Current, electric current in 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 cease as soon as the potential difference of these bodies becomes zero. The ordered motion of charges (electric current) will also exist in a conductor connecting the plates of a charged capacitor. In this case, the current is accompanied by the neutralization of charges on the plates of the capacitor, and continues until the potential difference of the capacitor plates becomes zero.

These examples show that the electric current in the conductor arises only when 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 can not be long, since in the process of charge transfer the potentials of the bodies quickly equalize and the electric field in the conductor disappears.

Consequently, to obtain the current, it is necessary to maintain different potentials at the ends of the conductor. For this, it is possible to 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, transfer is possible only by forces of non-electrical 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 a lower potential to the body with a larger potential and perform the work in this case. Therefore, the current source must have energy.

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

So, the basic conditions for the appearance of an 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 observable phenomena. Thus, for example, in some liquids, when a current flows through them, a release of matter is observed at the electrodes lowered into the liquid. The current in gases is often accompanied by glow of gases, etc. The outstanding electrician in gases and vacuum was the outstanding French physicist and mathematician, André Marie Amper, thanks to whom we now know the nature of such phenomena.

As is known, 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.

We take the vessel from which the air is pumped. In this vessel are soldered two metal plates - two electrodes. One of them A (anode) is connected with a positive current source, the other K (cathode) - with a negative one. The voltage between the cathode and the anode is sufficient to apply 80 - 100 V.

We switch on the sensitive milliammeter. The device does not show any current; This indicates that there is no electric current in a vacuum.

We will modify the experience. As a cathode we solder into the wire a wire - a thread, with the ends pulled out. This thread will remain a cathode. With the help of another source of current, we will heat it. We note that, once the filament is heated, the device included in the circuit shows an electric current in a vacuum, and the greater the stronger the filament is heated. Hence, the filament, when heated, ensures the presence of charged particles in vacuum, it is their source.

How are these particles charged? The answer to this question can give experience. Let us change the poles of the electrodes soldered into the vessel-we make the filament anode, and the opposite pole-the cathode. And although the filament is hot and sends charged particles into the vacuum, there is no current.

It follows that these particles are negatively charged, because they repel the 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. As the filament grows, this movement intensifies. At the same time, some electrons, acquiring energy, which is sufficient to make an exit, fly out of the filament, forming an "electronic cloud" near it. When an electric field is formed between the filament and the anode, the electrons fly to the electrode A, if it is attached to the positive pole of the battery, and repel back to the thread if it is attached to the negative pole, ie, it has a charge of the same name with electrons.

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