Vacuum conditions electric current in vacuum. What is electric current in a vacuum

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 on the capacitor plates, and continues until the potential difference between the capacitor plates becomes zero.

These examples show that an electric current in a conductor occurs only if there are different potentials at the ends of the conductor, that is, when there is an electric field in it.

But in the examples considered, the current cannot be long-term, 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 action of the forces of the same electric field, such a transfer of charges is impossible, since the potential of the second body is less than the potential of the first. Therefore, the 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. Therefore, it must have energy.

Current sources are galvanic cells, batteries, generators, etc.

So, the main conditions for the occurrence of electric current: 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. So, for example, in some liquids, when a current passes through them, a release of a substance is observed on electrodes immersed in a liquid. The current in gases is often accompanied by the glow of gases, etc. Electric current in gases and vacuum was studied by the outstanding French physicist and mathematician - André Marie Ampère, thanks to whom we now know the nature of such phenomena.

As you know, vacuum is the best insulator, that is, the space from which air is 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 the air is pumped out. Two metal plates are soldered into this vessel - two electrodes. One of them A (anode) is connected to a positive current source, the other K (cathode) - to a negative one. The voltage between is enough to apply 80 - 100 V.

We include a sensitive milliammeter in 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 - a thread, with the ends brought out to the outside - into the vessel. This filament will still remain the cathode. With the help of another current source, we heat it up. We will notice that as soon as the filament is heated, the instrument connected to the circuit shows an electric current in a vacuum, and the greater, the more heated the filament. This means that when heated, the filament ensures the presence of charged particles in 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 the anode, and the opposite pole - the 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 the electronic theory, free electrons in a metal are in chaotic motion. When the thread is heated, this movement increases. At the same time, some electrons, acquiring enough energy 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, the electrons fly towards the electrode A, if it is connected to the positive pole of the battery, and are repelled back to the filament if it is attached to the negative pole, i.e., has a charge of the same name as the electrons.

So, an electric current in a vacuum is a directed flow of electrons.

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, we will consider the main technical devices operating on the principles of current in a 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.

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):

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, 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

vacuum diode

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

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

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.

Rice. 11. Oscilloscope ()

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.: Mnemozina, 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 ().
2. Cathedral.narod.ru ().

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?

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, 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.

This is a short summary.

Work on the full version continues

Lecture20

current in vacuum

1. A note about vacuum

There is no electric current in a vacuum, because there are no particles in thermodynamic vacuum.

However, the best vacuum achieved in practice is

,

those. a huge number of particles.

Nevertheless, when talking about current in a vacuum, they mean an ideal vacuum in the thermodynamic sense, i.e. complete absence of particles. The particles obtained from any source are responsible for the flow of current.

2. Work function

As you know, in metals there is an electron gas, which is held by the force of attraction to the crystal lattice. Under normal conditions, the energy of electrons is not high, so they are kept inside the crystal.

If we approach the electron gas from classical positions, i.e. consider that it obeys the Maxwell-Boltzmann distribution, then it is obvious that there is a large proportion of particles whose velocities are above average. Consequently, these particles have sufficient energy to break out of the crystal and form an electron cloud near it.

The surface of the metal is positively charged. A double layer is formed, which prevents the removal of electrons from the surface. Therefore, in order to remove an electron, it is necessary to impart additional energy to it.

Definition: Work function of electrons from metal called the energy that must be imparted to an electron in order to remove it from the surface of the metal to infinity in a state with zeroE k.

For different metals, the work function is different.

Metal	Work function, eV
	1,81

3. Electronic emission.

Under normal conditions, the energy of electrons is quite small and they are bound inside the conductor. There are ways to impart additional energy to electrons. The phenomenon of electron emission under external influence is called electron emission, and was discovered by Edison in 1887. Depending on the method of energy transmission, 4 types of emission are distinguished:

1. Thermionic emission (TEE), method - heat supply (heating).

2. Photoelectronic emission (PEE), method - illumination.

3. Secondary electron emission (SEE), method - particle bombardment.

4. Autoelectronic emission (AEE), method - strong electric field.

4. Field emission

Under the action of a strong electric field, electrons can escape from the surface of the metal.

This magnitude of tension is enough to pull out an electron.

This phenomenon is called cold emission. If the field is strong enough, then the number of electrons can become large, and, consequently, the current can be large. According to the Joule-Lenz law, a large amount of heat will be released and AEE can turn into TEE.

5. Photoelectronic emission (PEE)

The phenomenon of the photoelectric effect has been known for a long time, see "Optics".

6. Secondary electron emission (SEE)

This phenomenon is used in photoelectron multiplications (PMTs).

During operation, an avalanche-like increase in the number of electrons occurs. It is used to register weak light signals.

7. Vacuum diode.

To study TEE, a device called a vacuum diode is used. Most often, structurally, it consists of two coaxial cylinders placed in a glass vacuum flask.

The cathode is heated by electric current directly or indirectly. With direct - the current passes through the cathode itself, with indirect - an additional conductor is placed inside the cathode - a filament. Heating occurs to sufficiently high temperatures, so the cathode is made complex. The base is a refractory material (tungsten), and the coating is a material with a low work function (cesium).

The diode refers to non-linear elements, i.e. it does not obey Ohm's law. It is said that a diode is an element with one-way conduction. Most of the CVC of a diode is described by the Boguslavsky-Langmuir law or the 3/2 law

As the filament temperature rises, the I–V characteristic shifts upwards and the saturation current increases. The dependence of saturation current density on temperature is described by the Richardson-Deshman law

Quantum statistics methods can be used to obtain this formula withconst= Bthe same for all metals. Experiment shows that the constants different.

8. Half-wave rectifier

9. full wave rectifier (independently).

10. Application of lamps.

The advantages of lamps include

· ease of control of the flow of electrons,

· big power,

· a large section of almost linear CVC.

· Tubes are used in powerful amplifiers.

The disadvantages include:

low efficiency,

· high energy consumption.

Electric current in a vacuum

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

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

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

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

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

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

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

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

2. Electric current in a vacuum

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

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

3. Vacuum diode

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

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

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

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

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

4. Cathode ray tube

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

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

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

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