Leads to the appearance of current carriers in a vacuum. Electric current in a vacuum. Electronic emission

Before radio engineering began to use semiconductor devices, vacuum tubes were used everywhere.

Vacuum concept

The electron tube was a glass tube sealed at both ends, with a cathode on one side and an anode on the other. Gas was released from the tube to a state in which gas molecules could fly from one wall to another without colliding. This state of gas is called vacuum. In other words, vacuum is a highly rarefied gas.

Under such conditions, conductivity inside the lamp can only be ensured by introducing charged particles into the source. In order for charged particles to appear inside the lamp, they used a property of bodies such as thermionic emission.

Thermionic emission is the phenomenon of the emission of electrons by a body under the influence of high temperature. For many substances, thermionic emission begins at temperatures at which the evaporation of the substance itself cannot yet begin. In lamps, cathodes were made from such substances.

Electric current in a vacuum

The cathode was then heated, causing it to continually emit electrons. These electrons formed an electron cloud around the cathode. When connecting a power source to the electrodes, a electric field.

Moreover, if the positive pole of the source is connected to the anode, and the negative pole to the cathode, then the voltage vector electric field will be directed towards the cathode. Under the influence of this force, some electrons escape from the electron cloud and begin to move towards the anode. Thus, they create an electric current inside the lamp.

If you connect the lamp differently, connecting the positive pole to the cathode and the negative pole to the anode, then the electric field strength will be directed from the cathode to the anode. This electric field will push the electrons back towards the cathode and there will be no conduction. The circuit will remain open. This property is called unilateral conductivity.

Vacuum diode

In the past, single-sided conduction was widely used in electronic devices with two electrodes. Such devices were called vacuum diodes. At one time they performed the role that semiconductor diodes now perform.

Most often used to straighten electric current. IN this moment Vacuum diodes are practically never used anywhere. Instead, all progressive humanity uses semiconductor diodes.

Vacuum is a state of rarefied gas in which the mean free path of moleculesλ is greater than the size of the vessel d in which the gas is located.

From the definition of vacuum it follows that there is practically no interaction between molecules, therefore ionization of molecules cannot occur, therefore, free charge carriers cannot be obtained in a vacuum, therefore, electric current is impossible in it;
To create an electric current in a vacuum, you need to place a source of free charged particles into it. Metal electrodes connected to a current source are placed in a vacuum. One of them is heated (it is called the cathode), as a result of which the ionization process occurs, i.e. Electrons are released from the substance and positive and negative ions are formed. The action of such a source of charged particles can be based on the phenomenon of thermionic emission.

Thermionic emission is the process of emitting electrons from a heated cathode. The phenomenon of thermionic emission causes a heated metal electrode to continuously emit electrons. The electrons form an electron cloud around the electrode. The electrode is charged positively, and under the influence of the electric field of the charged cloud, electrons from the cloud are partially returned to the electrode. In the equilibrium state, the number of electrons leaving the electrode per second is equal to the number of electrons returning to the electrode during this time. The higher the temperature of the metal, the higher the density of the electron cloud. The work that an electron must do to leave the metal is called the work function A out.

[A out] = 1 eV

1 eV is the energy that an electron acquires when moving in an electric field between points with a potential difference of 1 V.

1 eV = 1.6*10 -19 J

The difference between the temperatures of hot and cold electrodes sealed into a vessel from which air has been evacuated leads to one-way conduction of electric current between them.

When the electrodes are connected to a current source, an electric field arises between them. If the positive pole of the current source is connected to a cold electrode (anode), and the negative pole to a heated one (cathode), then the electric field strength vector is directed towards the heated electrode. Under the influence of this field, electrons partially leave the electron cloud and move towards the cold electrode. The electrical circuit is closed and an electric current is established in it. When the source is turned on in opposite polarity, the field strength is directed from the heated electrode to the cold one. The electric field pushes the cloud's electrons back toward the heated electrode. The circuit appears to be open.

A device that has one-way conductivity of electric current is called a vacuum diode. It consists of an electron tube (vessel), from which air has been pumped out and in which there are electrodes connected to a current source. Current-voltage characteristic of a vacuum diode. Sign the sections of the current-voltage characteristics of the diode throughput mode and closed?? At low anode voltages, 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 electrical current. Currently, vacuum diodes are practically not used.

If a hole is made in the anode of an electron tube, then some of the electrons accelerated by the electric field will fly into this hole, forming an electron beam behind the anode. An electron beam is flow of rapidly flying electrons in vacuum tubes and gas-discharge devices.

Properties of electron beams:
- deviate in electric fields;
- deflect in magnetic fields under the influence of the Lorentz force;
- when a beam hitting a substance is decelerated, X-ray radiation appears;
- causes glow (luminescence) of some solids and liquid bodies;
- heat the substance by contacting it.

Cathode ray tube (CRT).
CRTs use thermionic emission phenomena and the properties of electron beams.

In an electron gun, electrons emitted by a heated cathode pass through a control grid electrode and are accelerated by the anodes. An electron gun focuses an electron beam into a point and changes the brightness of the light 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 tube screen is covered with a phosphor, which begins to glow when bombarded with electrons.

There are two types of tubes:
1) with electrostatic control of the electron beam (deflection of the electron beam only by an electric field);
2) with electromagnetic control (magnetic deflection coils are added).
In cathode ray tubes, narrow electron beams are formed, controlled by electrical and magnetic fields. These beams are used in: TV picture tubes, computer displays, electronic oscilloscopes in measuring equipment.

Under vacuum understand the state of a gas in a vessel in which the free path of charged particles exceeds the dimensions of the vessel where the gas is located.

Vacuum is an ideal insulator, since there are no free charge carriers in it. In order for current to flow through the space in which a high vacuum is created, it is necessary to artificially introduce a source of free charges into this space. This can be done using thermionic emission by placing a metal wire in a vacuum, which can be included in electrical circuit. When an electric current is passed through it, the wire heats up and the free electrons of the metal acquire energy sufficient to perform the work function and, leaving the metal, form an electron cloud near it. In this case, the wire becomes positively charged, and under the influence of the electric field, electrons from the cloud are partially returned to the electrode. In the equilibrium state, the number of electrons leaving the electrode per second is equal to the number of electrons returning to the electrode during this time. The higher the temperature of the metal, the higher the density of the electron cloud.

For a current to occur, an additional condition is necessary - the creation of an electric field, under the influence of which the electrons will move directionally.

Current in vacuum is a flow electrons. The difference between hot and cold electrodes sealed into the vessel leads to one-way conduction of electric current between them. When the electrodes are connected to a current source, an electric field arises between them. If the positive pole of the source is connected to a cold electrode (anode), and the negative pole to a heated one (cathode), then the electric field strength is directed towards the heated electrode. Under the influence of this field, electrons partially leave the electron cloud and move towards the cold electrode. The electrical circuit is closed and an electric current is established in it. When the source is turned on in the opposite direction, the field strength is directed from the cathode to the anode. The electric field pushes the cloud's electrons back toward the cathode. The circuit is open and there is no current in the circuit. Therefore, the diode has one-way conductivity.

Literature

Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyhavanne, 2004. - P. 294-295.

The most important devices in electronics of the first half of the twentieth century. There were vacuum tubes that used electric current in a vacuum. However, they were replaced by semiconductor devices. But even today, current in a vacuum is used in cathode ray tubes, in vacuum melting and welding, including in space, and in many other installations. This determines the importance of studying electric current in a vacuum.

Vacuum (from Latin vacuum - emptiness) is the state of gas at a pressure less than atmospheric. This concept applies to a gas in a closed vessel or in a vessel from which gas is pumped, and often to gas in free space, for example to space. Physical characteristics vacuum is the relationship between the free path of molecules and the size of the vessel, between the electrodes of the device, etc.

When we're talking about about vacuum, then for some reason they believe that this is completely empty space. In fact, this is not so. If air is pumped out of a vessel, the number of molecules in it will decrease over time, although it is impossible to remove all molecules from the vessel. So when can we consider that a vacuum has been created in the vessel?

Air molecules, moving chaotically, often collide with each other and with the walls of the vessel. Between such collisions, molecules fly certain distances, which are called the free path of molecules. It is clear that when air is pumped out, the concentration of molecules (their number per unit volume) decreases, and the mean free path increases. And then there comes a moment when the mean free path becomes equal to the size of the vessel: the molecule moves from wall to wall of the vessel, practically without encountering other molecules. It is then that they believe that a vacuum has been created in the vessel, although there may still be many molecules in it. It is clear that in smaller vessels, a vacuum is created at higher gas pressures in them than in larger vessels. If you continue to pump air out of the vessel, they say that a deeper vacuum is created in it. In a deep vacuum, a molecule can travel from wall to wall many times before it encounters another molecule. It is almost impossible to pump out all the molecules from the vessel. Where do free charge carriers come from in a vacuum? If a vacuum is created in a vessel, then there are still many molecules in it, some of them may be ionized. But there are few charged particles in such a vessel to detect a noticeable current. How can we obtain a sufficient number of free charge carriers in a vacuum? If you heat a conductor by passing an electric current through it or in some other way, then part free electrons in the metal will have sufficient energy to exit the metal (perform a work function).

Thermionic emission. Let's connect the rod of a charged electrometer to one electrode of a vacuum glass flask, and the body of the electrometer to another electrode, which is a thin metal thread (Fig. 12). Experience will show that the electrometer does not discharge.

Rice. 12

Between two electrodes located in a sealed vessel from which air has been removed and under voltage, there is no electric current, since there are no free electric charge carriers in a vacuum. The American scientist and inventor Thomas Edison (1847-1931) discovered (1879) that an electric current arises in a vacuum glass flask if one of the electrodes is heated to a high temperature.

Let's connect a current source to the terminals of the metal thread. If the thread is connected to the negative pole of the source, then when it heats up, the electrometer quickly discharges. When the thread is connected to the positive pole, the electrometer does not discharge even when the thread is heated by current. These experiments prove that a heated cathode emits particles that have a negative electric charge. These particles are electrons. The phenomenon of the emission of free electrons from the surface of heated bodies is called thermionic emission.

Diode. Thermionic emission is used in various electronic devices. The simplest of them is an electric vacuum diode. This device consists of a glass cylinder containing two electrodes: a cathode and an anode. The anode is made of a metal plate, the cathode is made of a thin metal wire rolled into a spiral. The ends of the spiral are mounted on metal rods that have two terminals for connection to an electrical circuit. By connecting the cathode terminals to a current source, it is possible to cause the cathode wire helix to be heated by the passing current to a high temperature. A wire helix heated by electric current is called a lamp filament. The symbol for a vacuum diode is shown in Figure 13.

Rice. 13

Application of a diode. By connecting a vacuum diode in an electrical circuit in series with the source direct current and an ammeter, you can detect the main property of the diode used in various electronic devices - one-way conductivity. When a current source is connected with a positive pole to the anode and a negative pole to the cathode, the electrons emitted by the heated cathode move under the influence of an electric field to the anode - an electric current flows in the circuit. If you connect a current source with the positive pole to the cathode, and the negative pole to the anode, then the electric field will prevent the movement of electrons from the cathode to the anode - there is no electric current in the circuit. The property of one-way conductivity of the diode is used in radio-electronic devices for converting alternating current to permanent.

Triode. The flow of electrons moving in a vacuum tube from the cathode to the anode can be controlled using electric and magnetic fields. The simplest electric vacuum device in which the flow of electrons is controlled using an electric field is a triode. The container, anode and cathode of a vacuum triode have the same design as that of a diode, however, in the path of electrons from the cathode to the anode in the triode there is a third electrode called a grid. Usually the mesh is a spiral of several turns thin wire around the cathode.

If a positive potential is applied to the grid relative to the cathode (Fig. 14a), then a significant part of the electrons flies from the cathode to the anode, and an electric current exists in the anode circuit. When fed to the grid negative potential relative to the cathode, the electric field between the grid and the cathode prevents the movement of electrons from the cathode to the anode (Fig. 14b), the anode current decreases. Thus, by changing the voltage between the grid and the cathode, you can regulate the current in the anode circuit.

Rice. 14

The vacuum triode device is shown in Figure 15, it symbol on the diagrams - in Figure 16.

Rice. 15

Electron beams and their properties. Electrons emitted by a heated cathode can be accelerated using electric fields to high speeds. Beams of electrons moving at high speeds can be used to produce X-rays and melt and cut metals. The ability of electron beams to be deflected by electric and magnetic fields and cause crystals to glow is used in cathode ray tubes.

Cathode-ray tube. If a hole is made in anode 2 of a vacuum diode, then part of the electrons emitted by cathode 1 will fly through the hole and form a stream of parallel flying electrons in the space behind the anode - electron beam 5 (Fig. 15).

Rice. 16

An electric vacuum device that uses such a flow of electrons is called a cathode ray tube.

The inner surface of the glass cylinder of the cathode ray tube against the anode is coated thin layer crystals that can glow when hit by fast electrons. This part of the tube is called the screen (6).

Using electric and magnetic fields, you can control the movement of electrons on the path from the anode to the screen and force the electron beam to “draw” any picture on the screen. This electron beam ability is used to create images on the screen of a television cathode ray tube called a kinescope. Changing the brightness of a spot on the screen is achieved by controlling the intensity of the electron beam using an additional electrode located between the cathode and anode and operating on the principle of the control grid of an electric vacuum triode.

In the tube of a cathode ray oscilloscope, between the anode and the screen there are two pairs of parallel metal plates. These plates are called deflection plates. Applying voltage to vertically located plates 4 causes a displacement of the electron beam in the horizontal direction, applying voltage to horizontal plates 3 causes vertical deflection of the beam. The beam displacements on the tube screen are proportional to the applied voltage, so an electronic oscilloscope can be used as an electrical measuring instrument.

To study rapidly changing electrical processes in an oscilloscope, a scan is carried out - a uniform movement of the electron beam horizontally. In order for the beam to move along the horizontal axis with constant speed, the voltage on the horizontal deflection plates must change linearly in time, and to return the beam to initial position The voltage should drop to zero very quickly. This form of stress is called sawtooth (Fig. 17).

The most important devices in electronics of the first half of the twentieth century. There were vacuum tubes that used electric current in a vacuum. However, they were replaced by semiconductor devices. But even today, current in a vacuum is used in cathode ray tubes, in vacuum melting and welding, including in space, and in many other installations. This determines the importance of studying electric current in a vacuum.

Vacuum (from lat.vacuum– emptiness) – the state of a gas at a pressure less than atmospheric. This concept applies to gas in a closed vessel or in a vessel from which gas is pumped, and often to gas in free space, such as space. The physical characteristic of vacuum is the relationship between the free path of molecules and the size of the vessel, between the electrodes of the device, etc.

Fig.1. Evacuation of air from a vessel

When it comes to vacuum, for some reason they think that it is completely empty space. In fact, this is not so. If air is pumped out of a vessel (Fig.1 ), then the number of molecules in it will decrease over time, although it is impossible to remove all molecules from the vessel. So when can we consider that a vacuum has been created in the vessel?

Air molecules, moving chaotically, often collide with each other and with the walls of the vessel. Between such collisions, molecules fly certain distances, which are called the free path of molecules. It is clear that when air is pumped out, the concentration of molecules (their number per unit volume) decreases, and the mean free path increases. And then there comes a moment when the mean free path becomes equal to the size of the vessel: the molecule moves from wall to wall of the vessel, practically without encountering other molecules. It is then that they believe that a vacuum has been created in the vessel, although there may still be many molecules in it. It is clear that in smaller vessels, a vacuum is created at higher gas pressures in them than in larger vessels.

If you continue to pump air out of the vessel, they say that a deeper vacuum is created in it. In a deep vacuum, a molecule can travel from wall to wall many times before it encounters another molecule.

It is almost impossible to pump out all the molecules from the vessel.

Where do free charge carriers come from in a vacuum?

If a vacuum is created in a vessel, then there are still many molecules in it, some of them may be ionized. But there are few charged particles in such a vessel to detect a noticeable current.

How can we obtain a sufficient number of free charge carriers in a vacuum? If you heat a conductor by passing an electric current through it or in some other way (Fig.2 ), then some of the free electrons in the metal will have sufficient energy to leave the metal (perform the work function). The phenomenon of electron emission from incandescent bodies is called thermionic emission.

Rice. 2. Emission of electrons by a hot conductor

Electronics and radio are almost the same age. True, at first radio did without its peer, but later electronic devices became the material basis of radio, or, as they say, its elementary basis.

The beginning of electronics can be traced back to 1883, when the famous Thomas Alpha Edison, trying to extend the life of a lighting lamp with a carbon filament, introduced a metal electrode into the lamp cylinder, from which the air had been evacuated.

It was this experience that led Edison to his only fundamental scientific discovery, which formed the basis of all vacuum tubes and all electronics before the transistor period. The phenomenon he discovered later became known as thermionic emission.

On the surface, Edison's experiment looked quite simple. He connected a battery and a galvanometer to the terminal of the electrode and one of the terminals of the filament heated by electric current.

The galvanometer needle deflected whenever the plus of the battery was connected to the electrode, and the minus to the thread. If the polarity was changed, the current in the circuit stopped.

Edison publicized this effect and received a patent for the discovery. True, he, as they say, did not bring his work to fruition and did not explain the physical picture of the phenomenon. At this time, the electron had not yet been discovered, and the concept of “thermionic emission,” naturally, could appear only after the discovery of the electron.

That's the essence of it. In a hot metal thread, the speed and energy of electrons increase so much that they break away from the surface of the thread and rush into the space surrounding it in a free flow. The electrons escaping from the thread can be likened to rockets that have overcome the force of gravity. If a plus battery is connected to the electrode, then the electric field inside the cylinder between the filament and the electrode will direct electrons towards it. That is, an electric current will flow inside the lamp.

The flow of electrons in a vacuum is a type of electric current. Such an electric current in a vacuum can be obtained if a heated cathode, which is a source of “evaporating” electrons, and an anode are placed in a vessel from which air is carefully pumped out. An electric field is created between the cathode and anode, which imparts speed to the electrons in a certain direction.

In television tubes, radio tubes, installations for melting metals with an electron beam, and many other installations, electrons move in a vacuum. How are electron flows obtained in a vacuum? How are these flows managed?

Fig.3

We know that metals have conduction electrons. average speed The movement of these electrons depends on the temperature of the metal: the higher the temperature, the greater it is. Let us place two metal electrodes in a vacuum at a certain distance from each other (Fig.3 ) and create a certain potential difference between them. There will be no current in the circuit, which indicates the absence of free electric charge carriers in the space between the electrodes. Consequently, there are free electrons in metals, but they are retained inside the metal and at ordinary temperatures practically

can't get out of it. In order for electrons to escape beyond the metal (similar to the escape of molecules from a liquid during its evaporation), they must overcome the forces of electrical attraction from the excess positive charge that has arisen in the metal as a result of the escape of electrons, as well as the repulsive forces from the electrons that have escaped earlier and formed an electron “cloud” near the metal surface. In other words, in order to fly out of a metal into a vacuum, an electron must do a certain amount of work.Aagainst these forces, naturally, different for different metals. This work is calledwork function electrons from metal. The work function is performed by electrons due to their kinetic energy. Therefore, it is clear that slow electrons cannot escape from the metal, and only those whose kinetic energyE To exceeds the work function, that isE To ≥ A. The release of free electrons from a metal is calledelectron emission .

In order for electron emission to exist, it is necessary to impart kinetic energy to the conduction electrons of metals sufficient to perform the work function. Depending on the method of imparting the necessary kinetic energy to electrons, there are different types of electron emission. If energy is imparted to conduction electrons due to bombardment of the metal from the outside by some other particles (electrons, ions),secondary electron emission . Electron emission can occur under the influence of irradiation of the metal with light. In this case it is observedphotoemission , orphotoelectric effect . It is also possible for electrons to be ejected from a metal under the influence of a strong electric field -auto-electronic emissions . Finally, electrons can gain kinetic energy by heating the body. In this case they talk aboutthermionic emission .

Let us consider in more detail the phenomenon of thermionic emission and its application.

At ordinary temperatures, a tiny number of electrons can have kinetic energy comparable to the work function of electrons from a metal. With increasing temperature, the number of such electrons increases and when the metal is heated to temperatures of the order of 1000 - 1500 degrees, a significant number of electrons will already have an energy exceeding the work function of the metal. It is these electrons that can fly out of the metal, but they do not move away from its surface, since the metal becomes positively charged and attracts electrons. Therefore, a “cloud” of electrons is created near the heated metal. Some of the electrons from this “cloud” return back to the metal, and at the same time new electrons fly out of the metal. In this case, a dynamic equilibrium is established between the electron “gas” and the electron “cloud”, when the number of electrons escaping from the metal in a certain time is compared with the number of electrons that return from the “cloud” to the metal in the same time.