What is electric current in a vacuum. Electric current in gases. Electric current in vacuum. How electric current can appear in a vacuum

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 influence 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. At the moment, vacuum diodes are practically not used anywhere. Instead, all progressive mankind uses semiconductor diodes.

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?

Subject. Electric current in a vacuum

The purpose of the lesson: to explain to students the nature of electric current in a vacuum.

Type of lesson: lesson learning new material.

LESSON PLAN

STUDY NEW MATERIAL

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.

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.

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 volt-ampere 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.

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.

QUESTION TO STUDENTS DURING THE PRESENTATION OF NEW MATERIAL

First level

1. What is the purpose of high vacuum in electron tubes?

2. Why does a vacuum diode only conduct current in one direction?

3. What is the purpose of the electron gun?

4. How are electron beams controlled?

Second level

1. What features does the current-voltage characteristic of a vacuum diode have?

2. Will a radio lamp with broken glass work in space?

CONFIGURATION OF THE STUDYED MATERIAL

1. What needs to be done so that the trielectrode lamp can be used as a diode?

2. How can: a) increase the speed of electrons in the beam; b) change the direction of electron movement; c) stop moving electrons?

1. The maximum anode current in the vacuum diode is 50 mA. How many electrons are emitted from the cathode every second?

2. A beam of electrons, which are accelerated by a voltage U 1 \u003d 5 kV, flies into a flat capacitor in the middle between the plates and parallel to them. Capacitor length l = 10 cm, distance between plates d = 10 mm. For what minimum voltage U 2 on the capacitor will electrons not fly out of it?

Solutions. The motion of an electron resembles the motion of a body thrown horizontally.

The horizontal component v of the electron velocity does not change, it coincides with the electron velocity after acceleration. This speed can be determined using the law of conservation of energy: Here e is the elementary electric charge, me is the mass of the electron. The vertical acceleration a transfers to the electron the force F acting from the electric field of the capacitor. According to Newton's second law,

where is the electric field strength in the capacitor.

Electrons will not fly out of the capacitor if they are displaced by a distance d / 2.

So, is the time of electron movement in the capacitor. From here

After checking the units of quantities and substituting the numerical values, we get U 2 \u003d 100 B.

WHAT WE LEARNED IN THE LESSON

Vacuum is a gas so rarefied that the mean free path of molecules exceeds the linear dimensions of the vessel.

The energy that an electron needs to expend in order to leave the surface of the metal is called the work function.

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

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

The vacuum diode has one-way conduction.

A cathode ray tube allows you to control the movement of electrons. It was the CRT that made television possible.

Homework

1. Sub-1: § 17; sub-2: § 9.

Riv1 No. 6.12; 6.13; 6.14.

Riv2 No. 6.19; 6.20; 6.22, 6.23.

3. D: prepare for independent work No. 4.

ASSIGNMENTS FROM INDEPENDENT WORK No. 4 "LAWS OF DIRECT CURRENT"

Task 1 (1.5 points)

The movement of what particles creates an electric current in liquids?

A Movement of atoms.

Would the movement of molecules.

In The movement of electrons.

D Movement of positive and negative ions.

The figure shows an electric discharge in the air, created using a Tesla transformer.

And the electric current in any gas is directed in the direction where the negative ions move.

The conductivity of any gas is due to the movement of electrons only.

The conductivity of any gas is due to the movement of ions only.

D The conductivity of any gas is due to the movement of only electrons and ions.

Task 3 aims to establish a correspondence (logical pair). For each line marked with a letter, match the statement marked with a number.

A n-type semiconductors.

B Semiconductors p-type.

electronic conductivity.

D Hole conductivity.

1 Semiconductors in which holes are the majority charge carriers.

2 Semiconductors in which the majority charge carriers are electrons.

3 Conductivity of a semiconductor due to the movement of holes.

4 Conductivity of a semiconductor due to the movement of electrons.

5 Semiconductors in which the majority charge carriers are electrons and holes.

At what current strength was the electrolysis of an aqueous solution of CuSO 4 carried out, if in 2 min. 160 g of copper was released at the cathode?

Lesson #40-169 Electric current in gases. Electric current in vacuum.

Air-dielectric

Recombination of charged particles

4. arc - occurs between closely shifted electrodes - high current density, low voltage between the electrodes, (in searchlights, projection film equipment, welding, mercury lamps)

Electric current in vacuum.

vacuum diode.

CVC (voltage characteristic) of a vacuum diode.

electron beams is a stream of fast-flying electrons in vacuum 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 that hits a substance, X-ray radiation occurs; - causes glow (luminescence) of some solid and liquid bodies (phosphors); - heat the substance, falling on it.

Cathode Ray Tube (CRT)