Current, electric current in a vacuum. Electrovacuum devices Electric currents in vacuum gases

10.08.2021

An electric current can be formed not only in metals, but also in a vacuum, for example, in radio tubes, in cathode ray tubes. Let us find out the nature of the current in vacuum.

Metals have a large number of free, randomly moving electrons. When an electron approaches the surface of a metal, the attractive forces acting on it from the side of positive ions and directed inwards prevent the electron from leaving the metal. The work that must be done to remove an electron from a metal in a vacuum is called exit work. It is different for different metals. So, for tungsten it is equal to 7.2 * 10 -19 j. If the energy of an electron is less than the work function, it cannot leave the metal. many electrons, even room temperature, whose energy is not much greater than the work function. After leaving the metal, they move away from it for a short distance and, under the action of the forces of attraction of the ions, return to the metal, as a result of which a thin layer of outgoing and returning electrons is formed near the surface, which are in dynamic equilibrium. Due to the loss of electrons, the surface of the metal becomes positively charged.

In order for an electron to leave the metal, it must do work against the repulsive forces of the electric field of the electron layer and against the forces of the electric field of the positively charged surface of the metal (Fig. 85. a). At room temperature, there are almost no electrons that could escape the double charged layer.

In order for electrons to fly out of the double layer, they need to have an energy much greater than the work function. To do this, energy is imparted to the electrons from the outside, for example, by heating. The emission of electrons by a heated body is called thermionic emission. It is one of the proofs of the presence of free electrons in the metal.

The phenomenon of thermionic emission can be observed in such an experiment. Having charged the electrometer positively (from an electrified glass rod), we connect it with a conductor to electrode A of a demonstration vacuum lamp (Fig. 85, b). The electrometer does not discharge. Having closed the circuit, we will glow the thread K. We see that the needle of the electrometer falls - the electrometer is discharged. The electrons emitted by the heated filament are attracted to the positively charged electrode A and neutralize its charge. The flow of thermoelectrons from the filament to electrode A under the action of an electric field formed electricity in a vacuum.

If the electrometer is charged negatively, then it will not be discharged in such an experiment. The electrons flying out of the filament are no longer attracted by electrode A, but on the contrary, they are repelled from it and returned back to the filament.

Let's assemble the electrical circuit (Fig. 86). With an unheated thread K, the circuit between it and electrode A is open - the galvanometer needle is at zero. There is no current in its circuit. Having closed the key, we heat the filament. A current went through the galvanometer circuit, as the thermoelectrons closed the circuit between the filament and electrode A, thereby forming an electric current in a vacuum. An electric current in a vacuum is a directed flow of electrons under the action of an electric field. The speed of the directed motion of electrons that form a current in vacuum is billions of times greater than the speed of the directed motion of electrons that form a current in metals. Thus, the speed of the flow of electrons at the anode of the lamps of a radio receiver reaches several thousand kilometers per second.

Motion of charged free particles produced as a result of emission in a vacuum under the action of an electric field

Description

To obtain an electric current in a vacuum, the presence of free carriers is necessary. They can be obtained by emitting electrons from metals - electron emission (from the Latin emissio - release).

As you know, at ordinary temperatures, electrons are held inside the metal, despite the fact that they perform thermal motion. Consequently, near the surface there are forces acting on electrons and directed inside the metal. These are the forces that arise due to the attraction between electrons and positive ions of the crystal lattice. As a result, an electric field appears in the surface layer of metals, and the potential increases by a certain value Dj when moving from the outer space into the metal. Accordingly, the potential energy of the electron decreases by e Dj .

Distribution potential energy electron U for a limited metal is shown in fig. one.

Electron potential energy diagram U in a bounded metal

Rice. one

Here W0 is the energy level of an electron at rest outside the metal, F is the Fermi level (the energy value below which all states of the system of particles (fermions) are occupied at absolute zero), E c is the lowest energy of conduction electrons (the bottom of the conduction band). The distribution has the form of a potential well, its depth e Dj =W 0 - E c (electron affinity); Ф \u003d W 0 - F - thermionic work function (work function).

The condition for an electron to escape from a metal is W і W 0 , where W is the total energy of an electron inside the metal.

At room temperatures, this condition is satisfied only for an insignificant part of the electrons, which means that in order to increase the number of electrons leaving the metal, it is necessary to spend a certain amount of work, that is, to give them additional energy sufficient to pull out from the metal, observing electron emission: when the metal is heated - thermionic, when bombarded electrons or ions - secondary, when illuminated - photoemission.

Consider thermionic emission.

If the electrons emitted by a hot metal are accelerated by an electric field, then they form a current. Such an electron current can be obtained in a vacuum, where collisions with molecules and atoms do not interfere with the movement of electrons.

To observe thermionic emission, a hollow lamp containing two electrodes can serve: one in the form of a wire made of a refractory material (molybdenum, tungsten, etc.), heated by a current (cathode), and the other, a cold electrode that collects thermoelectrons (anode). The anode is most often given the shape of a cylinder, inside which an incandescent cathode is located.

Let us consider a circuit for observing thermionic emission (Fig. 2).

Electrical circuit for observing thermionic emission

Rice. 2

The circuit contains a diode D, the heated cathode of which is connected to the negative pole of the battery B, and the anode to its positive pole; milliammeter mA, which measures the current through the diode D, and a voltmeter V, which measures the voltage between the cathode and the anode. With a cold cathode, there is no current in the circuit, since the highly discharged gas (vacuum) inside the diode does not contain charged particles. If the cathode is heated with an additional source, then the milliammeter will register the appearance of a current.

At a constant temperature of the cathode, the strength of the thermionic current in the diode increases with an increase in the potential difference between the anode and cathode (see Fig. 3).

Current-Voltage Characteristics of a Diode at Different Cathode Temperatures

Rice. 3

However, this dependence is not expressed by a law similar to Ohm's law, according to which the current strength is proportional to the potential difference; this dependence is more complex, graphically presented in Figure 2, for example, curve 0-1-4 (voltage characteristic). With an increase in the positive potential of the anode, the current strength increases in accordance with the 0-1 curve, with a further increase in the anode voltage, the current strength reaches a certain maximum value i n, called the diode saturation current, and almost ceases to depend on the anode voltage (section of the curve 1-4).

Qualitatively, this dependence of the diode current on voltage is explained as follows. When the potential difference is zero, the current through the diode (with a sufficient distance between the electrodes) is also zero, since the electrons that have left the cathode form an electron cloud near it, creating an electric field that slows down the newly emitted electrons. The emission of electrons stops: how many electrons leave the metal, the same number returns to it under the action of the reverse field of the electron cloud. With an increase in the anode voltage, the concentration of electrons in the cloud decreases, its inhibitory effect decreases, and the anode current increases.

The dependence of the diode current i on the anode voltage U has the form:

where a is a coefficient depending on the shape and location of the electrodes.

This equation describes the 0-1-2-3 curve, and is called the Boguslavsky-Langmuir law or “3/2 law”.

When the anode potential becomes so high that all the electrons leaving the cathode in every unit of time hit the anode, the current reaches its maximum value and ceases to depend on the anode voltage.

With an increase in the temperature of the cathode, the current-voltage characteristic is depicted by curves 0-1-2-5, 0-1-2-3-6, etc., that is, at different temperatures the values of the saturation current i n turn out to be different, which rapidly increase with increasing temperature. At the same time, the anode voltage increases, at which the saturation current is set.

An electric current in a vacuum can pass, provided that free charge carriers are placed in it. After all, vacuum is the absence of any substance. This means that there are no charge carriers to provide current. The concept of vacuum can be defined as when the free path of the molecule is greater than the size of the vessel.

In order to find out how it is possible to ensure the passage of current in a vacuum, we will conduct an experiment. For him, we need an electrometer and a vacuum lamp. That is, a glass flask with a vacuum, in which there are two electrodes. One of which is made in the form of a metal plate, let's call it an anode. And the second in the form of a wire spiral of refractory material, let's call it the cathode.

Connect the electrodes of the lamp to the electrometer in such a way that the cathode is connected to the body of the electrometer, and the anode to the rod. Let's report the charge to the electrometer. By placing a positive charge on its rod. We will see that the charge will remain on the electrometer despite the presence of the lamp. This is not surprising because there are no charge carriers between the electrodes in the lamp, that is, no current can occur to discharge the electrometer.

Figure 1 - vacuum tube connected to a charged electrometer

Now we connect a current source to the cathode in the form of a wire spiral. This heats up the cathode. And we will see that the charge of the electrometer will begin to decrease until it disappears altogether. How could this happen because there were no charge carriers in the gap between the electrodes of the lamp to provide the conduction current.

Obviously, charge carriers somehow appeared. And this happened because when the cathode was heated, electrons were emitted from the cathode surface into the space between the electrodes. As you know, metals have free conduction electrons. Which are able to move in the volume of the metal between the nodes of the lattice. But they don't have enough energy to leave the metal. Since they are held by the Coulomb forces of attraction between the positive ions of the lattice and electrons.

Electrons perform chaotic thermal motion, moving along the conductor. Approaching the border of the metal, where there are no positive ions, they slow down and eventually return inside under the influence of the Coulomb force, which tends to bring two opposite charges closer. But if the metal is heated, then the thermal motion increases, and the electron acquires enough energy to leave the surface of the metal.

In this case, a so-called electron cloud is formed around the cathode. These are electrons that have left the surface of the conductor, and in the absence of an external electric field, they will return back to it. Since, by losing electrons, the conductor becomes positively charged. This is the case if we first heated the cathode, and the electrometer would be discharged. The field would be absent inside.

But since there is a charge on the electrometer, it creates a field that makes the electrons move. Remember on the anode we have a positive charge to it, and the electrons tend to be affected by the field. Thus, an electric current is observed in a vacuum.

If we say, we connect the electrometer in reverse, which will happen. It turns out that there will be a negative potential at the anode of the lamp, and a positive potential at the cathode. All electrons emitted from the surface of the cathode will immediately return back under the action of the field. Since the cathode will now have an even greater positive potential, it will attract electrons. And on the anode there will be an excess of electrons repelling electrons from the surface of the cathode.

Figure 2 - current versus voltage for a vacuum lamp

Such a lamp is called a vacuum diode. It can only pass current in one direction. The current-voltage characteristic of such a lamp consists of two sections. Ohm's law is fulfilled in the first section. That is, with increasing voltage, more and more electrons emitted from the cathode reach the anode and thereby the current increases. In the second section, all the electrons emitted from the cathode reach the anode, and with a further increase in voltage, the current does not increase. It just doesn't have the right amount of electrons. This area is called saturation.

Subject. Electric current in a vacuum

The purpose of the lesson: to explain to students the nature of electric current in a vacuum.

Type of lesson: lesson learning new material.

LESSON PLAN

STUDY NEW MATERIAL

Vacuum is the state of a gas where the pressure is less than atmospheric pressure. Distinguish between low, medium and high vacuum.

To create a high vacuum, a rarefaction is necessary, for which in the gas that remains, the mean free path of molecules is greater than the size of the vessel or the distance between the electrodes in the vessel. Consequently, if a vacuum is created in the vessel, then the molecules in it almost do not collide with each other and fly freely through the interelectrode space. In this case, they experience collisions only with the electrodes or with the walls of the vessel.

In order for a current to exist in a vacuum, it is necessary to place a source of free electrons in the vacuum. The highest concentration of free electrons in metals. But at room temperature, they cannot leave the metal, because they are held in it by the Coulomb attraction forces of positive ions. To overcome these forces, an electron must expend a certain amount of energy in order to leave the metal surface, which is called the work function.

If the kinetic energy of an electron exceeds or is equal to the work function, then it will leave the surface of the metal and become free.

The process of emitting electrons from the surface of a metal is called emission. Depending on how the energy needed was transferred to the electrons, there are several types of emission. One of them is thermoelectronic emission.

Ø The emission of electrons by heated bodies is called thermoelectronic emission.

The phenomenon of thermionic emission leads to the fact that a heated metal electrode continuously emits electrons. The electrons form an electron cloud around the electrode. In this case, the electrode is positively charged, and under the influence of the electric field of the charged cloud, the electrons from the cloud partially return to the electrode.

In the equilibrium state, the number of electrons that leave the electrode in a second is equal to the number of electrons that return to the electrode during this time.

For the existence of a current, two conditions must be met: the presence of free charged particles and an electric field. To create these conditions, two electrodes (cathode and anode) are placed in the balloon and air is pumped out of the balloon. As a result of heating the cathode, electrons fly out of it. A negative potential is applied to the cathode, and a positive potential is applied to the anode.

A modern vacuum diode consists of a glass or ceramic-metal cylinder, from which air is evacuated to a pressure of 10-7 mm Hg. Art. Two electrodes are soldered into the balloon, one of which - the cathode - has the form of a vertical metal cylinder made of tungsten and usually coated with a layer of alkaline earth metal oxides.

An insulated conductor is located inside the cathode, which is heated by alternating current. The heated cathode emits electrons that reach the anode. The lamp anode is a round or oval cylinder having a common axis with the cathode.

The one-way conduction of a vacuum diode is due to the fact that, due to heating, electrons fly out of the hot cathode and move to the cold anode. Electrons can only move through the diode from the cathode to the anode (that is, electric current can only flow in the opposite direction: from the anode to the cathode).

The figure reproduces the current-voltage characteristic of a vacuum diode (a negative voltage value corresponds to the case when the cathode potential is higher than the anode potential, that is, the electric field “tries” to return the electrons back to the cathode).

Vacuum diodes are used to rectify alternating current. If one more electrode (grid) is placed between the cathode and the anode, then even a slight change in the voltage between the grid and the cathode will significantly affect the anode current. Such a vacuum tube (triode) allows you to amplify weak electrical signals. Therefore, for some time these lamps were the main elements of electronic devices.

Electric current in a vacuum was used in a cathode ray tube (CRT), without which for a long time it was impossible to imagine a TV or an oscilloscope.

The figure shows a simplified view of the design of a CRT.

The electron "gun" at the neck of the tube is the cathode, which emits an intense beam of electrons. A special system of cylinders with holes (1) focuses this beam, making it narrow. When the electrons hit the screen (4), it starts to glow. The electron flow can be controlled using vertical (2) or horizontal (3) plates.

Significant energy can be transferred to electrons in a vacuum. Electron beams can even be used to melt metals in a vacuum.

QUESTION TO STUDENTS DURING THE PRESENTATION OF NEW MATERIAL

First level

1. What is the purpose of high vacuum in electron tubes?

2. Why does a vacuum diode only conduct current in one direction?

3. What is the purpose of the electron gun?

4. How are electron beams controlled?

Second level

1. What features does the current-voltage characteristic of a vacuum diode have?

2. Will a radio lamp with broken glass work in space?

CONFIGURATION OF THE STUDYED MATERIAL

1. What needs to be done so that the trielectrode lamp can be used as a diode?

2. How can: a) increase the speed of electrons in the beam; b) change the direction of electron movement; c) stop moving electrons?

1. The maximum anode current in the vacuum diode is 50 mA. How many electrons are emitted from the cathode every second?

2. A beam of electrons, which are accelerated by a voltage U 1 \u003d 5 kV, flies into a flat capacitor in the middle between the plates and parallel to them. Capacitor length l = 10 cm, distance between plates d = 10 mm. For what minimum voltage U 2 on the capacitor will electrons not fly out of it?

Solutions. The motion of an electron resembles the motion of a body thrown horizontally.

The horizontal component v of the electron velocity does not change, it coincides with the electron velocity after acceleration. This speed can be determined using the law of conservation of energy: Here e is the elementary electric charge, me is the mass of the electron. The vertical acceleration a transfers to the electron the force F acting from the electric field of the capacitor. According to Newton's second law,

where is the electric field strength in the capacitor.

Electrons will not fly out of the capacitor if they are displaced by a distance d / 2.

So, is the time of electron movement in the capacitor. From here

After checking the units of quantities and substituting numerical values, we get U 2 \u003d 100 B.

WHAT WE LEARNED IN THE LESSON

Vacuum is a gas so rarefied that the mean free path of molecules exceeds the linear dimensions of the vessel.

The energy that an electron needs to expend in order to leave the surface of the metal is called the work function.

The emission of electrons by heated bodies is called thermoelectronic emission.

Electric current in vacuum is a directed movement of electrons produced as a result of thermionic emission.

The vacuum diode has one-way conduction.

A cathode ray tube allows you to control the movement of electrons. It was the CRT that made television possible.

Homework

1. Sub-1: § 17; sub-2: § 9.

Riv1 No. 6.12; 6.13; 6.14.

Riv2 No. 6.19; 6.20; 6.22, 6.23.

3. D: prepare for independent work No. 4.

ASSIGNMENTS FROM INDEPENDENT WORK No. 4 "LAWS OF DIRECT CURRENT"

Task 1 (1.5 points)

The movement of what particles creates an electric current in liquids?

A Movement of atoms.

Would the movement of molecules.

In The movement of electrons.

D Movement of positive and negative ions.

The figure shows an electric discharge in the air, created using a Tesla transformer.

And the electric current in any gas is directed in the direction where the negative ions move.

The conductivity of any gas is due to the movement of electrons only.

The conductivity of any gas is due to the movement of ions only.

D The conductivity of any gas is due to the movement of only electrons and ions.

Task 3 aims to establish a correspondence (logical pair). For each line marked with a letter, match the statement marked with a number.

A n-type semiconductors.

B Semiconductors p-type.

electronic conductivity.

D Hole conductivity.

1 Semiconductors in which holes are the majority charge carriers.

2 Semiconductors in which the majority charge carriers are electrons.

3 Conductivity of a semiconductor due to the movement of holes.

4 Conductivity of a semiconductor due to the movement of electrons.

5 Semiconductors in which the majority charge carriers are electrons and holes.

At what current strength was the electrolysis of an aqueous solution of CuSO 4 carried out, if in 2 min. 160 g of copper was released at the cathode?

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ElElectric current in vacuum

1. Cathode ray tube

Vacuum is a state of gas in a vessel in which the molecules fly from one wall of the vessel to another without ever colliding with each other.

A vacuum insulator, the current in it can only arise due to the artificial introduction of charged particles; for this, the emission (emission) of electrons by substances is used. In vacuum lamps with heated cathodes, thermionic emission occurs, and in a photodiode, photoelectron emission occurs.

Let us explain why there is no spontaneous emission of free electrons by a metal. The existence of such electrons in a metal is a consequence of the close proximity of atoms in a crystal. However, these electrons are free only in the sense that they do not belong to specific atoms, but remain belonging to the crystal as a whole. Some of the free electrons, being as a result of chaotic movement at the surface of the metal, fly out of it. A micro area of the metal surface, which was previously electrically neutral, acquires a positive uncompensated charge, under the influence of which the emitted electrons return to the metal. The departure-return processes occur continuously, as a result of which a replaceable electron cloud is formed above the metal surface, and the metal surface forms a double electric layer, against the confining forces of which the work function must be performed. If electron emission occurs, then some external influences (heating, lighting) have done such work

Thermionic emission is the property of bodies heated to a high temperature to emit electrons.

The cathode ray tube is a glass flask in which a high vacuum is created (10 to -6 degrees-10 to -7 degrees mm Hg). The source of electrons is a thin wire spiral (it is also a cathode). Opposite the cathode there is an anode in the form of a hollow cylinder, to which the electron beam enters after passing through a focusing cylinder containing a diaphragm with a narrow aperture. A voltage of several kilovolts is maintained between the cathode and anode. Electrons accelerated by an electric field fly out of the aperture of the diaphragm and fly to a screen made of a substance that glows under the action of electron impacts.

To control the electron beam, two pairs of metal plates are used, one of which is located vertically and the other horizontally. If the left of the plates has a negative potential, and the right one has a positive potential, then the beam will deviate to the right, and if the polarity of the plates is changed, then the beam will deviate to the left. If voltage is applied to these plates, then the beam will oscillate in the horizontal plane. Similarly, the beam will oscillate in the vertical plane if there is an alternating voltage on the vertically deflecting plates. The previous plates are horizontally deflecting.

2. Electric current in a vacuum

What is a vacuum?

This is such a degree of gas rarefaction at which there are practically no collisions of molecules;

Electric current is not possible, because. the possible number of ionized molecules cannot provide electrical conductivity;

You can create an electric current in a vacuum if you use a source of charged particles; beam tube vacuum diode

The action of a source of charged particles can be based on the phenomenon of thermionic emission.

3. vacuum diode

Electric current in a vacuum is possible in electron tubes.

A vacuum tube is a device that uses the phenomenon of thermionic emission.

A vacuum diode is a two-electrode (A-anode and K-cathode) electron tube.

Very low pressure is created inside the glass container

H - filament placed inside the cathode to heat it. The surface of the heated cathode emits electrons. If the anode is connected to the + current source, and the cathode to -, then the circuit flows

constant thermionic current. The vacuum diode has one-way conduction.

Those. current in the anode is possible if the anode potential is higher than the cathode potential. In this case, the electrons from the electron cloud are attracted to the anode, creating an electric current in vacuum.

4. Volt-amperevacuum diode characteristic

At low voltages at the anode, not all the electrons emitted by the cathode reach the anode, and the electric current is small. At high voltages, the current reaches saturation, i.e. maximum value.

A vacuum diode is used to rectify alternating current.

Current at the input of the diode rectifier

Rectifier output current

5. electron beams

This is a stream of fast-flying electrons in vacuum tubes and gas-discharge devices.

Properties of electron beams:

Deviate in electric fields;

Deviate in magnetic fields under the action of the Lorentz force;

When decelerating a beam that hits a substance, X-rays are produced;

Causes glow (luminescence) of some solid and liquid bodies (phosphors);

They heat the substance, falling on it.

6. Cathode Ray Tube (CRT)

Phenomena of thermionic emission and properties of electron beams are used.

The CRT consists of an electron gun, horizontal and vertical deflecting electrode plates, and a screen.

In the electron gun, the electrons emitted by the heated cathode pass through the control grid electrode and are accelerated by the anodes. The electron gun focuses the electron beam to a point and changes the brightness of the glow on the screen. Deflecting horizontal and vertical plates allow you to move the electron beam on the screen to any point on the screen. The screen of the tube is covered with a phosphor, which glows when bombarded with electrons.

There are two types of tubes:

1) with electrostatic control of the electron beam (deviation of the electron beam only by the electric field);

2) with electromagnetic control (magnetic deflection coils are added).

Main application of CRT:

kinescopes in television equipment;

computer displays;

electronic oscilloscopes in measuring technology.

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Current, electric current in a vacuum. Electrovacuum devices Electric currents in vacuum gases

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