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 fly from wall to wall many times before meeting 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, imparting 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. The average speed of 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 kept inside the metal and at ordinary temperatures practically
can't get out of it. In order for electrons to escape from 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, is 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.
In this lesson we continue to study the flow of currents in various media, specifically in a vacuum. We will consider the mechanism of formation of free charges, consider the main technical devices that operate on the principles of current in a vacuum: a diode and a cathode ray tube. We will also indicate the basic properties of electron beams.
The result of the experiment is explained as follows: as a result of heating, the metal begins to emit electrons from its atomic structure, similar to the emission of water molecules during evaporation. The heated metal is surrounded by an electron cloud. This phenomenon is called thermionic emission.
Rice. 2. Scheme of Edison's experiment
Property of electron beams
In technology, the use of so-called electron beams is very important.
Definition. An electron beam is a stream of electrons whose length is much greater than its width. It's pretty easy to get. It is enough to take a vacuum tube through which current flows and make a hole in the anode, to which the accelerated electrons go (the so-called electron gun) (Fig. 3).
Rice. 3. Electron gun
Electron beams have a number of key properties:
As a result of their high kinetic energy, they have a thermal effect on the material they impact. This property is used in electronic welding. Electronic welding is necessary in cases where maintaining the purity of materials is important, for example, when welding semiconductors.
- When colliding with metals, electron beams slow down and emit X-rays used in medicine and technology (Fig. 4).
Rice. 4. Photo taken using X-rays ()
- When an electron beam hits certain substances called phosphors, a glow occurs, which makes it possible to create screens that help monitor the movement of the beam, which, of course, is invisible to the naked eye.
- The ability to control the movement of beams using electric and magnetic fields.
It should be noted that the temperature at which thermionic emission can be achieved cannot exceed the temperature at which the metal structure is destroyed.
At first, Edison used the following design to generate current in a vacuum. A conductor connected to a circuit was placed on one side of the vacuum tube, and a positively charged electrode was placed on the other side (see Fig. 5):
Rice. 5
As a result of the passage of current through the conductor, it begins to heat up, emitting electrons that are attracted to the positive electrode. In the end, a directed movement of electrons occurs, which, in fact, is an electric current. However, the number of electrons thus emitted is too small, resulting in too little current for any use. This problem can be overcome by adding another electrode. Such a negative potential electrode is called an indirect filament electrode. With its use, the number of moving electrons increases significantly (Fig. 6).
Rice. 6. Using an indirect filament electrode
It is worth noting that the conductivity of current in a vacuum is the same as that of metals - electronic. Although the mechanism for the appearance of these free electrons is completely different.
Based on the phenomenon of thermionic emission, a device called a vacuum diode was created (Fig. 7).
Rice. 7. Designation of a vacuum diode on an electrical diagram
Vacuum diode
Let's take a closer look at the vacuum diode. There are two types of diodes: a diode with a filament and anode and a diode with a filament, anode and a cathode. The first is called a direct filament diode, the second is called an indirect filament diode. In technology, both the first and second types are used, however, the direct filament diode has the disadvantage that when heated, the resistance of the filament changes, which entails a change in the current through the diode. And since some operations using diodes require a completely constant current, it is more advisable to use the second type of diodes.
In both cases, the filament temperature for effective emission must be equal to 
.
Diodes are used to rectify alternating currents. If a diode is used to convert industrial currents, then it is called a kenotron.
The electrode located near the electron-emitting element is called the cathode (), the other is called the anode (). When connected correctly, the current increases as the voltage increases. When connected in reverse, no current will flow at all (Fig. 8). In this way, vacuum diodes compare favorably with semiconductor diodes, in which, when turned back on, the current, although minimal, is present. Due to this property, vacuum diodes are used to rectify alternating currents.
Rice. 8. Current-voltage characteristic of a vacuum diode
Another device created based on the processes of current flow in a vacuum is an electric triode (Fig. 9). Its design differs from the diode design in the presence of a third electrode, called a grid. A device such as a cathode ray tube, which makes up the bulk of devices such as an oscilloscope and tube televisions, is also based on the principles of current in a vacuum.
Rice. 9. Vacuum triode circuit
Cathode-ray tube
As mentioned above, based on the properties of current propagation in a vacuum, such an important device as a cathode ray tube was designed. It bases its work on the properties of electron beams. Let's look at the structure of this device. A cathode ray tube consists of a vacuum flask with an expansion, an electron gun, two cathodes and two mutually perpendicular pairs of electrodes (Fig. 10).
Rice. 10. Structure of a cathode ray tube
The operating principle is as follows: electrons emitted from the gun due to thermionic emission are accelerated due to the positive potential at the anodes. Then, by applying the desired voltage to the control electrode pairs, we can deflect the electron beam as desired, horizontally and vertically. After which the directed beam falls on the phosphor screen, which allows us to see the image of the beam trajectory on it.
A cathode ray tube is used in an instrument called an oscilloscope (Fig. 11), designed to study electrical signals, and in CRT televisions, with the only exception that the electron beams there are controlled by magnetic fields.
Rice. 11. Oscilloscope ()
In the next lesson we will look at the passage of electric current in liquids.
Bibliography
- Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemosyne, 2012.
- Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Ilexa, 2005.
- Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.
- Physics.kgsu.ru ().
- Cathedral.narod.ru ().
Homework
- What is electronic emission?
- What are the ways to control electron beams?
- How does the conductivity of a semiconductor depend on temperature?
- What is an indirect filament electrode used for?
- *What is the main property of a vacuum diode? What is it due to?
Electric current can pass in a vacuum provided that free charge carriers are placed in it. After all, a vacuum is the absence of any substance. This means that there are no charge carriers to provide the current. The concept of vacuum can be defined as when the free path of a 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 this we need an electrometer and a vacuum tube. That is, a glass flask with a vacuum containing two electrodes. One of which is made in the form of a metal plate, let's call it the anode. And the second in the form of a wire spiral made of refractory material will be called the cathode.
Let's connect the electrodes of the lamp to the electrometer so that the cathode is connected to the body of the electrometer, and the anode to the rod. Let's give 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 arise for the electrometer to discharge.
Figure 1 - a vacuum tube connected to a charged electrometer
Now let's connect a current source to the cathode in the form of a wire spiral. In this case, the cathode will heat up. And we will see that the charge of the electrometer will begin to decrease until it disappears completely. How could this happen because there were no charge carriers in the gap between the electrodes of the lamp to provide conduction current.
It is obvious that charge carriers somehow appeared. This happened because when the cathode was heated, electrons were emitted from the surface of the cathode into the space between the electrodes. As you know, metals have free conduction electrons. Which are capable of moving in the volume of metal between lattice nodes. 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 undergo chaotic thermal motion as they move along a conductor. Approaching the metal boundary, 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 unlike charges closer together. But if the metal is heated, the thermal movement 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 escaped from the surface of the conductor, and in the absence of an external electric field they will return back into 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. There would be no field inside.
But because there is a charge on the electrometer, it creates a field that causes the electrons to move. Remember, at the anode we have a positive charge towards it, and electrons tend to flow under the influence of the field. Thus, an electric current is observed in a vacuum.
If we say we connect the electrometer in reverse, what will happen? It turns out that there will be a negative potential at the anode of the lamp, and a positive one at the cathode. All electrons emitted from the cathode surface will immediately return back under the influence of the field. Since the cathode will now have an even greater positive potential, it will attract electrons. And at 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 tube
This lamp is called a vacuum diode. It is capable of passing current only in one direction. The current-voltage characteristic of such a lamp consists of two sections. In the first section, Ohm's law is satisfied. That is, as the voltage increases, more and more electrons emitted from the cathode reach the anode and thereby the current increases. In the second section, all electrons emitted from the cathode reach the anode and with a further increase in voltage, the current does not increase. There just aren't the right number of electrons. This area is called saturation.
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 becomes positively charged, 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 is 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 electronic 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 liquids;
- 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 coated with a phosphor that 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).
Cathode ray tubes produce narrow electron beams controlled by electric and magnetic fields. These beams are used in: TV picture tubes, computer displays, electronic oscilloscopes in measuring equipment.
Electric current is the ordered movement of electric charges. It can be obtained, for example, in a conductor that connects a charged and uncharged body. However, this current will stop as soon as the potential difference between these bodies becomes zero. An ordered current will also exist in the conductor connecting the plates of a charged capacitor. In this case, the current is accompanied by the neutralization of the charges located on the capacitor plates and continues until the potential difference of the capacitor plates becomes zero.
These examples show that an electric current in a conductor occurs only when there are different potentials at the ends of the conductor, i.e., when there is an electric field in it.
But in the examples considered, the current cannot be long-lasting, since in the process of moving charges, the potentials of the bodies quickly equalize and the electric field in the conductor disappears.
Therefore, to obtain current, it is necessary to maintain different potentials at the ends of the conductor. To do this, you can transfer charges from one body to another back through another conductor, forming a closed circuit for this. However, under the influence of the forces of the same electric field, such charge transfer is impossible, since the potential of the second body is less than the potential of the first. Therefore, transfer is possible only by forces of non-electric origin. The presence of such forces is provided by a current source included in the circuit.
The forces acting in the current source transfer charge from a body with a lower potential to a body with a higher potential and do work at the same time. Therefore, it must have energy.
Current sources are galvanic cells, batteries, generators, etc.
So, the main conditions for the occurrence of electric current are: the presence of a current source and a closed circuit.
The passage of current in a circuit is accompanied by a number of easily observable phenomena. For example, in some liquids, when a current passes through them, a release of a substance is observed on the electrodes lowered into the liquid. Current in gases is often accompanied by glow of gases, etc. Electric current in gases and vacuum was studied by the outstanding French physicist and mathematician Andre Marie Ampere, thanks to whom we now know the nature of such phenomena.
As you know, vacuum is the best insulator, i.e. the space from which air has been pumped out.
But it is possible to obtain an electric current in a vacuum, for which it is necessary to introduce charge carriers into it.
Let's take a vessel from which air has been pumped out. Two metal plates are soldered into this vessel - two electrodes. We connect one of them A (anode) to a positive current source, the other K (cathode) to a negative one. The voltage between is sufficient to apply 80 - 100 V.
Let's connect a sensitive milliammeter to the circuit. The device does not show any current; this indicates that electric current does not exist in a vacuum.
Let's change the experience. As a cathode, we solder a wire into the vessel - a thread, with the ends brought out. This filament will still be the cathode. Using another current source, we heat it up. We will notice that as soon as the filament is heated, the device connected to the circuit shows an electric current in a vacuum, and the greater the more the filament is heated. This means that when heated, the thread ensures the presence of charged particles in the vacuum; it is their source.
How are these particles charged? Experience can provide the answer to this question. Let's change the poles of the electrodes soldered into the vessel - we will make the thread an anode, and the opposite pole - a cathode. And although the filament is heated and sends charged particles into the vacuum, there is no current.
It follows that these particles are negatively charged because they are repelled from electrode A when it is negatively charged.
What are these particles?
According to electronic theory, free electrons in a metal are in chaotic motion. When the filament is heated, this movement intensifies. At the same time, some electrons, acquiring energy that is sufficient to exit, fly out of the thread, forming an “electron cloud” around it. When an electric field is formed between the filament and the anode, electrons fly to electrode A if it is connected to the positive pole of the battery, and are repelled back to the filament if it is connected to the negative pole, i.e., it has the same charge as the electrons.
So, electric current in a vacuum is a directed flow of electrons.