Electric current is an 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 of these bodies becomes zero. The ordered movement of charges (electric current) will also exist in the conductor connecting the plates of the charged capacitor. In this case, the current is accompanied by the neutralization of the charges located on the capacitor plates, and continues until the potential difference of the capacitor plates becomes zero.
These examples show that an electric current in a conductor arises only if there are different potentials at the ends of the conductor, i.e., when there is an electric field in it.
But in the examples considered, the current cannot be long, since during the movement of 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 along another conductor, forming a closed circuit for this. However, under the influence of the forces of the same electric field, such a charge transfer 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 provides a current source included in the circuit.
The forces acting in the current source transfer the charge from the body with lower potential to the body with high potential and do the job. Therefore, the current source must have energy.
Current sources are electrical machines, 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 the circuit is accompanied by a number of easily observed phenomena. So, for example, in some liquids, when current flows through them, a substance is released on electrodes lowered into the liquid. The current in gases is often accompanied by a glow of gases, etc. The electric current in gases and vacuum was studied by the outstanding French physicist and mathematician Andre Marie Ampère, thanks to whom we now know the nature of such phenomena.
As you know, vacuum is the best insulator, i.e. 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.
Take a vessel from which air is pumped out. Two metal plates are soldered into this vessel - two electrodes. We connect one of them A (anode) to a positive current source, and the other K (cathode) to a negative one. The voltage between the cathode and the anode 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.
Change the experience. As a cathode, we solder a wire into the vessel - a thread, with the ends brought out. This thread will still remain the cathode. Using another current source we glow it. We will notice that as soon as the filament is heated, the device connected to the circuit shows an electric current in vacuum, and the greater the stronger the filament is heated. This means that the thread, when heated, ensures the presence of charged particles in a vacuum, it is their source.
How are these particles charged? The answer to this question can give experience. We change the poles of the electrodes soldered into the vessel - we make the thread an anode, and the opposite pole - a cathode. And although the thread is heated and sends charged particles into a vacuum, there is no current.
It follows that these particles are negatively charged because they are repelled by electrode A when it is negatively charged.
What are these particles?
According to electronic theory, free electrons in a metal are in a chaotic motion. When the filament is heated, this movement intensifies. At the same time, some electrons, acquiring energy, which is enough to complete the exit, fly out of the filament, forming an “electron cloud” near it. When an electric field is formed between the filament and the anode, the electrons fly to electrode A if it is attached to the positive pole of the battery, and are repelled back to the filament if it is attached to the negative pole, i.e. it has a charge of the same name as electrons.
So, an electric current in a vacuum is a directed flow of electrons.