How to get an electric current in a vacuum. Subject. Electric current in a vacuum
Subject. Electricity 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 a kinetic energy electron will exceed or be 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 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.
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 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?
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 a physical phenomenon as 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.
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 action 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 the 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.
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.
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 you connect the lamp differently, connect the positive pole to the cathode, and the negative 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 this moment Vacuum diodes are practically not used anywhere. Instead, all progressive mankind uses semiconductor diodes.