Electrolytes it is customary to call conductive media in which the flow of electric current is accompanied by the transfer of matter. The carriers of free charges in electrolytes are positively and negatively charged ions.
The main representatives of electrolytes, widely used in technology, are aqueous solutions of inorganic acids, salts, and bases. The passage of electric current through the electrolyte is accompanied by the release of substances at the electrodes. This phenomenon is called electrolysis (fig 9.10) .
Electric current in electrolytes is the movement of ions of both signs in opposite directions. Positive ions move towards the negative electrode ( cathode), negative ions - to the positive electrode ( anode). Ions of both signs appear in aqueous solutions of salts, acids and alkalis as a result of the splitting of some of the neutral molecules. This phenomenon is called electrolytic dissociation .
The law of electrolysis was experimentally established by the English physicist M. Faraday in 1833.
Faraday's first law determines the amount of primary products released at the electrodes during electrolysis: the mass m of the substance released at the electrode is directly proportional to the charge q passed through the electrolyte:
m = kq = kIt,
where k – electrochemical equivalent of a substance:
F = eN A \u003d 96485 C / mol. - faraday constant.
Faraday's second lawelectrochemical equivalents of various substances refer to their chemical equivalents :
Faraday's combined lawfor electrolysis:
Electrolytic processes are classified as follows:
obtaining inorganic substances (hydrogen, oxygen, chlorine, alkalis, etc.);
obtaining metals (lithium, sodium, potassium, beryllium, magnesium, zinc, aluminum, copper, etc.);
cleaning of metals (copper, silver, ...);
obtaining metal alloys;
obtaining electroplated coatings;
metal surface treatment (nitriding, boriding, electropolishing, cleaning);
obtaining organic substances;
electrodialysis and water demineralization;
application of films using electrophoresis.
Practical application of electrolysis
Electrochemical processes are widely used in various fields of modern technology, in analytical chemistry, biochemistry, etc. In the chemical industry, chlorine and fluorine, alkalis, chlorates and perchlorates, persulfuric acid and persulfates, chemically pure hydrogen and oxygen, etc. are obtained by electrolysis. In this case, some substances are obtained by reduction at the cathode (aldehydes, paraaminophenol, etc.), others by electrooxidation at the anode (chlorates, perchlorates, potassium permanganate, etc.).
Electrolysis in hydrometallurgy is one of the stages in the processing of metal-containing raw materials, which ensures the production of commercial metals. Electrolysis can be carried out with soluble anodes - the electrorefining process or with insoluble - the electroextraction process. The main task in the electrorefining of metals is to ensure the required purity of the cathode metal with acceptable energy consumption. In non-ferrous metallurgy, electrolysis is used to extract metals from ores and refine them.
Aluminum, magnesium, titanium, zirconium, uranium, beryllium, etc. are obtained by electrolysis of molten media. For refining (purification) of metal by electrolysis, plates are cast from it and placed as anodes 1 in electrolyzer 3 (Fig. 9.11). When a current is passed, the metal to be cleaned 1 undergoes anodic dissolution, i.e., goes into solution in the form of cations. Then these metal cations are discharged at cathode 2, due to which a compact deposit of already pure metal is formed. The impurities in the anode either remain insoluble 4 or pass into the electrolyte and are removed.
Figure 9.11 shows a schematic diagram of copper electrolytic refining.
Electroplating - the field of applied electrochemistry, dealing with the processes of applying metal coatings to the surface of both metallic and non-metallic products when a direct electric current is passed through solutions of their salts. Electroplating is divided into electroplating and electroplating.
Electroplating (from the Greek. to cover) - it is electrodeposition on the metal surface of another metal, which firmly binds (adheres) to the metal (object) to be coated, which serves as the cathode of the electrolyzer (Figure 9.12).
Electroplating can be used to coat the part with a thin layer of gold or silver, chromium or nickel. Electrolysis can be used to apply the finest metal coatings on various metal surfaces. With this method of coating, the part is used as a cathode placed in a salt solution of the metal to be coated. A plate of the same metal is used as the anode.
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| Figure: 9.12 | Figure: 9.13 |
Electrotype – obtaining precise, easily separable metal copies by electrolysissignificant thickness from various non-metallic and metallic objects called matrices (Fig. 9.13).
Busts, statues, etc. are made using electroforming. Electroforming is used to apply relatively thick metallic coatings to other metals (for example, the formation of a "patch" layer of nickel, silver, gold, etc.).
What is called amperage? This question has come up more than once or twice in our discussion of various issues. Therefore, we decided to deal with it in more detail, and we will try to make it as accessible as possible without a huge number of formulas and incomprehensible terms.
So what is called electric shock? This is a directed flow of charged particles. But what are these particles, why are they suddenly moving, and where? This is all not very clear. Therefore, let's take a closer look at this issue.
- Let's start with the question about charged particles, which, in fact, are carriers of electric current.... They are different in different substances. For example, what is electric current in metals? They are electrons. In gases - electrons and ions; in semiconductors - holes; and in electrolytes, these are cations and anions.
- These particles have a certain charge. It can be positive or negative. The definition of positive and negative charge is given conditionally. Particles with the same charge are repelled, and opposite ones are attracted.
- Based on this, it turns out logical that the movement will occur from the positive pole to the negative. And the more charged particles there is at one charged pole, the more their number will move to the pole with a different sign.
- But this is all deep theory, so let's take a concrete example. Let's say we have an outlet to which not a single device is connected. Is there a current there?
- To answer this question, we need to know what voltage and current are. To make it clearer, let's analyze this using the example of a pipe with water. To put it simply, the pipe is our wire. The cross-section of this pipe is the voltage of the electrical network, and the flow rate is our electrical current.
- We return to our outlet. If we draw an analogy with a pipe, then a socket without electrical appliances connected to it is a pipe closed by a valve. That is, there is no electric current there.
- But there is tension there. And if in the pipe, in order for the flow to appear, it is necessary to open the valve, then in order to create an electric current in the conductor, it is necessary to connect the load. This can be done by plugging the plug into an outlet.
- Of course, this is a very simplified presentation of the question, and some professionals will find fault with me and point out inaccuracies. But it gives an idea of \u200b\u200bwhat is called electric current.
DC and AC
The next question that we propose to understand is: what is alternating current and direct current. After all, many do not quite understand these concepts correctly.
Constant is a current that does not change its magnitude and direction over time. Quite often, pulsating current is also referred to as constant, but let's talk about everything in order.
- Direct current is characterized by the fact that the same number of electric charges are constantly replacing each other in one direction. The direction is from one pole to the other.
- It turns out that the conductor always has either a positive or negative charge. And this is unchanged over time.
Note! When determining the direction of the direct current, there may be inconsistencies. If the current is formed by the movement of positively charged particles, then its direction corresponds to the movement of the particles. If the current is formed by the movement of negatively charged particles, then its direction is considered to be opposite to the movement of particles.
- But under the concept of what a direct current is, the so-called pulsating current is often referred to. It differs from constant only in that its value changes over time, but at the same time it does not change its sign.
- Let's say we have a current of 5A. For direct current, this value will remain unchanged during the entire period of time. For a pulsating current, in one time interval it will be 5, in another 4, and in the third 4.5. But at the same time, it in no way drops below zero, and does not change its sign.
- This ripple current is very common when converting AC to DC. This is the ripple current that your inverter or diode bridge in electronics produces.
- One of the main advantages of DC is that it can be stored. You can do this yourself, using batteries or capacitors.
Alternating current
To understand what an alternating current is, we need to imagine a sinusoid. It is this flat curve that best describes the change in DC current, and is the standard.
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Like a sinusoid, an alternating current at a constant frequency reverses its polarity. In one period of time it is positive, and in another period of time it is negative. |
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Therefore, directly in the conductor of movement, there are no charge carriers as such. To understand this, imagine a wave running ashore. It moves in one direction and then in the opposite direction. As a result, the water seems to move, but remains in place. |
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Based on this, for alternating current, its rate of polarity reversal becomes a very important factor. This factor is called frequency. The higher this frequency, the more often the polarity of the alternating current changes per second. In our country, there is a standard for this value - it is equal to 50Hz. That is, the alternating current changes its value from extreme positive to extreme negative 50 times per second. |
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But there is not only alternating current with a frequency of 50 Hz. Many equipment operates on alternating current of excellent frequencies. Indeed, by changing the frequency of the alternating current, you can change the speed of rotation of the motors. You can also get better data processing rates - such as in the chipsets of your computers, and much more. |
Note! You can clearly see what alternating and direct current is, using the example of an ordinary light bulb. This can be seen especially well on low-quality diode lamps, but if you look closely, you can also see it on an ordinary incandescent lamp. When operating on DC, they glow with an even light, and when operating on AC, they flicker slightly.
What is power and current density?
Well, we found out what is direct current and what is alternating current. But you probably still have a lot of questions. We will try to consider them in this section of our article.
From this video you can learn more about what power is.
- And the first of these questions will be: what is the voltage of an electric current? Voltage is the potential difference between two points.
- The question immediately arises, what is potential? Now professionals will again find fault with me, but let's just say: it's an excess of charged particles. That is, there is one point at which there is an excess of charged particles - and there is a second point, where these charged particles are either more or less. This difference is called voltage. It is measured in volts (V).
- Let's take a regular outlet as an example. You all probably know that its voltage is 220V. We have two wires in the outlet, and the voltage of 220V means that the potential of one wire is greater than the potential of the second wire just by these 220V.
- We need to understand the concept of voltage in order to understand what the power of an electric current is. Although from a professional point of view, this statement is not entirely true. Electric current has no power, but is its derivative.
- To understand this point, let's go back to our water pipe analogy. As you remember, the cross section of this pipe is voltage, and the flow velocity in the pipe is current. So: power is the amount of water that flows through this pipe.
- It is logical to assume that with equal cross-sections, that is, voltages, the stronger the flow, that is, the electric current, the greater the flow of water to move through the pipe. Accordingly, the more power will be transferred to the consumer.
- But if, in analogy with water, we can transfer a strictly defined amount of water through a pipe of a certain section, since water does not compress, then everything is not so with an electric current. Through any conductor, we can theoretically transmit any current. But in practice, a conductor of small cross-section at a high current density will simply burn out.
- In this regard, we need to understand what the current density is. Roughly speaking, this is the number of electrons that moves through a certain section of the conductor per unit of time.
- This number should be optimal. After all, if we take a conductor of large cross-section, and we transmit a small current through it, then the price of such an electrical installation will be high. At the same time, if we take a conductor of small cross-section, then, due to the high current density, it will overheat and quickly burn out.
- In this regard, the PUE has a corresponding section that allows you to select conductors based on the economic current density.
- But back to the concept, what is current power? As we understood by our analogy, with the same pipe section, the transmitted power depends only on the current strength. But if the cross-section of our pipe is increased, that is, the voltage is increased, in this case, at the same values \u200b\u200bof the flow rate, completely different volumes of water will be transmitted. It's the same with electrics.
- The higher the voltage, the less current is required to transmit the same power. That is why high-voltage power lines are used to transmit large powers over long distances.
After all, a line with a wire cross-section of 120 mm 2 for a voltage of 330 kV is capable of transmitting many times greater power in comparison with a line of the same cross-section, but with a voltage of 35 kV. Although what is called the current strength, they will be the same.
Electric current transmission methods
We figured out what current and voltage are. It's time to figure out how to distribute electric current. This will allow you to feel more confident in dealing with electrical appliances in the future.
As we have already said, the current can be alternating and constant. In industry, and in your socket, AC is used. It is more common because it is easier to transmit over wires. The fact is that changing the DC voltage is quite difficult and expensive, and you can change the AC voltage using ordinary transformers.
Note! No AC transformer will operate on DC. Since the properties that he uses are inherent only in alternating current.
- But this does not mean at all that direct current is not used anywhere. It has another useful property that is not inherent in a variable. It can be accumulated and stored.
- In this regard, direct current is used in all portable electrical appliances, in railway transport, as well as in some industrial facilities where it is necessary to maintain performance even after a complete power outage.
- The most common way to store electrical energy is batteries. They have special chemical properties that allow them to accumulate and then, if necessary, release direct current.
- Each battery has a strictly limited amount of stored energy. This is called the capacity of the battery and is partly determined by the starting current of the battery.
- What is Battery Inrush Current? This is the amount of energy that the battery is able to give at the very initial moment of connecting the load. The fact is that, depending on the physicochemical properties, batteries differ in the way they return the accumulated energy.
- Some can give a lot at once. Because of this, they, of course, will be quickly discharged. And the second give a long time, but little by little. In addition, the ability to maintain voltage is an important aspect of a battery.
- The fact is that, as the instruction says, in some batteries, as the capacity is released, their voltage gradually decreases. And other batteries are capable of giving almost all the capacity with the same voltage. Based on these basic properties, these storage facilities are chosen for electricity.
- For direct current transmission, in all cases two wires are used. This is a positive and negative vein. Red and blue.
Alternating current
But with alternating current, everything is much more complicated. It can be transmitted over one, two, three or four wires. To explain this, we need to deal with the question: what is three-phase current?
- Alternating current is generated by a generator. Usually, almost all of them have a three-phase structure. This means that the generator has three leads and each of these leads outputs an electric current that differs from the previous ones by an angle of 120⁰.
- In order to understand this, let's remember our sinusoid, which is a model for describing alternating current, and according to the laws of which it changes. Let's take three phases - "A", "B" and "C", and take a certain point in time. At this point, the sinusoid of phase “A” is at the zero point, the sinusoid of phase “B” is at the extreme positive point, and the sinusoid of phase “C” is at the extreme negative point.
- Each subsequent unit of time, the alternating current in these phases will change, but synchronously. That is, after a certain time, there will be a negative maximum in phase "A". In phase "B" there will be zero, and in phase "C" - a positive maximum. And after a while, they will change again.
- As a result, it turns out that each of these phases has its own potential, which is different from the potential of the neighboring phase. Therefore, there must necessarily be something between them that does not conduct an electric current.
- This potential difference between the two phases is called line voltage. In addition, they have a potential difference relative to ground - this voltage is called phase voltage.
- And now, if the line voltage between these phases is 380V, then the phase voltage is 220V. It differs by a value of √3. This rule is always valid for any voltage.
- Based on this, if we need a voltage of 220V, then we can take one phase wire, and a wire rigidly connected to the ground. And we get a single-phase 220V network. If we need a 380V network, then we can only take any 2 phases, and connect some kind of heating device as in the video.
But in most cases, all three phases are used. All powerful consumers are connected to a three-phase network.
Conclusion
What is induction current, capacitive current, starting current, no-load current, negative sequence currents, stray currents and much more, we simply cannot consider within the framework of one article.
After all, the question of electric current is quite voluminous, and a whole science of electrical engineering has been created to consider it. But we really hope that we were able to explain the main aspects of this issue in an accessible language, and now the electric current will not be something terrible and incomprehensible for you.
Electric current is charged particles that can move in an orderly manner in any conductor. This movement takes place under the influence of an electric field. The emergence of electric charges occurs almost constantly. This is especially pronounced when various substances are in contact with each other.
If full free movement of charges relative to each other is possible, then these substances are conductors. When such movement is impossible, this category of substances is considered insulators. Conductors include all metals with varying degrees of conductivity, as well as salt and acid solutions. Insulators can be natural substances in the form of ebonite, amber, various gases and quartz. They can be of artificial origin, for example, PVC, polyethylene and others.
Electric current values
As a physical quantity, the current can be measured according to its main parameters. Based on the measurement results, the possibility of using electricity in a particular area is determined.
There are two types of electric current - direct and alternating. The first, always remains unchanged in time and direction, and in the second case, changes occur in these parameters over a certain period of time.
Directional movement of charged particles in an electric field.
Charged particles can be electrons or ions (charged atoms).
An atom that has lost one or more electrons acquires a positive charge. - Anion (positive ion).
An atom that has attached one or more electrons acquires a negative charge. - Cation (negative ion).
Ions are considered as mobile charged particles in liquids and gases.
In metals, charge carriers are free electrons, like negatively charged particles.
In semiconductors, the movement (movement) of negatively charged electrons from one atom to another is considered and, as a result, the movement between the atoms of the formed positively charged vacant places - holes.
Per direction of electric current the direction of movement of positive charges is conventionally accepted. This rule was established long before the study of the electron and is still valid. Likewise, the electric field strength is determined for a positive test charge.
Any single charge q in an electric field of strength E force acting F \u003d qE, which moves the charge in the direction of the vector of this force.
The figure shows that the force vector F - \u003d -qEacting on a negative charge -q, directed in the direction opposite to the field strength vector, as the product of the vector E by a negative amount. Consequently, negatively charged electrons, which are carriers of charges in metal conductors, in reality have a direction of motion opposite to the vector of the field strength and the generally accepted direction of the electric current.
Charge amount Q \u003d 1 Pendant displaced across the conductor cross-section in time t \u003d 1 second, determined by the current I \u003d 1 Ampere from the ratio:
I \u003d Q / t.
Current ratio I \u003d 1 Ampere in a conductor to its cross-sectional area S \u003d 1 m 2 will determine the current density j \u003d 1 A / m 2:
Job A \u003d 1 Joule spent on transporting the charge Q \u003d 1 The pendant from point 1 to point 2 will determine the value of the electrical voltage U \u003d 1 Volt, as a potential difference φ 1 and φ 2 between these points based on:
U = A / Q = φ 1 - φ 2
The electric current can be direct or alternating.
Direct current is an electric current, the direction and magnitude of which do not change over time.
Alternating current is an electric current whose magnitude and direction change over time.
Back in 1826, the German physicist Georg Ohm discovered an important law of electricity that quantifies the relationship between electric current and the properties of a conductor, which characterize their ability to resist electric current.
These properties later became known as electrical resistance, denoted by the letter R and measured in Ohms in honor of the discoverer.
Ohm's law in the modern interpretation with the classical U / R ratio determines the amount of electric current in a conductor based on the voltage U at the ends of this conductor and its resistance R:
Electric current in conductors
Conductors have free charge carriers, which, under the action of the electric field force, set in motion and create an electric current.
Free electrons are charge carriers in metallic conductors.
With increasing temperature, the chaotic thermal movement of atoms prevents the directional movement of electrons and the resistance of the conductor increases.
When cooling and the temperature tends to absolute zero, when the thermal movement stops, the resistance of the metal tends to zero.
Electric current in liquids (electrolytes) exists as a directed movement of charged atoms (ions), which are formed in the process of electrolytic dissociation.
Ions move towards the opposite electrodes in sign and are neutralized, settling on them. - Electrolysis.
Anions are positive ions. They move to the negative electrode - the cathode.
Cations are negative ions. Move to the positive electrode - the anode.
Faraday's laws of electrolysis determine the mass of the substance released on the electrodes.
When heated, the resistance of the electrolyte decreases due to an increase in the number of molecules decomposed into ions.
Electric current in gases is plasma. An electrical charge is carried by positive or negative ions and free electrons, which are generated by radiation.
There is an electric current in a vacuum, like the flow of electrons from the cathode to the anode. Used in cathode-ray devices - lamps.
Electric current in semiconductors
Semiconductors occupy an intermediate position between conductors and dielectrics in terms of their specific resistance.
The significant difference between semiconductors and metals is the dependence of their resistivity on temperature.
With a decrease in temperature, the resistance of metals decreases, while in semiconductors, on the contrary, it increases.
As the temperature tends to absolute zero, metals tend to become superconductors, and semiconductors - insulators.
The fact is that at absolute zero the electrons in semiconductors will be busy creating a covalent bond between the atoms of the crystal lattice and, ideally, free electrons will be absent.
With an increase in temperature, some of the valence electrons can receive energy sufficient to break covalent bonds and free electrons appear in the crystal, and vacancies are formed at the break points, which are called holes.
A vacant position can be occupied by a valence electron from a neighboring pair and the hole will move to a new position in the crystal.
When a free electron meets a hole, the electronic bond between the atoms of the semiconductor is restored and the reverse process occurs - recombination.
Electron-hole pairs can appear and recombine when the semiconductor is illuminated by the energy of electromagnetic radiation.
In the absence of an electric field, electrons and holes participate in chaotic thermal motion.
In the electric field, the ordered motion involves not only the formed free electrons, but also holes, which are considered as positively charged particles. Current I in a semiconductor consists of an electronic I n and hole I p currents.
Semiconductors include such chemical elements as germanium, silicon, selenium, tellurium, arsenic, etc. The most common semiconductor in nature is silicon.
Comments and suggestions are welcome and welcome!
Current conditions
Modern science has created theories that explain natural processes. Many processes are based on one of the atomic structure models, the so-called planetary model. According to this model, an atom consists of a positively charged nucleus and a negatively charged cloud of electrons that surrounds the nucleus. Various substances consisting of atoms, for the most part, are stable and unchanged in their properties under constant environmental conditions. But in nature there are processes that can change the stable state of substances and cause a phenomenon in these substances called electric current.
Friction is such a basic process for nature. Many people know that if the hair is combed with a comb made of certain types of plastic, or when wearing clothes made of certain types of fabric, the sticking effect occurs. Hair attracts and sticks to the comb, and so does clothing. This effect is explained by friction, which disturbs the stability of the comb material or fabric. The electron cloud can be displaced relative to the nucleus or partially destroyed. As a result, the substance acquires an electric charge, the sign of which is determined by the structure of this substance. The electric charge resulting from friction is called electrostatic.
A pair of charged substances is obtained. Each of the substances has a specific electrical potential. An electric field acts on the space between two charged substances, in this case an electrostatic field. The effectiveness of the electrostatic field depends on the magnitudes of the potentials and is defined as the potential difference or voltage.
- When a voltage arises, a directed movement of charged particles of substances appears in the space between the potentials - an electric current.
Where does the electric current flow?
In this case, the potentials will decrease if friction ceases. And, in the end, the potentials will disappear, and the substances will regain stability.
But if the process of formation of potentials and voltage continues in the direction of their increase, the current will also increase in accordance with the properties of the substances filling the space between the potentials. The clearest demonstration of this process is lightning. The friction of the upward and downward air currents against each other creates tremendous tension. As a result, one potential is formed by updrafts in the sky and another by downdrafts in the ground. And, in the end, due to the properties of air, an electric current occurs in the form of lightning.
- The first cause of electric current is voltage.
- The second reason for the appearance of electric current is the space in which the voltage acts - its size and what it is filled with.
Tension does not come from friction alone. Other physical and chemical processes that disrupt the balance of the atoms of a substance also lead to the appearance of tension. Tension arises only as a result of interaction either
- one substance with another substance;
- one or more substances with a field or radiation.
Voltage can appear from:
- a chemical reaction that occurs in a substance, such as in all batteries and accumulators, as well as in all living things;
- electromagnetic radiation, such as in solar panels and thermal power generators;
- electromagnetic field, such as in all dynamos.
Electric current has a nature corresponding to the substance in which it flows. Therefore it differs:
- in metals;
- in liquids and gases;
- in semiconductors
In metals, electric current consists only of electrons, in liquids and gases - of ions, in semiconductors - of electrons and "holes".
DC and AC
The voltage relative to its potentials, the signs of which remain unchanged, can only change in magnitude.
- In this case, a constant or pulsed electric current appears.
The electric current depends on the duration of this change and the properties of the space filled with matter between the potentials.
- But if the signs of the potentials change and this leads to a change in the direction of the current, it is called variable, like the voltage that determines it.
Life and electric current
For quantitative and qualitative assessments of electric current in modern science and technology, certain laws and quantities are used. The main laws are:
- coulomb's law;
- ohm's law.
Charles Coulomb in the 80s of the 18th century determined the appearance of voltage, and Georg Ohm in the 20s of the 19th century determined the appearance of an electric current.
In nature and human civilization, it is used mainly as a carrier of energy and information, and the topic of its study and use is as immense as life itself. For example, studies have shown that all living organisms live because the muscles of the heart contract from the effects of impulses of electric current generated in the body. All other muscles work the same way. When dividing, the cell uses information based on ultrahigh-frequency electric current. The list of such facts with clarifications can be continued in the volume of the book.
There have already been many discoveries related to electric current, and more remains to be done. Therefore, with the advent of new research tools, new laws, materials and other results appear for the practical use of this phenomenon.