Current, electric current in a vacuum. Electron emission Vacuum conditions electric current in vacuum

Before talking about the mechanism by which it spreads electricity in a vacuum, you need to understand what kind of environment 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 was placed 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

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

Physics.kgsu.ru ().
Cathedral.narod.ru ().
Encyclopedia of Physics and Technology ().

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?

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 free charges, consider the main technical devices operating on the principles of current in vacuum: a diode and a cathode ray tube. We also indicate the main 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, by analogy with the emission of water molecules during evaporation. The heated metal surrounds the electron cloud. This phenomenon is called thermionic emission.

Rice. 2. Scheme of the Edison experiment

Property of electron beams

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

Rice. 3. Electron gun

Electron beams have a number of key properties:

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.

Rice. 5

Rice. 6. Using an indirect glow plug

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 the vacuum diode on the electrical circuit

vacuum diode

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.

Rice. 8. Current-voltage characteristic of a vacuum diode

Rice. 9. Diagram of a vacuum triode

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. 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

Rice. 11. Oscilloscope ()

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

Bibliography

Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemozina, 2012.
Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 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?

Any current appears only in the presence of a source with free charged particles. This is due to the fact that there are no substances in vacuum, including electric charges. Therefore, the vacuum is considered the best. In order for it to become possible for the passage of an electric current a, it is necessary to ensure the presence of a sufficient number of free charges. In this article we will look at what constitutes an electric current in a vacuum.

How electric current can appear in a vacuum

In order to create a full-fledged electric current in a vacuum, it is necessary to use such physical phenomenon like thermionic emission. It is based on the property of a certain substance to emit free electrons when heated. Such electrons emerging from a heated body are called thermoelectrons, and the entire body is called an emitter.

Thermionic emission underlies the operation of vacuum devices, better known as vacuum tubes. The simplest design contains two electrodes. One of them is the cathode, which is a spiral, the material of which is molybdenum or tungsten. It is he who is heated by an electric current ohm. The second electrode is called the anode. It is in a cold state, performing the task of collecting thermionic electrons. As a rule, the anode is made in the form of a cylinder, and a heated cathode is placed inside it.

Application of current in vacuum

In the last century, vacuum tubes played a leading role in electronics. And, although they have long been replaced by semiconductor devices, the principle of operation of these devices is used in cathode ray tubes. This principle is used in welding and melting work in vacuum and other areas.

Thus, one of the varieties of current a is an electron flow flowing in vacuum. When the cathode is heated, an electric field appears between it and the anode. It is this that gives the electrons a certain direction and speed. According to this principle, an electronic lamp with two electrodes (diode) works, which is widely used in radio engineering and electronics.

The modern device is a cylinder made of glass or metal, from which air has been previously pumped out. Two electrodes, a cathode and an anode, are soldered inside this cylinder. For amplification specifications additional grids are installed, with the help of which the electron flux is increased.

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. There are many electrons, even at 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 an electric current 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 .

The distribution of the potential energy of an electron U for a limited metal is shown in fig. 1.

Electron potential energy diagram U in a bounded metal

Rice. 1

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 the hot metal are accelerated 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 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 cathode temperature, 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.