4.1
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4.1
Average rating: 4.1
Total ratings received: 927.
Electric current is a flow that is caused by the ordered movement of electrically charged particles. The movement of charges is taken as the direction of the electric current. Electric current can be short-term or long-term.
Electric current concept
During a lightning discharge, an electric current can occur, which is called short-term. And to maintain current for a long time, the presence of an electric field and free electric charge carriers is necessary.
An electric field is created by differently charged bodies. The current strength is the ratio of the charge transferred through the cross-section of a conductor over a time interval to this time interval. It is measured in Amperes.
Rice. 1. Current formula
The concept of plasma
- Plasma
is a partially or fully ionized gas in which the densities of positive and negative charges are almost equal. Therefore, in general, plasma is an electrically neutral system.
The degree of plasma ionization
α
is determined by the ratio of the number of ionized atoms to their total number\[~\alpha = \frac{N_i}{N}\].
Depending on the degree of ionization, plasma is divided into weakly ionized
(α - fractions of a percent),
partially ionized
(α - several percent) and
completely ionized
(α = 100%). The ionosphere, the upper layer of the earth's atmosphere, is a weakly ionized plasma. The Sun and hot stars are in a state of completely ionized plasma. The sun and stars are giant clots of hot plasma, where the temperature is very high, about 106 - 107 K. Artificially created plasma of varying degrees of ionization is plasma in gas discharges and gas-discharge lamps.
The existence of plasma is associated either with the heating of the gas, or with radiation of various kinds, or with the bombardment of the gas by fast charged particles.
A number of properties of plasma allow us to consider it as a special state of matter. Plasma is the most common state of matter. Plasma exists not only as the substance of stars and the Sun, it also fills the space between stars and galaxies. The upper layer of the Earth's atmosphere is also weakly ionized plasma.
Controlling the motion of plasma in electric and magnetic fields is the basis for its use as a working fluid in various engines for directly converting internal energy into electrical energy - plasma sources of electricity, magnetohydrodynamic generators. The use of low-power plasma engines is promising for spacecraft. A powerful jet of dense plasma produced in a plasma torch is widely used for cutting and welding metals, drilling wells, and accelerating many chemical reactions. Large-scale research is being conducted on the use of high-temperature plasma to create controlled thermonuclear reactions.
Self-sustaining and non-self-sustaining gas discharges
Discharges in gas can be independent or non-self-sustaining. Current begins to exist when free charges appear. Non-self-sustaining discharges exist as long as an external force acts on it, that is, an external ionizer. That is, if the external ionizer stops working, then the current stops.
Self-discharge of electric current in gases exists even after the cessation of the external ionizer. Independent discharges in physics are divided into quiet, glow, arc, spark, corona.
- Quiet is the weakest of the independent discharges. The current strength in it is very small (no more than 1 mA). It is not accompanied by sound or light phenomena.
- Glow - if you increase the voltage in a quiet discharge, it goes to the next level - into a glow discharge. In this case, a glow appears, which is accompanied by recombination. Recombination is the reverse process of ionization, the meeting of an electron and a positive ion. Used in bactericidal and lighting lamps.
Rice. 2. Glow discharge
- Arc - the current strength ranges from 10 A to 100 A. Ionization is almost 100%. This type of discharge occurs, for example, when operating a welding machine.
Rice. 3. Arc discharge
- Spark - can be considered one of the types of arc discharge. During such a discharge, a certain amount of electricity flows in a very short time.
- Corona discharge - ionization of molecules occurs near electrodes with small radii of curvature. This type of charge occurs when the electric field strength changes suddenly.
Ionization and recombination
Gases only become conductors when they are ionized in some way. The process of ionization of gases involves the removal of one or more electrons from an atom for some reason. As a result, a positive ion
and
electron
.
- The breakdown of molecules into ions and electrons is called gas ionization
.
Some of the resulting electrons can be captured by other neutral atoms, and then negatively charged ions
.
Thus, in an ionized gas there are three types of charge carriers: electrons, positive ions and negative ones.
The removal of an electron from an atom requires the expenditure of a certain amount of energy— the ionization energy
Wi
. The ionization energy depends on the chemical nature of the gas and the energy state of the electron in the atom. Thus, to remove the first electron from a nitrogen atom, the energy required is 14.5 eV, to remove the second electron - 29.5 eV, and to remove the third - 47.4 eV.
Factors that cause gas ionization are called ionizers
.
There are three types of ionization: thermal ionization, photoionization and impact ionization.
- Thermal ionization
occurs as a result of the collision of atoms or molecules of a gas at high temperature if the kinetic energy of the relative motion of the colliding particles exceeds the binding energy of the electron in the atom.
- Photoionization
occurs under the influence of electromagnetic radiation (ultraviolet, x-ray or γ-radiation), when the energy required to remove an electron from an atom is transferred to it by a radiation quantum.
- Electron impact ionization
(or
impact ionization
) is the formation of positively charged ions as a result of collisions of atoms or molecules with fast electrons with high kinetic energy.
The process of gas ionization is always accompanied by the opposite process of reduction of neutral molecules from oppositely charged ions due to their electrical attraction. This phenomenon is called recombination
. During recombination, energy is released equal to the energy spent on ionization. This can cause, for example, gas to glow.
If the action of the ionizer is unchanged, then a dynamic equilibrium is established in the ionized gas, in which the same number of molecules are restored per unit time as they disintegrate into ions. In this case, the concentration of charged particles in the ionized gas remains unchanged. If the action of the ionizer is stopped, then recombination will begin to dominate over ionization and the number of ions will quickly decrease to almost zero. Consequently, the presence of charged particles in a gas is a temporary phenomenon (while the ionizer is operating).
In the absence of an external field, charged particles move chaotically.
Literature
- Aksenovich L. A. Physics in secondary school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyakhavanne, 2004. - P. 287-292.
- Burov L.I., Strelchenya V.M. Physics from A to Z: for students, applicants, tutors. - Mn.: Paradox, 2000. - P. 232-239.
- Myakishev G. Ya. Physics: Electrodynamics. 10 – 11 grades: textbook for in-depth study of physics / G.Ya. Myakishev, A.Z. Sinyakov, B.A. Slobodskov. - M.: Bustard, 2005. - P. 276-284.
Interstellar plasma
Space is filled with plasma
Not so long ago, scientists from all over the world agreed that interstellar space is a perfect vacuum. Moreover, many experts still adhere to this point of view, but as recent studies show, this is not entirely true.
- Space is not empty and its space is filled with plasma, very rarefied, but still.
- These are mainly light molecules of helium and hydrogen - their ions and electrons. The concentration is one particle per 1 cubic centimeter, which is 1013 times less than in earthly air.
- Space exploration has shown that Bickerland currents constantly flow between celestial bodies, and this is not prevented in any way by the low concentration of plasma, which, as we have found out, is an excellent conductor.
- There is active debate among scientists today about the charge of cosmic plasma. Thus, Hennes Alfvén and James McCanney consider it almost neutral and only slightly positive. This contradicts the official theory that the solar wind is completely neutral.
- For the first time, the positively charged cosmic plasma that makes up the solar wind was announced back in 1930 by geophysicist and mathematician Sidney Chapman. 1968 Nobel Prize laureate Luis Alvarez recently came to the same conclusion in his research. The same opinion is shared by many eminent scientists around the world.
In the photo - the Bickerland current flows through space
Behavior of electric current in plasma
Electric charges coil into threads
We already know that a plasma current discharge looks like a luminous thread connecting the electrodes. This chapter will explain why folding occurs.
- To make this phenomenon clear, it is necessary to recall the school physics course. In particular, we are interested in electromagnetism, and how the electromagnetic field is generated.
Magnetic field: right and left hand rules
- The figure above shows how current flowing through a wire creates a magnetic field perpendicular to it.
- The same thing happens in plasma, but it, unlike a rigid wire, does not have a specific shape.
- Plasma is collected into beams precisely thanks to the magnetic field, that is, it pulls it together, as if into a wire, and directs it to a certain point. This type of filamentary discharges is called Bickerland current.
Pulling plasma current into a cord
- What happens when two plasma filaments are nearby?
- The magnetic fields from them first begin to attract, trying to merge together. But the threads do not join into one, due to the fact that the magnetic fields rotate.
- As a result of interaction, the threads wrap around, creating a simple spiral. The resulting structure is called a plasma vortex.
Plasma vortex structure
- As soon as the threads come together at a sufficient distance, a certain repulsive force is formed, which prevents the flows from merging. At the same time, attraction and repulsion give a very stable structure, which allows the threads to be held at a certain distance. That is, they can neither merge nor separate.
- This phenomenon is very common in nature. With its help, you can explain the structure of hurricanes, vortexes, the rotation of stars, planets, the shape of galaxies and much, much more.
Electrical conductivity of gases
Gases under normal conditions are dielectrics.
Air is used in technology as an insulator: – power lines;
– between the plates of the capacitor;
– in switch contacts.
At high temperatures and under the influence of ultraviolet, x-ray and gamma radiation (external ionizers), gases become conductors.
This can be easily verified if you take a charged flat air capacitor with an electrometer connected to it, and heat the air between the plates.
