Metals in the solid state have a crystalline structure.
The metal model is a crystal lattice, in the nodes of which the particles perform a chaotic oscillatory motion.

Pay attention!

Positive ions are located at the nodes of the crystal lattice. Free electrons move in the space between them.

The negative charge of all free electrons is equal in absolute value to the positive charge of all ions of the lattice. Therefore, under normal conditions, the metal is electrically neutral. Free electrons move randomly in it. If an electric field is created in the metal, then free electrons will begin to move in a direction (orderly), i.e. an electric current will occur. However, the random motion of electrons is preserved.

Pay attention!

Electric current in metals is an ordered movement of free electrons.

What is the speed of electrons in a conductor under the action of an electric field? It is small - only a few millimeters per second, and sometimes even less.
If an electric field arises in a conductor, it propagates with great speed along the entire length of the conductor (close to the speed of light - 300,000 km / s), at the same time, electrons begin to move in one direction along the entire length of the conductor.
Experiments proved that the current in metals is due to electrons. The experiment of Mandelstam and Papaleksi was carried out in 1916. The purpose of the experiment was to check whether the carrier of electric current, the electron, has mass. If the electron has mass, then it must obey the laws of mechanics, in particular, the law of inertia. For example, if a moving conductor is abruptly slowed down, then the electrons will move in the same direction for some time by inertia.
For this test, the researchers rotated the coil with a passing current, and then abruptly stopped it. The resulting inrush current was recorded using a telephone.
By clicking the current in the phones, Mandelstam and Papaleksi found that the electron has mass. But they could not measure this mass. Therefore, this experience is of high quality. Later, the American physicists Tolman and Stewart, using the same idea of ​​the rotation of a coil, measured the mass of an electron. To do this, they measured the charge that occurs during braking of the coil at its terminals.

Electric current can exist not only in metals, but also in other media: in semiconductors, gases and electrolyte solutions. Carriers of electric charges in different environments are different.

Pay attention!

So, in solutions of electrolytes (salts, acids and alkalis), carriers are positive and negative ions, in gases - positive and negative ions, as well as electrons. In semiconductors, charge carriers are electrons and holes (a hole is an invented particle to explain the conduction mechanism, in fact, it is a free space not occupied by an electron).

Semiconductors are made from semiconductors. Here is some of them:

Photocell	photoresistor	Photodiodes	integrated circuits	transistors

Semiconductors do not conduct electricity at low temperatures; are dielectrics. As the temperature rises, the number of electric charge carriers increases, the semiconductor becomes a conductor. Why is this happening? The valence electrons located on the outer shell of the atom become free, and under the action of an electric field, an electric current arises in the semiconductor. A similar process occurs in a semiconductor when exposed to light, impurities, etc.
The change in the electrical conductivity of semiconductors under the action of temperature makes it possible to use them as thermometers.

The change in the electrical conductivity of semiconductors under the influence of light is used in photoresistors. They are used for signaling, remote control of production processes, sorting of parts. In emergency situations, they allow you to automatically stop machines and conveyors, preventing accidents.

The following has been historically accepted:

The direction of the current coincides with the direction of movement of positive charges in the conductor.

In this case, if the only current carriers are negatively charged particles (for example, electrons in a metal), then the direction of the current is opposite to the direction of movement of the electrons.

The passage of current through the conductor is accompanied by the following actions:

Magnetic (observed in all conductors).

Using this property, you can find the place of a phase wire break with devices that respond to changes in the electromagnetic field, for example, an indicator screwdriver with a phase detector.

If a wire frame carrying current is placed between the poles of a magnet, it will rotate. This phenomenon is used in the galvanometer device.

The needle of a galvanometer is connected to a moving coil in a magnetic field. When current flows through the coil, the needle deflects. Thus, using a galvanometer, we can conclude that there is a current in the circuit. The magnetic effect of the current manifests itself regardless of the state of aggregation of the substance. When the key is closed, one can observe how the wire wound around the nail begins to attract small iron objects.

Topic "Electric current in metals"

The purpose of the lesson: Continue the study of the nature of the electric current in metals, experimentally study the effect of the electric current.

Lesson objectives:

Educational - the formation of common views on the nature of electric current, the formation of the ability to work with electrical circuits, to assemble electrical circuits.

Educational- the formation of the ability to find errors and avoid them when applying knowledge in practice, as well as logically explain new phenomena, apply their knowledge in non-standard situations.

Educational - formation of the ability to concentrate attention, conduct a dialogue, defend one's opinion with reason.

Equipment and materials: power sources, electric bulb for a flashlight, electric bell, switches, lead wires, copper sulphate solution, electromagnet, copper and zinc plates, crystal lattice model, galvanometer.

TSO: computer presentation, multimedia projector.

Demos:

1) Assembly of the simplest electrical circuits.

2) Isolation of copper during the electrolysis of copper sulfate

3) The action of a coil with current, like an electromagnet.

Lesson plan.

1. Updating knowledge (10 min).
2. Studying the new material "Electric current in metals" (10 min)

"Actions of electric current" (12 min)

1. Fixing (9 min)
2. Homework (2min)
3. Summing up (2 min)

During the classes.

Hello guys!

How would our planet live,

How would people live on it?

Without heat, magnet, light

And electric rays.

This quatrain mentions electric rays. What do you think it is? (electricity)

1) What is called electric current?

2) What is necessary for an electric current to exist in a circuit?

3) Work with diagrams: name the proposed main parts of the electrical circuit

Designations proposed: electric lamp, key, ammeter, voltmeter, current source, bells, etc.

4) And now let's check how you see violations in the compilation of electrical circuits.

Before you are two electrical circuits, the diagrams of which are presented on the screen.

1. What violations did you notice? Why does not a working lamp in the first circuit burn when the key is closed? Answer. The electrical circuit is broken. In order for the lamp to light up, an electric current must exist in the circuit, and this is possible with a closed circuit consisting only of conductors of electricity.

2) How are conductors different from non-conductors or insulators? Answer. Students bridge the gap. The lamp lights up.

2. Why doesn't the bell ring in the second circuit when the circuit is closed? Answer. To obtain an electric current in a conductor, it is necessary to create an electric field in it. Under the influence of this field, free charged particles will begin to move in an orderly manner, and this is an electric current. An electric field in conductors is created and can be maintained for a long time by electric field sources. The electrical circuit must have a current source. We connect the circuit to a current source and the bell rings. For the existence of an electric current, the following conditions are necessary: ​​-------- the presence of free electric charges in the conductor; -the presence of an external electric field for the conductor. The student, having connected a current source to the circuit, demonstrates the correct answer.

2. Learning new things material "Electric current in metals" - 10 min . Slide number 1 The topic of our lesson: “Electric current in metals. Actions of electric current »Guys, who knows how to avoid the action of electric current if you accidentally touch an electrical appliance that turned out to be energized? Answer. This requires grounding, since the earth is a conductor and, due to its huge size, can hold a large charge. Teacher. What materials are grounding made of? Answer. Grounding is made of metal. Teacher. Why are metals preferred? We will answer this question after studying the new topic “Electric current in metals”. Write the topic of the lesson in your notebook.

The most famous of the early definitions of metal was given in the middle of the 18th century by M.V. Lomonosov: “Metal is a light body that can be forged. There are only six such bodies: gold, silver, copper, tin, iron and lead.” Two and a half centuries later, much has become known about metals. More than 75% of all elements of the table of D. I. Mendeleev belong to the number of metals.

Today we will get acquainted with an important property of metals - electrical conductivity. Consider the structure of metals. Demonstration model of the crystal lattice, an image of the model of the structure of metals is projected on the screen.

The metal model is a crystal lattice, in the nodes of which the particles perform a chaotic oscillatory motion.

Free electrons move under the influence of an electric field. The final confirmation of this fact was an experiment carried out in 1913 by the physicists of our country L. I. Mandelstam and N. D. Papaleksi, as well as by the American physicists B. Stuart and R. Tolman. Look at the picture on the screen

Scientists brought a multi-turn coil around its axis into a very fast rotation. Then, with a sharp deceleration of the coil, its ends were closed to a galvanometer, and the device recorded a short-term electric current. The reason for the occurrence, which is caused by the movement by inertia of free charged particles between the nodes of the crystal lattice of the metal. Since the direction of the initial velocity and the direction of the resulting current are known from experience, the sign of the charge of the carriers can be found: it turns out to be negative. Therefore, free charge carriers in a metal are free electrons. By the deviation of the galvanometer needle, one can judge the magnitude of the electric charge flowing in the circuit. Experience confirmed the theory. The triumph of the classical theory of electricity took place.


An electrical signal sent, for example, by wire from Moscow to Vladivostok (s = 8000 km), arrives there in about 0.03 s. And now you can move on to the knowledge of the external world. Finished electric current in metals. Let's move on to the next block "Actions of electric current"

We cannot see electrons moving in a metallic conductor. We can judge the presence of current in a circuit by the various phenomena that an electric current causes. Such phenomena are called current actions. Some of these actions are easy to observe experimentally.

Thermal effect of current.

Chemical action of current. The chemical action of electric current was first discovered in 1800. An experience. We will conduct an experiment with a solution of copper sulfate. We lower two carbon electrodes into distilled water and close the circuit. We observe that the light bulb does not light up. We take a solution of copper sulfate and connect it to a power source. The light bulb lights up. Conclusion. Chemical the effect of the current is that in some solutions of acids (salts, alkalis) when an electric current passes through them, a release of substances is observed. The substances contained in the solution are deposited on the electrodes dipped into this solution. When current is passed through a solution of copper sulfate (CuSO 4), pure copper (Cu) will be released on a negatively charged electrode. This is used to obtain pure metals. Aluminum, chemically pure metals are obtained by electrolysis, nickel plating, chromium plating, gilding are produced. To protect metals from corrosion, their surface is often coated with metals that are difficult to oxidize, that is, nickel or chromium plating is performed. This process is called electroplating. Guys, what methods of protecting metals from corrosion do you know?

The Chinese philosopher Confucius once said, "It's good to have natural talent, but exercise, friends, gives us more than natural talent." A Russian proverb says: "Learning is always useful." .1) Why can't you touch bare electrical wires with your bare hands? (Moisture on the hands always contains a solution of various salts and is an electrolyte. Therefore, it creates good contact between the wires and the skin.)

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Physics lesson in 8th grade.

Topic "Electric current in metals"

The purpose of the lesson : Continue the study of the nature of the electric current in metals, experimentally study the effect of the electric current.

Lesson objectives:

Educational -the formation of common views on the nature of electric current, the formation of the ability to work with electrical circuits, to assemble electrical circuits.

Educational - the formation of the ability to find errors and avoid them when applying knowledge in practice, as well as logically explain new phenomena, apply their knowledge in non-standard situations.

Educational -formation of the ability to concentrate attention, conduct a dialogue, defend one's opinion with reason.

Equipment and materials: power sources, electric bulb for a flashlight, electric bell, switches, lead wires, copper sulphate solution, electromagnet, copper and zinc plates, crystal lattice model, galvanometer.

TSO : computer presentation, multimedia projector.

Demos:

1) Assembly of the simplest electrical circuits.

2) Isolation of copper during the electrolysis of copper sulfate

3) The action of a coil with current, like an electromagnet.

Lesson plan.

1. Updating knowledge (10 min).
2. Studying the new material "Electric current in metals" (10 min)

"Actions of electric current" (12 min)

1. Fixing (9 min)
2. Homework (2min)
3. Summing up (2 min)

During the classes.

Announcement of the topic, objectives of the lesson.

1) Actualization of knowledge -10 min.

Hello guys!

How would our planet live,

How would people live on it?

Without heat, magnet, light

And electric rays.

This quatrain mentions electric rays. What do you think it is? (electricity)

Questions:

1. What is called electric current?
2. What is necessary for an electric current to exist in a circuit?

3) Work with diagrams: name the proposed main parts of the electrical circuit

Designations proposed: electric lamp, key, ammeter, voltmeter, current source, bells, etc.

4) And now let's check how you see violations in the compilation of electrical circuits.

Before you are two electrical circuits, the diagrams of which are presented on the screen.

1. What violations did you notice? Why does not a working lamp in the first circuit burn when the key is closed? Answer. The electrical circuit is broken. In order for the lamp to light up, an electric current must exist in the circuit, and this is possible with a closed circuit consisting only of conductors of electricity.

2) How are conductors different from non-conductors or insulators? Answer. Students bridge the gap. The lamp lights up.

2. Why doesn't the bell ring in the second circuit when the circuit is closed? Answer. To obtain an electric current in a conductor, it is necessary to create an electric field in it. Under the influence of this field, free charged particles will begin to move in an orderly manner, and this is an electric current. An electric field in conductors is created and can be maintained for a long time by electric field sources. The electrical circuit must have a current source. We connect the circuit to a current source and the bell rings. For the existence of an electric current, the following conditions are necessary: ​​-------- the presence of free electric charges in the conductor; -the presence of an external electric field for the conductor. The student, having connected a current source to the circuit, demonstrates the correct answer.

2. Learning new thingsmaterial "Electric current in metals" - 10 min. Slide number 1 The topic of our lesson: “Electric current in metals. Actions of electric current »Guys, who knows how to avoid the action of electric current if you accidentally touch an electrical appliance that turned out to be energized? Answer. This requires grounding, since the earth is a conductor and, due to its huge size, can hold a large charge. Teacher. What materials are grounding made of? Answer. Grounding is made of metal. Teacher. Why are metals preferred? We will answer this question after studying the new topic “Electric current in metals”. Write the topic of the lesson in your notebook.

The most famous of the early definitions of metal was given in the middle of the 18th century by M.V. Lomonosov: “Metal is a light body that can be forged. There are only six such bodies: gold, silver, copper, tin, iron and lead.” Two and a half centuries later, much has become known about metals. More than 75% of all elements of the table of D. I. Mendeleev belong to the number of metals.

Today we will get acquainted with an important property of metals - electrical conductivity. Consider the structure of metals. Demonstration model of the crystal lattice, an image of the model of the structure of metals is projected on the screen.

The metal model is a crystal lattice, in the nodes of which the particles perform a chaotic oscillatory motion.

Metals in the solid state have a crystalline structure. Particles in crystals are arranged in a certain order, forming a spatial (crystal) lattice. As you already know, in any metal, some of the valence electrons leave their places in the atom, as a result of which the atom turns into a positive ion. Positive ions are located at the nodes of the crystal lattice of the metal, and free electrons (electron gas) move in the space between them, i.e. not bound to the nuclei of their atoms.
The negative charge of all free electrons is equal in absolute value to the positive charge of all ions of the lattice. Therefore, under normal conditions, the metal is electrically neutral.
What electric charges move under the action of an electric field in metal conductors?Free electrons move under the influence of an electric field. The final confirmation of this fact was an experiment carried out in 1913 by the physicists of our country L. I. Mandelstam and N. D. Papaleksi, as well as by the American physicists B. Stuart and R. Tolman. Look at the picture on the screen

Scientists brought a multi-turn coil around its axis into a very fast rotation. Then, with a sharp deceleration of the coil, its ends were closed to a galvanometer, and the device recorded a short-term electric current. The reason for the occurrence, which is caused by the movement by inertia of free charged particles between the nodes of the crystal lattice of the metal. Since the direction of the initial velocity and the direction of the resulting current are known from experience, the sign of the charge of the carriers can be found: it turns out to be negative. Therefore, free charge carriers in a metal are free electrons. By the deviation of the galvanometer needle, one can judge the magnitude of the electric charge flowing in the circuit. Experience confirmed the theory. The triumph of the classical theory of electricity took place.Electric current in metal conductors is an ordered movement of free electrons, under the influence of an electric field
If there is no electric field in the conductor, then the electrons move randomly, similar to how the molecules of gases or liquids move. At each moment of time, the speeds of various electrons differ in modules and in directions. If an electric field is created in the conductor, then the electrons, retaining their chaotic movement, begin to shift towards the positive pole of the source. Together with the chaotic movement of electrons, their ordered transfer arises - drift. The speed of the ordered movement of electrons in a conductor under the action of an electric field is a few millimeters per second, and sometimes even less. But as soon as an electric field arises in the conductor, it propagates along the entire length of the conductor at a tremendous speed close to the speed of light in vacuum (300,000 km / s).
Simultaneously with the propagation of the electric field, all electrons begin to move in the same direction along the entire length of the conductor. So, for example, when the circuit of an electric lamp is closed, the electrons present in the lamp spiral also begin to move in an orderly manner.
It will help to understand this by comparing the electric current with the flow of water in a water supply system, and the propagation of an electric field with the propagation of water pressure. When water rises into the water tower, pressure (pressure) of water spreads very quickly throughout the entire plumbing system. When we turn on the faucet, the water is already under pressure and starts to flow. But the water that was in it flows from the tap, and the water from the tower will reach the tap much later, because. the movement of water occurs at a lower speed than the propagation of pressure.
When they talk about the speed of propagation of an electric current in a conductor, they mean the speed of propagation of an electric field along the conductor.
An electrical signal sent, for example, by wire from Moscow to Vladivostok (s = 8000 km), arrives there in about 0.03 s. And now you can move on to the knowledge of the external world. Finished electric current in metals. Let's move on to the next block "Actions of electric current"

The study of new material "Actions of electric current"We cannot see electrons moving in a metallic conductor. We can judge the presence of current in a circuit by the various phenomena that an electric current causes. Such phenomena are called current actions. Some of these actions are easy to observe experimentally.

Thermal effect of current.Program disk Physics lessons Grade 8. Virtual School of Cyril and Methodius

Chemical action of current.The chemical action of electric current was first discovered in 1800. An experience. We will conduct an experiment with a solution of copper sulfate. We lower two carbon electrodes into distilled water and close the circuit. We observe that the light bulb does not light up. We take a solution of copper sulfate and connect it to a power source. The light bulb lights up.Conclusion. Chemicalthe effect of the current is that in some solutions of acids (salts, alkalis) when an electric current passes through them, a release of substances is observed. The substances contained in the solution are deposited on the electrodes dipped into this solution. When current is passed through a solution of copper sulphate (CuSO 4 ) pure copper (Cu) will be released on the negatively charged electrode. This is used to obtain pure metals. Aluminum, chemically pure metals are obtained by electrolysis, nickel plating, chromium plating, gilding are produced. To protect metals from corrosion, their surface is often coated with metals that are difficult to oxidize, that is, nickel or chromium plating is performed. This process is called electroplating. Guys, what methods of protecting metals from corrosion do you know?

Magnetic action of current. An experience.We include a coil with an iron core in a circuit and observe the attraction of metal objects. Use of the magnetic action of current in galvanometers. Galvanometer. Schematic notationConsolidation of the studied material. Questions on a new topic. ToThe Chinese philosopher Confucius once said, "It's good to have natural talent, but exercise, friends, gives us more than natural talent." A Russian proverb says: "Learning is always useful." .1) Why can't you touch bare electrical wires with your bare hands? (Moisture on the hands always contains a solution of various salts and is an electrolyte. Therefore, it creates good contact between the wires and the skin.)

Homework. P. 34.35L. No. 1260, 1261. Prepare a report on the metals "Aluminum", "Gold", "Iron"

Let's look at what elements can be connected with wires to make an electrical circuit: a galvanic cell, a battery of cells, a light bulb, a bell, resistance, a switch (or key), an ammeter and a voltmeter.

The drawing, which shows the methods of connecting elements in a circuit, is called a diagram. This is what an electric flashlight looks like.

And this is how the circuit looks like, consisting of a source, one call and two (or more) buttons, by which you can independently turn on the call, for example, in a hospital (or on an airplane) when you need to call a nurse for a sick person.

Let us recall the structure of metals: there are positive ions at the nodes of the crystal lattice, and electrons move freely between these nodes, creating an “electron gas” that occupies the entire volume of the metal conductor. Therefore, the electric current in metals is an ordered movement of electrons. In the absence of an electric field, electrons move randomly, chaotically, with sufficiently high speeds.

But when an electric field is supplied from a source, and its propagation speed is 300,000 km / s, then all electrons in the entire volume of the metal conductor begin to move in an orderly manner at a low speed, which is several mm / s.

For the existence of an electric current, it is necessary: ​​the presence of free charged particles, an electric field (source), a consumer and conductors of an electric current.

Electric current, when passing through a load, has different actions. What actions can we observe?

Thermal action. To observe this action, we will conduct an experiment.

We will place a long wire on two insulated racks. In several places we will attach easily hanging tassels of pieces of paper. We connect the wire to an adjustable source (such as LATR, so that the voltage can be gradually increased). We turn on the installation, slowly increase the voltage, at a certain value the wire starts to heat up, and the pieces of paper light up. Let's pay attention to the fact that during the experiment the wire sags more strongly. This is due to the fact that it has heated up, and when heated, all bodies expand, and the wire lengthens.

mechanical action. Connect a small fan. Why are the blades spinning? Because when an electric current passes through the motor, the frames in the magnetic field rotate (mechanical movement) and rotate the fan blades.

magnetic action. Consider the experiment of Oersted, which he conducted in 1820. On the installation according to the first experiment, we will bring a magnetic needle on the rack while turning on the current. The needle will deviate from its usual direction in the Earth's magnetic field and turn perpendicular to the conductor, fixing the presence of a stronger magnetic field near the conductor through which the current flows. When the current is turned off, we see that the arrow deviates and again shows the direction to the "north".

Chemical action. As a load, we now include in the electrical circuit two carbon electrodes inserted into a glass beaker in which a solution of copper sulphate is poured.

It is first necessary to clean the electrodes with sandpaper to remove any impurities. We turn on the circuit in a regulated source ... and after a while we turn it off and we see that a thin layer of copper has separated on the negative electrode (cathode).

Is there some more physiological action of electric current: effect on living organisms. For the first time, during the preparation of the legs of a frog, Luigi Galvani discovered a contraction of the muscles of the leg. That is, when current passes through the body, all muscles contract, trying to protect the body from unpleasant consequences.

The direction of the electric current was invented by the American banker Benjamin Franklin, who was engaged in electricity in his spare time.

He believed that money from a large positive pile flows into small negative pockets of customers. Therefore, he suggested: the current goes from the positive pole to the negative.

This rule has been accepted all over the world.

Only much later, after the discovery of the electron by Thomson, they realized that the physical (true) direction of the current is from “minus” to “plus”. The current flows from places on the source where an excess amount of electrons has accumulated to those places where there are not enough electrons.

But rules have already been invented: the gimlet rule, the left hand rule, the right hand rule, Ampère's rule and others for directing the current from "plus" to "minus". And it was decided not to change anything, but to assume that the current goes from “plus” to “minus”.

Thus, we have considered what the current in metals is, what actions the current has, and what is the difference between the generally accepted direction of the current from “plus” to “minus” from the true physical direction.

Metals in the solid state, as is known, have a crystalline structure. Particles in crystals are arranged in a certain order, forming a spatial (crystal) lattice.

Positive ions are located at the nodes of the metal crystal lattice, and free electrons move in the space between them. Free electrons are not bound to the nuclei of their atoms (Fig. 53).

Rice. 53. Metal lattice

The negative charge of all free electrons is equal in absolute value to the positive charge of all ions of the lattice. Therefore, under normal conditions, the metal is electrically neutral. Free electrons in it move randomly. But if an electric field is created in the metal, then free electrons will begin to move in a direction under the action of electric forces. There will be an electric current. At the same time, the random movement of electrons is preserved, just as the random movement is preserved in a flock of midges when, under the influence of the wind, it moves in one direction.

So, electric current in metals is an ordered movement of free electrons.

Mandelstam Leonid Isaakovich (1879-1944)
Russian physicist, academician. He made a significant contribution to the development of radiophysics and radio engineering.

Papaleksi Nikolai Dmitrievich (1880-1947)
Russian physicist, academician. He was engaged in research in the field of radio engineering, radiophysics, radio astronomy.

The proof that the current in metals is due to electrons was the experiments of the physicists of our country Leonid Isaakovich Mandelstam and Nikolai Dmitrievich Papaleksi, as well as the American physicists Balfour Stewart and Robert Tolman.

The speed of movement of the electrons themselves in the conductor under the action of an electric field is small - a few millimeters per second, and sometimes even less. But as soon as an electric field arises in the conductor, it propagates along the entire length of the conductor at a tremendous speed close to the speed of light in vacuum (300,000 km / s).

Simultaneously with the propagation of the electric field, all electrons begin to move in the same direction along the entire length of the conductor. So, for example, when the circuit of an electric lamp is closed, the electrons present in the lamp spiral also begin to move in an orderly manner.

It will help to understand this by comparing the electric current with the flow of water in a water supply system, and the propagation of an electric field with the propagation of water pressure. When water rises into the water tower, the pressure (pressure) of the water spreads very quickly throughout the entire plumbing system. When we turn on the faucet, the water is already under pressure and immediately begins to flow. But the water that was in it flows from the tap, and the water from the tower will reach the tap much later, since the movement of water occurs at a lower speed than the pressure spreads.

When they talk about the speed of propagation of an electric current in a conductor, they mean the speed of propagation of an electric field along the conductor.

An electrical signal sent, for example, by wire from Moscow to Vladivostok (s = 8000 km) arrives there in approximately 0.03 s.

Questions

1. How to explain that under normal conditions the metal is electrically neutral?
2. What happens to the electrons of a metal when an electric field appears in it?
3. What is electric current in metal?
4. What speed is meant when talking about the speed of propagation of electric current in a conductor?

Exercise

Using the Internet, find how fast electrons move in metals. Compare it to the speed of light.

In this lesson, we will get acquainted with why an electric current occurs in metals, explain why metals are good conductors. In addition, we will study the effects of electric current and its direction. We will consider the Rikke experiment, which confirms that a metal conductor practically does not change when an electric current flows through it, we will find out what actions of the current are most used by a person in technology and everyday life, and we will also understand why the direction of the current does not coincide with the direction of movement of electrons.

Topic: Electrical Phenomena

Lesson: Electric current in metals. Actions of electric current. Current direction

In previous lessons, we have studied almost all the concepts related to the occurrence of electric current: electric charges, electric field, current sources, the simplest electrical circuits and electrical circuits. Now we have to find out how the electric current flows in metals, what effects the electric current has, and also the direction of the current.

Metals, as we found out from experiments in previous lessons, conduct electricity well. In order to clarify this fact, let us ask ourselves the question: what are metals?

Metals, as a rule, are polycrystalline substances (consisting of many crystals) (Fig. 1, 2).

Rice. 2. Iron structure ()

That is, in metals we are dealing with an ordered structure of atoms: each atom is in its specific place.

As we already know, electrons move around the nucleus of atoms.

What gives rise to free electric charges?

The fact is that distant electrons (those that are on the orbits farthest from the nucleus) are rather weakly bound to the nucleus. Therefore, they can move quite easily from one atom to another. This random movement of electrons is somewhat reminiscent of an electron gas. If there is no electric field inside the metal, then the movement of these free electrons is somewhat reminiscent of the movement of midges raised into the air on a summer day (Fig. 3).

Rice. 3. Movement of electrons inside a metal conductor ()

Everything changes when an electric field appears inside the metal. An electric field causes charged particles to move. The nuclei of atoms remain in place, but the electrons begin to move in an orderly manner.

Electrons, jumping from one atom to another, move in the direction in which the electric field points them. This movement is called electric current in metals.

We know that electric current is a directed, ordered movement of charged particles. In metals, the role of moving charged particles is played by electrons. In other substances, these may be ions or ions and electrons.

The movement of charged particles (in metals - electrons) is very slow (fractions of millimeters per second). The question arises: why, when we press the switch, the light bulb lights up almost instantly?

The fact is that an electric field propagates inside the conductors at great speed (at the speed of light - approximately 300,000 kilometers per second).

When the circuit is closed, the field propagates almost instantly. And already after the field, electrons begin to move slowly, and at once along the entire circuit. This situation can be compared with the movement of water in a water pipe. Water is forced to move by pressure in the pipes, which, when the tap is opened, spreads almost instantly, forcing the water “nearest” to the tap to pour out. At the same time, all the water under this same pressure moves through the pipes. It turns out that pressure is an analog of an electric field, and water is an analog of electrons. As soon as the action of the electric field stops, the ordered movement of electric charges immediately stops.

A logical question arises: does the conductor change due to the fact that electrons “left” it? An experiment to confirm that all electrons are the same was carried out by the German scientist Rikke (Fig. 4) when three different conductors were used on tram lines: aluminum and two copper.

Rice. 4. Carl Victor Rikke ()

Rikke over the course of a year observed the series connection of three conductors: copper + aluminum + copper. Since the current in the tram lines is quite large, the experiment made it possible to give an unambiguous answer: are the electrons that are carriers of a negative charge in different conductors the same?

During the year, the mass of the conductors did not change, diffusion did not occur, that is, the structure of the conductors remained unchanged. From this followed the conclusion that electrons can pass from one conductor to another, but their structure does not change.

Let's talk now about what effect the electric current has. Due to what he received such a wide application in everyday life and technology?

There are three main actions of electric current:

1. Thermal. When current flows, the conductor heats up. This is one of the most important actions of the current, which is used by man. The simplest example is some household heaters (Fig. 5).

Rice. 5. Electric heater ()

2.Chemical. A conductor can change its chemical composition when a current is passed through it. In particular, with the help of electric current, some metals are mined in their pure form, separating them from various compounds. For example, aluminum is obtained in this way (Fig. 6).

Rice. 6. Electrolysis shop of an aluminum plant ()

3. Magnetic. If current flows through the conductor, then the magnetic needle near such a conductor will change its position.

Now let's talk about direction of electric current.

The direction of movement of positive electric charges is taken as the direction of electric current.

But we just talked about the fact that the current in metals is created by moving electrons that have a negative charge. Why does such a contradiction arise?

When the question arose about the direction of the electric current, no one knew about the existence of electrons. It was decided to assume that the current moves in the direction of positive charges. Time passed, scientists found out that in metals, in particular, electrons move, but it was decided to leave everything as it was. This is due to the fact that the sign of the charge practically does not interest us, we are much more interested in the action of the current itself.

The movement of electrons in a conductor is opposite to the direction of the electric field (Fig. 7).

Rice. 7. Movement of electrons in a conductor ()

In this lesson, we figured out how current flows in metals, learned about the actions of electric current, and also determined the direction of the current.

In the next lesson, we will begin to get acquainted with the numerical characteristics of the current.

Bibliography

1. Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
2. Peryshkin A. V. Physics 8. - M .: Bustard, 2010.
3. Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M .: Education.

Additional precommended links to Internet resources

1. Festival of pedagogical ideas "Open Lesson" ().
2. Festival of pedagogical ideas "Open Lesson" ().

Homework

1. P. 34-36, questions 1-4, p. 81, questions 1-7, p. 83, questions 1-3, p. 84. Peryshkin A. V. Physics 8. - M .: Bustard, 2010.
2. What devices use the thermal effect of current? Magnetic action?
3. What effects of current can be observed by passing current through sea water?