What is electric current in a vacuum. Electric current in a vacuum What is an electric current in a vacuum

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

It follows from the definition of vacuum that there is practically no interaction between molecules, therefore, ionization of molecules cannot occur, therefore, free charge carriers cannot be obtained in vacuum, therefore, an electric current in it is impossible;
To create an electric current in a vacuum, you need to place a source of free charged particles in it. Metal electrodes connected to a current source are placed in a vacuum. One of them is heated (it is called a cathode), as a result of which an ionization process occurs, i.e. electrons are emitted from the substance, 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 leads to the fact that a heated metal electrode continuously emits electrons. The electrons form an electron cloud around the electrode. 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 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 \u003d 1.6 * 10 -19 J

The difference between the temperatures of hot and cold electrodes soldered into a vessel from which air is evacuated leads to one-sided 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 is connected to a heated one (cathode), then the electric field strength vector is directed towards the heated electrode. Under the influence of this field, the 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. With the opposite polarity of the source switching on, the field strength is directed from the heated electrode to the cold one. The electric field repels the electrons of the cloud back to the heated electrode. The circuit is open.

A device that conducts electric current in one direction is called a vacuum diode. It consists of an electron lamp (vessel), from which the air is pumped out and in which there are electrodes connected to a current source. Current-voltage characteristic of a vacuum diode. Sign sections of the I–V characteristics of the throughput mode of the diode and closed ?? 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 electrical current. Currently, vacuum diodes are practically not used.

If a hole is made in the anode of a vacuum tube, then part of the electrons accelerated by the electric field will fly into this hole, forming an electron beam behind the anode. The electron beam is flow of fast-flying electrons in electron 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 falling on a substance, X-rays are produced;
- causes glow (luminescence) of some solid and liquid bodies;
- heat the substance, falling on it.

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

In an electron gun, electrons emitted by a 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).
Narrow electron beams controlled by electric and magnetic fields are formed in cathode ray tubes. These beams are used in TV kinescopes, computer displays, electronic oscilloscopes in measuring technology.

Before talking about the mechanism by which an electric current propagates in a vacuum, it is necessary to understand what kind of medium it is.

Definition. Vacuum is the state of a gas in which the free path of a particle is greater than the size of the vessel. That is, such a state in which a molecule or atom of a gas flies from one wall of the vessel to another without colliding with other molecules or atoms. There is also the concept of vacuum depth, which characterizes the small number of particles that always remains in vacuum.

For the existence of an electric current, the presence of free charge carriers is necessary. Where do they come from in a region of space with a very low content of matter? To answer this question, it is necessary to consider the experiment conducted by the American physicist Thomas Edison (Fig. 1). During the experiment, two plates were placed in a vacuum chamber and closed outside it into a circuit with an electrometer turned on. After one plate was heated, the electrometer showed a deviation from zero (Fig. 2).

The result of the experiment is explained as follows: as a result of heating, the metal begins to emit electrons from its atomic structure, by analogy with the emission of water molecules during evaporation. The heated metal surrounds the electron lake. This phenomenon is called thermionic emission.

Rice. 2. Scheme of the Edison experiment

In technology, the use of so-called electron beams is of great importance.

Definition. An electron beam is a stream of electrons whose length is much greater than its width. Getting it is pretty easy. It is enough to take a vacuum tube through which the current passes, and make a hole in the anode, to which the dispersed 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 the presence of high kinetic energy, they have a thermal effect on the material into which they crash. This property is used in electronic welding. Electronic welding is necessary when maintaining the purity of materials is important, for example, when welding semiconductors.

When colliding with metals, electron beams, slowing down, emit X-rays used in medicine and technology (Fig. 4).

Rice. 4. A picture taken using x-rays ()

When an electron beam hits some substances called phosphors, a glow occurs, which makes it possible to create screens that help monitor the movement of the beam, of course, 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 construction to obtain current in a vacuum. A conductor included in the circuit was placed on one side of the vacuum tube, and a positively charged electrode on the other side (see Fig. 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, there is a directed movement of electrons, which, in fact, is an electric current. However, the number of electrons thus emitted is too small, giving too little current for any use. This problem can be overcome by adding another electrode. Such a negative potential electrode is called an indirect incandescent electrode. With its use, the number of moving electrons increases many times (Fig. 6).

Rice. 6. Using an indirect glow plug

It should be noted 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 the vacuum diode on the electrical circuit

Let's take a closer look at the vacuum diode. There are two types of diodes: a diode with a filament and an anode and a diode with a filament, an anode and a cathode. The first is called a direct filament diode, the second - indirect filament. In technology, both the first and second types are used, however, the direct filament diode has such a drawback that when heated, the resistance of the thread 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 appropriate to use the second type of diodes.

In both cases, the temperature of the filament for efficient emission must be .

Diodes are used to rectify alternating currents. If the 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, as the voltage increases, the current increases. With the reverse connection, the current will not flow at all (Fig. 8). In this way, vacuum diodes compare favorably with semiconductor diodes, in which, when switched 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 on the basis of the processes of current flow in a vacuum is an electric triode (Fig. 9). Its design differs from the diode one by the presence of a third electrode, called a grid. Also based on the principles of current in a vacuum is an instrument such as a cathode ray tube, which forms the main part of such instruments as an oscilloscope and tube televisions.

Rice. 9. Diagram of a vacuum triode

As mentioned above, based on the properties of current propagation in a vacuum, such a important device like a cathode ray tube. At the heart of her work, she uses the properties of electron beams. Consider the structure of this device. The cathode-ray tube consists of a vacuum flask with an extension, an electron gun, two cathodes, and two mutually perpendicular pairs of electrodes (Fig. 10).

Rice. 10. The structure of a cathode ray tube

The principle of operation is as follows: the electrons emitted from the gun as a result of thermionic emission are accelerated due to the positive potential at the anodes. Then, by applying the desired voltage to the pairs of control electrodes, we can deflect the electron beam as we like, horizontally and vertically. After that, the directed beam falls on the phosphor screen, which allows us to see the image of the beam trajectory on it.

The cathode ray tube is used in an instrument called an oscilloscope (Fig. 11), designed to study electrical signals, and in kinescopic televisions, with the only exception that there the electron beams are controlled by magnetic fields.

In the next lesson, we will analyze the passage of electric current in liquids.

Bibliography

1. Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M .: Mnemosyne, 2012.
2. Gendenstein L.E., Dick Yu.I. Physics grade 10. – M.: Ileksa, 2005.
3. Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. – M.: 2010.

1. Physics.kgsu.ru ().
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Homework

1. What is electronic emission?
2. What are the ways to control electron beams?
3. How does the conductivity of a semiconductor depend on temperature?
4. What is an indirect filament electrode used for?
5. *What is the main property of a vacuum diode? What is it due to?

Before semiconductor devices were used in radio engineering, vacuum tubes were used everywhere.

Concept of vacuum

The vacuum tube was a glass tube sealed at both ends, with the cathode on one side and the anode on the other. Gas was pushed out of the tube to such a state that gas molecules could fly from one wall to another without colliding. This state of the 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 such a property of bodies as thermionic emission.

Thermionic emission is the phenomenon of the emission of electrons by a body, under the action of high temperature. In very 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, as a result of which it began to constantly emit electrons. These electrons formed an electron cloud around the cathode. When connected to the electrodes of the power source, an electric field was formed between them.

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

If the lamp is connected differently, the positive pole is connected 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 conduction.

vacuum diode

Previously, one-way conduction was widely used in electronic devices with two electrodes. Such devices were called vacuum diodes. They performed at one time the role that semiconductor diodes now play.

Most often used to rectify electric current. IN this moment Vacuum diodes are practically not used anywhere. Instead, all progressive mankind uses semiconductor diodes.