can be very large and cause the destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have a short circuit current of several hundred amperes. Particularly dangerous are short circuits in lighting networks powered by substations (thousands of amperes). To avoid the destructive effect of such high currents, fuses or special circuit breakers are included in the circuit.

In some cases, to prevent dangerous values ​​of the short circuit current, some external resistance is connected in series to the source. Then resistance r is equal to the sum of the internal resistance of the source and the external resistance, and in the event of a short circuit, the current strength will not be excessively large.

If the external circuit is open, then Δφ ba = – Δφ ab= , i.e., the potential difference at the poles of an open battery is equal to its EMF.

If the external load resistance R switched on and current flows through the battery I, the potential difference at its poles becomes equal to

Δφ ba = – Ir.

On fig. 1.8.3 is a schematic representation of a DC source with equal EMF and internal resistance r in three modes: "idle", work on load and short circuit mode (short circuit). The strength of the electric field inside the battery and the forces acting on positive charges are indicated: – electric force and – third-party force. In short circuit mode, the electric field inside the battery disappears.

To measure voltages and currents in DC electrical circuits, special devices are used - voltmeters And ammeters.

Voltmeter designed to measure the potential difference applied to its terminals. He connects parallel section of the circuit on which the measurement of the potential difference is made. Any voltmeter has some internal resistance. R B. In order for the voltmeter not to introduce a noticeable redistribution of currents when connected to the measured circuit, its internal resistance must be large compared to the resistance of the section of the circuit to which it is connected. For the circuit shown in Fig. 1.8.4, this condition is written as:

R B >> R 1 .

This condition means that the current I B = Δφ cd / R B, flowing through the voltmeter, is much less than the current I = Δφ cd / R 1 that flows through the tested section of the circuit.

Since there are no outside forces acting inside the voltmeter, the potential difference at its terminals coincides, by definition, with the voltage. Therefore, we can say that the voltmeter measures voltage.

Ammeter designed to measure the current in the circuit. The ammeter is connected in series to the break in the electrical circuit so that the entire measured current passes through it. The ammeter also has some internal resistance. R A. Unlike a voltmeter, the internal resistance of an ammeter must be sufficiently small compared to the total resistance of the entire circuit. For the circuit in fig. 1.8.4 the resistance of the ammeter must satisfy the condition

Conditions for the existence of direct electric current.

For the existence of a direct electric current, the presence of free charged particles and the presence of a current source are necessary. in which the conversion of any type of energy into the energy of an electric field is carried out.

Current source- a device in which any type of energy is converted into the energy of an electric field. In a current source, external forces act on charged particles in a closed circuit. The reasons for the appearance of external forces in various current sources are different. For example, in batteries and galvanic cells, external forces arise due to the flow of chemical reactions, in generators of power plants they arise when a conductor moves in a magnetic field, in photocells - when light acts on electrons in metals and semiconductors.

The electromotive force of the current sourcecalled the ratio of the work of external forces to the value of the positive charge transferred from the negative pole of the current source to the positive.

Basic concepts.

Current strength- a scalar physical quantity equal to the ratio of the charge that has passed through the conductor to the time for which this charge has passed.

Where I - current strength,q - amount of charge (amount of electricity),t - charge transit time.

current density- vector physical quantity equal to the ratio of the current strength to the cross-sectional area of ​​​​the conductor.

Where j -current density, S - cross-sectional area of ​​the conductor.

The direction of the current density vector coincides with the direction of motion of positively charged particles.

Voltage - scalar physical quantity equal to the ratio of the total work of the Coulomb and external forces when moving a positive charge in the area to the value of this charge.

WhereA - full work of third-party and Coulomb forces,q - electric charge.

Electrical resistance- physical quantity characterizing electrical properties section of the chain.

Where ρ - specific resistance of the conductor,l - length of the conductor section,S - cross-sectional area of ​​the conductor.

Conductivityis the reciprocal of the resistance

WhereG - conductivity.