Electric current in liquids. Movement of charges, anions cations. Electric current in liquids: its origin, quantitative and qualitative characteristics What creates a current in liquids
It is formed by the directed movement of free electrons and that in this case no changes in the substance from which the conductor is made do not occur.
Such conductors, in which the passage of an electric current is not accompanied by chemical changes in their substance, are called conductors of the first kind. These include all metals, coal and a number of other substances.
But there are also such conductors of electric current in nature, in which chemical phenomena occur during the passage of current. These conductors are called conductors of the second kind. These include mainly various solutions in water of acids, salts and alkalis.
If you pour water into a glass vessel and add a few drops of sulfuric acid (or some other acid or alkali) to it, and then take two metal plates and attach conductors to them by lowering these plates into the vessel, and connect a current source to the other ends of the conductors through a switch and an ammeter, then gas will be released from the solution, and it will continue continuously until the circuit is closed. acidified water is indeed a conductor. In addition, the plates will begin to be covered with gas bubbles. Then these bubbles will break away from the plates and come out.
When an electric current passes through the solution, chemical changes occur, as a result of which gas is released.
Conductors of the second kind are called electrolytes, and the phenomenon that occurs in the electrolyte when an electric current passes through it is.
Metal plates dipped into the electrolyte are called electrodes; one of them, connected to the positive pole of the current source, is called an anode, and the other, connected to the negative pole, is called cathode.
What causes the passage of electric current in a liquid conductor? It turns out that in such solutions (electrolytes), acid molecules (alkalis, salts) under the action of a solvent (in this case, water) decompose into two components, and one particle of the molecule has a positive electrical charge, and the other negative.
The particles of a molecule that have an electric charge are called ions. When an acid, salt or alkali is dissolved in water, a large number of both positive and negative ions appear in the solution.
Now it should become clear why an electric current passed through the solution, because between the electrodes connected to the current source, it was created, in other words, one of them turned out to be positively charged and the other negatively. Under the influence of this potential difference, positive ions began to move towards the negative electrode - the cathode, and negative ions - towards the anode.
Thus, the chaotic movement of ions has become an ordered counter-movement of negative ions in one direction and positive ones in the other. This charge transfer process constitutes the flow of electric current through the electrolyte and occurs as long as there is a potential difference across the electrodes. With the disappearance of the potential difference, the current through the electrolyte stops, the orderly movement of ions is disturbed, and chaotic movement sets in again.
As an example, consider the phenomenon of electrolysis when an electric current is passed through a solution of copper sulphate CuSO4 with copper electrodes lowered into it.
The phenomenon of electrolysis when current passes through a solution of copper sulphate: C - vessel with electrolyte, B - current source, C - switch
There will also be a counter movement of ions to the electrodes. The positive ion will be the copper (Cu) ion, and the negative ion will be the acid residue (SO4) ion. Copper ions, upon contact with the cathode, will be discharged (attaching the missing electrons to themselves), i.e., they will turn into neutral molecules of pure copper, and deposited on the cathode in the form of the thinnest (molecular) layer.
Negative ions, having reached the anode, are also discharged (give away excess electrons). But at the same time, they enter into a chemical reaction with the copper of the anode, as a result of which a molecule of copper Cu is attached to the acidic residue SO4 and a molecule of copper sulfate CuS O4 is formed, which is returned back to the electrolyte.
Since this chemical process takes a long time, copper is deposited on the cathode, which is released from the electrolyte. In this case, instead of the copper molecules that have gone to the cathode, the electrolyte receives new copper molecules due to the dissolution of the second electrode - the anode.
The same process occurs if zinc electrodes are taken instead of copper ones, and the electrolyte is a solution of zinc sulfate ZnSO4. Zinc will also be transferred from the anode to the cathode.
Thus, difference between electric current in metals and liquid conductors lies in the fact that in metals only free electrons, i.e., negative charges, are charge carriers, while in electrolytes it is carried by oppositely charged particles of matter - ions moving in opposite directions. Therefore they say that electrolytes have ionic conductivity.
The phenomenon of electrolysis was discovered in 1837 by B. S. Jacobi, who carried out numerous experiments on the study and improvement of chemical current sources. Jacobi found that one of the electrodes placed in a solution of copper sulphate, when an electric current passes through it, is covered with copper.
This phenomenon is called electroplating, finds extremely wide practical application now. One example of this is the coating of metal objects with a thin layer of other metals, i.e. nickel plating, gilding, silver plating, etc.
Gases (including air) do not conduct electricity under normal conditions. For example, naked, being suspended parallel to each other, are isolated from one another by a layer of air.
However, under the influence of high temperature, a large potential difference, and other reasons, gases, like liquid conductors, ionize, i.e., particles of gas molecules appear in them in large numbers, which, being carriers of electricity, contribute to the passage of electric current through the gas.
But at the same time, the ionization of a gas differs from the ionization of a liquid conductor. If a molecule breaks up into two charged parts in a liquid, then in gases, under the action of ionization, electrons are always separated from each molecule and an ion remains in the form of a positively charged part of the molecule.
One has only to stop the ionization of the gas, as it ceases to be conductive, while the liquid always remains a conductor of electric current. Consequently, the conductivity of a gas is a temporary phenomenon, depending on the action of external causes.
However, there is another one called arc discharge or just an electric arc. The phenomenon of an electric arc was discovered at the beginning of the 19th century by the first Russian electrical engineer V. V. Petrov.
V. V. Petrov, doing numerous experiments, discovered that between two charcoal connected to a current source, a continuous electric discharge occurs through the air, accompanied by a bright light. In his writings, V. V. Petrov wrote that in this case, "the dark peace can be quite brightly illuminated." So for the first time electric light was obtained, which was practically applied by another Russian electrical scientist Pavel Nikolaevich Yablochkov.
"Yablochkov's Candle", whose work is based on the use of an electric arc, made a real revolution in electrical engineering in those days.
The arc discharge is used as a source of light even today, for example, in searchlights and projectors. The high temperature of the arc discharge allows it to be used for . At present, arc furnaces powered by a very high current are used in a number of industries: for the smelting of steel, cast iron, ferroalloys, bronze, etc. And in 1882, N. N. Benardos first used an arc discharge for cutting and welding metal.
In gas-light tubes, fluorescent lamps, voltage stabilizers, to obtain electron and ion beams, the so-called glow gas discharge.
A spark discharge is used to measure large potential differences using a ball gap, the electrodes of which are two metal balls with a polished surface. The balls are moved apart, and a measured potential difference is applied to them. Then the balls are brought together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, the pressure, temperature and humidity of the air, they find the potential difference between the balls according to special tables. This method can be used to measure, to within a few percent, potential differences of the order of tens of thousands of volts.
Everyone is familiar with the definition of electric current. It is represented as a directed motion of charged particles. Such movement in different environments has fundamental differences. As a basic example of this phenomenon, one can imagine the flow and propagation of electric current in liquids. Such phenomena are characterized by different properties and are seriously different from the ordered movement of charged particles, which occurs under normal conditions not under the influence of various liquids.
Figure 1. Electric current in liquids. Author24 - online exchange of student papers
Formation of electric current in liquids
Despite the fact that the process of conduction of electric current is carried out by means of metal devices (conductors), the current in liquids depends on the movement of charged ions that have acquired or lost such atoms and molecules for some specific reason. An indicator of such a movement is a change in the properties of a certain substance, where the ions pass. Thus, it is necessary to rely on the basic definition of electric current in order to form a specific concept of the formation of current in various liquids. It is determined that the decomposition of negatively charged ions contributes to the movement to the region of the current source with positive values. Positively charged ions in such processes will move in the opposite direction - to a negative current source.
Liquid conductors are divided into three main types:
- semiconductors;
- dielectrics;
- conductors.
Definition 1
Electrolytic dissociation is the process of decomposition of molecules of a certain solution into negative and positive charged ions.
It can be established that an electric current in liquids can occur after a change in the composition and chemical properties of the liquids used. This completely contradicts the theory of the propagation of electric current in other ways when using a conventional metal conductor.
Faraday's experiments and electrolysis
The flow of electric current in liquids is a product of the movement of charged ions. The problems associated with the emergence and propagation of electric current in liquids led to the study of the famous scientist Michael Faraday. With the help of numerous practical studies, he was able to find evidence that the mass of a substance released during electrolysis depends on the amount of time and electricity. In this case, the time during which the experiments were carried out is important.
The scientist was also able to find out that in the process of electrolysis, when a certain amount of a substance is released, the same amount of electric charges is needed. This quantity was accurately established and fixed in a constant value, which was called the Faraday number.
In liquids, electric current has different propagation conditions. It interacts with water molecules. They significantly impede all movement of ions, which was not observed in experiments using a conventional metal conductor. It follows from this that the generation of current during electrolytic reactions will not be so large. However, as the temperature of the solution increases, the conductivity gradually increases. This means that the voltage of the electric current is increasing. Also in the process of electrolysis, it has been observed that the probability of a certain molecule disintegrating into negative or positive ion charges increases due to the large number of molecules of the substance or solvent used. When the solution is saturated with ions in excess of a certain norm, the reverse process occurs. The conductivity of the solution begins to decrease again.
Currently, the electrolysis process has found its application in many fields and fields of science and in production. Industrial enterprises use it in the production or processing of metal. Electrochemical reactions are involved in:
- salt electrolysis;
- electroplating;
- surface polishing;
- other redox processes.
Electric current in vacuum and liquids
The propagation of electric current in liquids and other media is a rather complex process that has its own characteristics, features and properties. The fact is that in such media there are completely no charges in the bodies, therefore they are usually called dielectrics. The main goal of the research was to create such conditions under which atoms and molecules could begin their movement and the process of generating an electric current began. For this, it is customary to use special mechanisms or devices. The main element of such modular devices are conductors in the form of metal plates.
To determine the main parameters of the current, it is necessary to use known theories and formulas. The most common is Ohm's law. It acts as a universal ampere characteristic, where the principle of current-voltage dependence is implemented. Recall that voltage is measured in units of amperes.
For experiments with water and salt, it is necessary to prepare a vessel with salt water. This will give a practical and visual representation of the processes that occur when an electric current is generated in liquids. Also, the installation should contain rectangular electrodes and power supplies. For full-scale preparation for experiments, you need to have an ampere installation. It will help conduct energy from the power supply to the electrodes.
Metal plates will act as conductors. They are dipped into the liquid used, and then the voltage is connected. The movement of particles begins immediately. It runs randomly. When a magnetic field arises between the conductors, the entire process of particle movement is ordered.
The ions begin to change charges and combine. Thus cathodes become anodes and anodes become cathodes. In this process, there are also several other important factors to consider:
- dissociation level;
- temperature;
- electrical resistance;
- use of alternating or direct current.
At the end of the experiment, a layer of salt is formed on the plates.
Almost every person knows the definition of electric current as However, the whole point is that its origin and movement in various media are quite different from each other. In particular, the electric current in liquids has somewhat different properties than the same metallic conductors.
The main difference is that the current in liquids is the movement of charged ions, that is, atoms or even molecules that have lost or gained electrons for some reason. At the same time, one of the indicators of this movement is a change in the properties of the substance through which these ions pass. Based on the definition of electric current, we can assume that during decomposition, negatively charged ions will move towards positive and positive, on the contrary, towards negative.
The process of decomposition of solution molecules into positive and negative charged ions is called electrolytic dissociation in science. Thus, an electric current in liquids arises due to the fact that, unlike the same metallic conductor, the composition and chemical properties of these liquids change, resulting in the process of movement of charged ions.
The electric current in liquids, its origin, quantitative and qualitative characteristics were one of the main problems studied by the famous physicist M. Faraday for a long time. In particular, with the help of numerous experiments, he was able to prove that the mass of the substance released during electrolysis directly depends on the amount of electricity and the time during which this electrolysis was carried out. From any other reasons, with the exception of the type of substance, this mass does not depend.
In addition, studying the current in liquids, Faraday experimentally found out that the same amount is needed to isolate one kilogram of any substance during electrolysis. This amount, equal to 9.65.10 7 k, was called the Faraday number.
Unlike metal conductors, the electric current in liquids is surrounded, which greatly complicates the movement of the ions of the substance. In this regard, in any electrolyte, only a small voltage can be generated. At the same time, if the temperature of the solution rises, then its conductivity increases, and the field increases.
Electrolysis has another interesting property. The thing is that the probability of the decay of a particular molecule into positive and negative charged ions is the higher, the greater the number of molecules of the substance itself and the solvent. At the same time, at a certain moment, the solution becomes supersaturated with ions, after which the conductivity of the solution begins to decrease. Thus, the strongest will take place in a solution where the concentration of ions is extremely low, but the electric current in such solutions will be extremely low.
The electrolysis process has found wide application in various industrial productions related to electrochemical reactions. Among the most important of them are the production of metal using electrolytes, the electrolysis of salts containing chlorine and its derivatives, redox reactions, the production of such a necessary substance as hydrogen, surface polishing, electroplating. For example, at many enterprises of mechanical engineering and instrument making, the refining method is very common, which is the production of metal without any unnecessary impurities.
Electric current in liquids is caused by the movement of positive and negative ions. Unlike current in conductors where electrons move. Thus, if there are no ions in a liquid, then it is a dielectric, for example, distilled water. Since charge carriers are ions, that is, molecules and atoms of a substance, when an electric current passes through such a liquid, it will inevitably lead to a change in the chemical properties of the substance.
Where do positive and negative ions come from in a liquid? Let us say at once that charge carriers are not capable of forming in all liquids. Those in which they appear are called electrolytes. These include solutions of salts of acids and alkalis. When dissolving salt in water, for example, take table salt NaCl, it decomposes under the action of a solvent, that is, water into a positive ion Na called a cation and a negative ion Cl called an anion. The process of formation of ions is called electrolytic dissociation.
Let's conduct an experiment, for it we need a glass bulb, two metal electrodes, an ammeter and a direct current source. We fill the flask with a solution of common salt in water. Then we put two rectangular electrodes into this solution. We connect the electrodes to a direct current source through an ammeter.
Figure 1 - Flask with salt solution
When the current is turned on between the plates, an electric field will appear under the action of which salt ions will begin to move. Positive ions will rush to the cathode, and negative ions to the anode. At the same time, they will make a chaotic movement. But at the same time, under the action of the field, an ordered one will also be added to it.
Unlike conductors in which only electrons move, that is, one type of charge, two types of charges move in electrolytes. These are positive and negative ions. They move towards each other.
When the positive sodium ion reaches the cathode, it will gain the missing electron and become a sodium atom. A similar process will occur with the chlorine ion. Only when reaching the anode, the chlorine ion will give up an electron and turn into a chlorine atom. Thus, current is maintained in the external circuit due to the movement of electrons. And in the electrolyte, ions seem to carry electrons from one pole to another.
The electrical resistance of electrolytes depends on the amount of ions formed. In strong electrolytes, the level of dissociation is very high when dissolved. The weak are low. Also, the electrical resistance of the electrolyte is affected by temperature. With its increase, the viscosity of the liquid decreases and heavy and clumsy ions begin to move faster. Accordingly, the resistance decreases.
If the salt solution is replaced with a solution of copper sulfate. Then, when a current is passed through it, when the copper cation reaches the cathode and receives the missing electrons there, it will be restored to a copper atom. And if after that you remove the electrode, you can find copper deposits on it. This process is called electrolysis.
« Physics - Grade 10 "
What are the carriers of electric current in a vacuum?
What is the nature of their movement?
Liquids, like solids, can be dielectrics, conductors, and semiconductors. Dielectrics include distilled water, conductors - solutions and melts of electrolytes: acids, alkalis and salts. Liquid semiconductors are molten selenium, sulfide melts, etc.
electrolytic dissociation.
When electrolytes are dissolved under the influence of the electric field of polar water molecules, electrolyte molecules decompose into ions.
The disintegration of molecules into ions under the influence of the electric field of polar water molecules is called electrolytic dissociation.
Degree of dissociation- the proportion of molecules in the dissolved substance that have decayed into ions.
The degree of dissociation depends on the temperature, the concentration of the solution, and the electrical properties of the solvent.
With increasing temperature, the degree of dissociation increases and, consequently, the concentration of positively and negatively charged ions increases.
Ions of different signs, when meeting, can again unite into neutral molecules.
Under constant conditions, a dynamic equilibrium is established in the solution, at which the number of molecules that decay into ions per second is equal to the number of pairs of ions that recombine into neutral molecules in the same time.
Ionic conduction.
Charge carriers in aqueous solutions or electrolyte melts are positively and negatively charged ions.
If a vessel with an electrolyte solution is included in an electrical circuit, then negative ions will begin to move towards the positive electrode - the anode, and positive - towards the negative - cathode. As a result, an electric current will flow through the circuit.
The conductivity of aqueous solutions or electrolyte melts, which is carried out by ions, is called ionic conductivity.
Electrolysis. With ionic conductivity, the passage of current is associated with the transfer of matter. On the electrodes, substances that make up electrolytes are released. At the anode, the negatively charged ions donate their extra electrons (in chemistry, this is called an oxidative reaction), and at the cathode, the positive ions gain the missing electrons (reduction reaction).
Liquids can also have electronic conductivity. Such conductivity is possessed, for example, by liquid metals.
The process of release of a substance at the electrode, associated with redox reactions, is called electrolysis.
What determines the mass of a substance released in a given time? Obviously, the mass m of the released substance is equal to the product of the mass m 0i of one ion by the number N i of ions that have reached the electrode during the time Δt:
m = m 0i N i . (16.3)
The ion mass m 0i is:
where M is the molar (or atomic) mass of the substance, and N A is the Avogadro constant, i.e. the number of ions in one mole.
The number of ions reaching the electrode is
where Δq = IΔt is the charge passed through the electrolyte during the time Δt; q 0i is the charge of the ion, which is determined by the valence n of the atom: q 0i \u003d ne (e is the elementary charge). During the dissociation of molecules, for example KBr, consisting of monovalent atoms (n = 1), K + and Br - ions appear. The dissociation of copper sulfate molecules leads to the appearance of doubly charged Cu 2+ and SO 2- 4 ions (n = 2). Substituting expressions (16.4) and (16.5) into formula (16.3) and taking into account that Δq = IΔt, a q 0i = ne, we obtain
Faraday's law.
Let us denote by k the coefficient of proportionality between the mass m of the substance and the charge Δq = IΔt passing through the electrolyte:
where F \u003d eN A \u003d 9.65 10 4 C / mol - Faraday constant.
The coefficient k depends on the nature of the substance (the values of M and n). According to formula (16.6) we have
m = kIΔt. (16.8)
Faraday's law of electrolysis:
The mass of the substance released on the electrode during the time Δt. during the passage of electric current, is proportional to the strength of the current and time.
This statement, obtained theoretically, was first established experimentally by Faraday.
The value k in formula (16.8) is called electrochemical equivalent given substance and expressed in kilograms per pendant(kg/C).
From formula (16.8) it can be seen that the coefficient k is numerically equal to the mass of the substance released on the electrodes during the transfer of a charge of 1 C by ions.
The electrochemical equivalent has a simple physical meaning. Since M / N A \u003d m 0i and en \u003d q 0i, then according to formula (16.7) k \u003d rn 0i / q 0i, i.e. k is the ratio of the ion mass to its charge.
By measuring the values of m and Δq, one can determine the electrochemical equivalents of various substances.
You can verify the validity of Faraday's law by experience. Let's assemble the installation shown in Figure (16.25). All three electrolytic baths are filled with the same electrolyte solution, but the currents passing through them are different. Let's denote the strength of the currents through I1, I2, I3. Then I 1 = I 2 + I 3 . By measuring the masses m 1 , m 2 , m 3 of the substances released on the electrodes in different baths, one can make sure that they are proportional to the corresponding currents I 1 , I 2 , I 3 .
Determination of the electron charge.
Formula (16.6) for the mass of the substance released on the electrode can be used to determine the electron charge. From this formula it follows that the electron charge modulus is equal to:
Knowing the mass m of the released substance during the passage of the charge IΔt, the molar mass M, the valency of n atoms and the Avogadro constant N A, one can find the value of the electron charge modulus. It turns out to be equal to e = 1.6 10 -19 C.
It was in this way that the value of the elementary electric charge was obtained for the first time in 1874.
Application of electrolysis. Electrolysis is widely used in engineering for various purposes. Electrolytically cover the surface of one metal with a thin layer of another ( nickel plating, chrome plating, gold plating and so on.). This durable coating protects the surface from corrosion. If good peeling of the electrolytic coating is ensured from the surface on which the metal is deposited (this is achieved, for example, by applying graphite to the surface), then a copy can be obtained from the relief surface.
The process of obtaining peelable coatings - electrotype- was developed by the Russian scientist B. S. Jacobi (1801-1874), who in 1836 applied this method to make hollow figures for St. Isaac's Cathedral in St. Petersburg.
Previously, in the printing industry, copies from a relief surface (stereotypes) were obtained from matrices (an imprint of a set on a plastic material), for which a thick layer of iron or another substance was deposited on the matrices. This made it possible to reproduce the set in the required number of copies.
Electrolysis removes impurities from metals. Thus, crude copper obtained from the ore is cast in the form of thick sheets, which are then placed in a bath as anodes. During electrolysis, the anode copper dissolves, impurities containing valuable and rare metals fall to the bottom, and pure copper settles on the cathode.
Aluminum is obtained from molten bauxite by electrolysis. It was this method of obtaining aluminum that made it cheap and, along with iron, the most common in technology and everyday life.
With the help of electrolysis, electronic circuit boards are obtained, which serve as the basis of all electronic products. A thin copper plate is glued onto the dielectric, on which a complex pattern of connecting wires is applied with a special paint. Then the plate is placed in an electrolyte, where the areas of the copper layer that are not covered with paint are etched. After that, the paint is washed off, and the details of the microcircuit appear on the board.
