With a positive charge and the cathode. Electrochemistry and electroplating

Among the terms in electrics there are such concepts as anode and cathode. This applies to power supplies, electroplating, chemistry and physics. The term is also found in vacuum and semiconductor electronics. They designate the conclusions or contacts of devices and how electrical sign they possess. In this article, we will tell you what an anode and a cathode are, as well as how to determine where they are in an electrolyzer, a diode, and a battery, which of them are plus and which are minus.

Electrochemistry and electroplating

There are two main branches in electrochemistry:

1. Galvanic cells - the production of electricity through chemical reaction. These items include batteries and accumulators. They are often called chemical current sources.
2. Electrolysis is the effect of electrical energy on a chemical reaction. in simple words- With the help of a power source, some kind of reaction is triggered.

Consider a redox reaction in a galvanic cell, then what processes take place on its electrodes?

• Anode- the electrode on which it is observed oxidative reaction , that is, he donates electrons. The electrode at which the oxidation reaction takes place is called reducing agent.
• Cathode- the electrode on which flows reducing reaction , that is, he accepts electrons. The electrode at which the reduction reaction occurs is called oxidizing agent.

This raises the question - where is the plus, and where is the minus of the battery? Based on the definition, for a galvanic cell anode donates electrons.

Important! In GOST 15596-82, the official definition of the names of the conclusions of chemical current sources is given, in short, then plus on the cathode, and minus on the anode.

AT this case the flow of electric current is considered along the conductor of the external circuit from oxidizer (cathode) to reducer (anode). Since the electrons in the circuit flow from minus to plus, and electricity on the contrary, then the cathode is a plus, and the anode is a minus.

Attention: current always flows into the anode!

Or the same in the diagram:

The process of electrolysis or battery charging

These processes are similar and inverse to a galvanic cell, since here energy does not come from a chemical reaction, but vice versa - a chemical reaction occurs due to external source electricity.

In this case, the plus of the power source is also called the cathode, and the minus the anode. But the contacts of the charged galvanic cell or the electrodes of the electrolyzer will already have opposite names, let's see why!

Important! When discharging a galvanic cell, the anode is minus, the cathode is plus, and vice versa when charging.

Since the current from the positive terminal of the power source is supplied to the positive terminal of the battery, the latter can no longer be the cathode. Referring to the foregoing, we can conclude that in this case, the battery electrodes conditionally change places during charging.

Then through the electrode of the charged galvanic cell, into which the electric current flows, is called the anode. It turns out that when charging a battery, the plus becomes the anode, and the minus becomes the cathode.

The processes of metal deposition as a result of a chemical reaction under the influence of an electric current (during electrolysis) are called electroplating. Thus, the world received silver-plated, gilded, chrome-plated or other metal-plated jewelry and details. This process is used both for decorative and applied purposes - to improve the corrosion resistance of various components and assemblies of mechanisms.

The principle of operation of electroplating installations lies in the use of solutions of salts of the elements with which the part will be coated as an electrolyte.

In electroplating, the anode is also an electrode to which the positive output of the power source is connected, respectively, the cathode in this case is a minus. In this case, the metal is deposited (reduced) on the negative electrode (reduction reaction). That is, if you want to make a gold-plated ring with your own hands, connect the negative terminal of the power supply to it and place it in a container with the appropriate solution.

In electronics

Electrodes or legs of semiconductor and vacuum electronic devices are also often called the anode and cathode. Consider the conventional graphic designation of a semiconductor diode in the diagram:

As we can see, the anode of the diode is connected to the positive of the battery. It is so called for the same reason - in any case, current flows into this terminal of the diode. On a real element, there is a marking in the form of a strip or a dot on the cathode.

The LED is similar. On 5 mm LEDs, the insides are visible through the bulb. The larger half is the cathode.

This is also the case with the thyristor, the pin assignment and the "unipolar" use of these three-legged components make it a controlled diode:

For a vacuum diode, the anode is also connected to the plus, and the cathode to the minus, which is shown in the diagram below. Although when a reverse voltage is applied, the names of these elements will not change, despite the flow of electric current in reverse direction, albeit a small one.

With passive elements such as capacitors and resistors, the situation is different. The resistor does not have a separate cathode and anode, the current can flow in it in any direction. You can give any name to its conclusions, depending on the situation and the circuit in question. Ordinary non-polar capacitors also. Less commonly, such a separation by contact names is observed in electrolytic capacitors.

Conclusion

So, let's sum up by answering the question: how to remember where is the plus, where is the minus of the cathode with the anode? There is a convenient mnemonic rule for electrolysis, battery charging, electroplating and semiconductor devices. These words with similar names have the same number of letters, as illustrated below:

In all these cases, the current flows out of the cathode and flows into the anode.

Don’t be confused by the confusion: “why does the battery have a positive cathode, and when it is charged, does it become negative?”. Remember for all elements of electronics, as well as electrolyzers and in electroplating - in general, for all energy consumers, the anode is the output connected to the plus. This is where the differences end, now it’s easier for you to figure out what is the plus, what is the minus between the outputs of the elements and devices.

Now you know what an anode and a cathode are, and how to memorize them quickly enough. We hope that the information provided was useful and interesting for you!

materials

Any electrovacuum device has an electrode designed for the emission (emission) of electrons. This electrode is called the cathode. The electrode designed to receive the electrons emitted by the cathode is called the anode.

A higher and positive potential relative to the cathode is applied to the anode.

Cathode should give a large emission current per unit surface at the lowest possible heating temperature and have a long service life. The cathode is heated in an electrovacuum device by a current flowing through it.

Such thermionic cathodes are divided into two main groups:

• direct filament cathodes,
• cathodes of indirect heating (heated).

cathodes direct heating are a metal thread, which is directly heated by the filament current and serves to emit electrons ( rice. 6, a).

The radiation surface of direct-heated cathodes is small, so a large emission current cannot be obtained from them. The low heat capacity of the filament does not allow the use of alternating current for heating. In addition, when heated alternating current The cathode temperature is not constant in time, and therefore, the emission current also changes with time.

A positive property of a direct-heated cathode is its efficiency, which is achieved due to the small amount of heat radiated into the environment due to the small surface of the cathode.

cathodes direct heating are made of tungsten and nickel wire. However, a large work function (W 0 = 4.2 ÷ 4.5 V) determines the high operating temperature of the cathode, as a result of which the cathode becomes uneconomical. To increase the efficiency of the cathode, the tungsten or nickel wire (core) is “activated” - covered with a film of another element. Such cathodes are called activated.

If an electropositive film is deposited on the surface of the core (a film of cesium, thorium or barium, which have a lower work function than the core material), then the film is polarized: valence electrons pass into the core, and a potential difference arises between the positively charged film and the core, accelerating the movement electron when it leaves the core. The work function of a cathode with such a monomolecular electropositive film is less than the work function of an electron from both the base metal and the metal of the film. When the core is covered with an electronegative film, for example, with oxygen, the work function of the cathode increases.

Heated cathodes are made in the form of nickel sleeves, the surface of which is covered with an active metal layer having small job exit ( rice. 6b). Inside the cathode is placed heater- a tungsten filament or spiral, which can be heated by both direct and alternating current.

To isolate the heater from the sleeve, the inside of the latter is covered with alundum (Al 2 O 3).

Heated cathodes, due to their large thermal inertia, are usually fed with alternating current, a large surface of the sleeve provides a large emission current. Heated cathodes, however, are less economical and take much longer to heat up than directly heated cathodes.

Parameters and characteristics of cathodes

Cathodes are characterized by the following main parameters:

1. Specific emission, determined by the magnitude of the current from one square centimeter of the emitting surface of the cathode at normal operating temperature.

In vacuum tubes with activated cathodes, instead of specific emission, a parameter called the allowable cathode current density is often used. This parameter is characterized by the current that can be obtained from one square centimeter of the cathode surface at a normal (working) filament voltage. Work at currents from the cathode, equal to the current emission in these lamps, leads to the destruction of the surface of the cathode layer.

2. Efficiency, equal to cathode emission current per watt of power expended on heating:

H \u003d I e / P n (12)

where I e - cathode emission current, ma; P n - the power spent in the filament circuit, watts.

3. Service life cathode, measured in hours and characterizing the time during which the cathode retains the necessary performance properties. For simple cathodes, it is believed that a decrease in the cathode diameter by 10% leads to its death. Accordingly, their service life is also estimated.

The service life of activated cathodes is determined by a decrease in the cathode coverage area with an active film (and, consequently, a deterioration in the main parameters of the lamp) by 20%.

To select the optimal operating mode of the cathode, it is necessary to know the dependence of the cathode emission current on its temperature. Direct measurement of the temperature of an incandescent cathode is difficult, therefore, the so-called incandescent or emission characteristic of the cathode is used - graphically expressed dependences of the filament current or emission current on the filament voltage or current ( rice. 7, a).

There are two circuits in the circuit: anode and filament. The heating voltage is controlled by a voltmeter V1 directly connected to the cathode circuit; if you need to know the filament current, then an ammeter is included in it. In this case, the ammeter should be connected to the cathode terminal through which the filament and anode currents pass in the same direction: this end of the filament heats up more and works in the most difficult thermal conditions.

The magnitude of the filament current is determined by the difference between the readings of the ammeter and the readings of the milliammeter, but halved (since about half of the anode current passes through this part of the circuit).

Supporting constant voltage on the anode, remove the dependence of the emission current on the voltage (or current) of the filament. The emission current appears starting from a cathode voltage of 1-1.5 V and increases sharply at filament voltages close to normal (working) values.

Characteristic I n \u003d ƒ (U n) (see. rice. 7, a) should be removed with the anode circuit open. The incandescent characteristic is non-linear, since with an increase in the temperature of the cathode, its resistance increases. In this case, the filament current increases less than the filament voltage increases.

The study of industries such as electrochemistry and non-ferrous metallurgy, is impossible without a full understanding of the terms cathode and anode. At the same time, these terms are an integral part of vacuum and semiconductor electronic devices.

Cathode and anode in electrochemistry

Electrochemistry should be understood as the section physical chemistry who studies the chemical processes caused by the action of an electric current, as well as electrical phenomena, called chemical processes. There are two main types of electrochemical operations:

• Conversion procedure electrical impact into a chemical reaction called electrolysis;
• The process of converting a chemical reaction into an electrical current, called the galvanic process.

In electrochemistry, the terms anode and cathode mean the following:

1. The electrode at which the oxidation reaction takes place is called the anode;
2. The electrode on which the reduction procedure is carried out is called the cathode.

Oxidation processes should be understood as a procedure in which a particle gives up electrons. The recovery process implies the procedure for accepting electrons by a particle. Accordingly, particles that donate electrons are referred to as "reductants" and are subject to oxidation. The particles that accept electrons are called "oxidizers" and are reduced.

Non-ferrous metallurgy makes extensive use of the electrolysis process to isolate metals from mined ores and further refine them. In the electrolysis procedure, soluble and insoluble anodes are used, and the processes themselves are called electrorefining and electroextraction, respectively.

Cathode in vacuum devices

One of the varieties of electrovacuum devices is an electron lamp. The purpose of electric lamps is to regulate the flow of electrons drifting in a vacuum between other electrodes. Structurally, the electric lamp looks like a sealed vessel-cylinder, with small metal leads placed in the middle. The number of leads depends on the type of radio tube.

As part of any radio tube, the following elements:

• Cathode;
• Anode;
• Net.

The cathode of an electric lamp is a heated electrode connected to the "minus" of the power supply and emitting electrons, being heated. These electrons move towards the anode connected to the "plus". The process of emitting electrons from a heated cathode is called thermal emission, and the current that has arisen in this case is called thermal emission current. The heating method determines the types of cathodes:

• Direct heating cathode;
• Indirect heating cathode.

The cathode of direct heating is a strong tungsten conductor of high resistance. The cathode is heated by applying voltage to it.

Important! The features of directly heated vacuum tubes include a quick start-up of the lamp with less power consumption, although at the expense of service life. Since the supply current of such lamps is constant, their use in an alternating current environment is limited.

Electric lamps, in which a heating filament is placed inside the cathode, made in the form of a cylinder, are called radio lamps of indirect heating.

Structurally, the anode looks like a plate or a box placed around the cathode with a grid and having a potential opposite to the cathode. Additional electrodes placed between the anode and cathode, called the grid, are used to control the flow of electrons.

Cathode in semiconductor devices

Semiconductor devices include devices consisting of a substance, specific electrical resistance which is greater than the resistance of the conductor, but less than the resistance of the dielectric. The features of such devices include a large dependence of electrical conductivity on the concentration of additives and the effect of electric current. Properties p-n junction and determine the principles of operation of most semiconductor components.

The simplest representative of semiconductor components is a diode. This is an element that has two outputs and one p-n junction, distinctive feature which favors the flow of current in one direction.

The cathode is the electrode of the device that is connected to the negative pole of a current source. Anode is its opposite. This is the electrode of the device connected to the positive pole of the current source.

Note! To make it easier to remember the difference between them, use a cheat sheet. In the words "cathode" - "minus", "anode" - "plus" the same number letters.

Application in electrochemistry

In this branch of chemistry, a cathode is a negatively charged electrical conductor(electrode) that attracts positively charged ions (cations) to itself during the processes of oxidation and reduction.

Electrolytic refining is the electrolysis of alloys and aqueous solutions. Most non-ferrous metals undergo such cleaning. With the help of electrolytic purification, a metal with high purity is obtained. Thus, the degree of purity of copper after refining reaches 99.99%.

An electrolytic process takes place on the positive electrical conductor during refining or purification. During it, the metal with impurities is placed in an electrolytic cell and made an anode. Such processes are carried out with the help of an external source. electrical energy and are called electrolysis reactions. Carried out in electrolyzers. It performs the function of an electric pump that injects negatively charged particles (electrons) into the negative conductor and removes it from the anode. Where the current comes from is irrelevant.

At the cathode, the metal is cleaned from foreign impurities. A simple cathode is made of tungsten, sometimes tantalum. The advantage of the tungsten negative electrode is the stability of its manufacture. Among the shortcomings - it has low efficiency and uneconomical. Complex cathodes have miscellaneous device. Many of these types of conductors have pure metal a special layer is applied on top, which activates the receipt of greater performance at a relatively low temperatures. They are very economical. Their disadvantage is a small stability of performance.

The finished pure metal is also called the cathode. For example, a zinc or platinum cathode. In production, the negative conductor is separated from the cathode base using cathode stripping machines.

When negatively charged particles are removed from an electrical conductor, an anode is created on it, and when negatively charged particles are injected onto an electrical conductor, a cathode is created. During the electrolysis of the metal to be purified, its positive ions attract negatively charged particles to themselves on the negative conductor, and a reduction process occurs. The most commonly used anodes are:

• zinc;
• cadmium;
• copper;
• nickel;
• tin;
• gold;
• silver;
• platinum.

Most often, zinc anodes are used in production. They are:

• rolled;
• cast;
• spherical.

Rolled zinc anodes are most commonly used. Still use nickel and copper. But cadmium is almost never used because of their toxicity to the environment. Bronze and tin anodes are used in the manufacture of electronic printed circuit boards.

Galvanization (electroplating) is the process of applying a thin layer of metal to another object in order to prevent product corrosion, oxidation of contacts in electronics, wear resistance, decoration. The essence of the process is the same as in refining.

Zinc and tin are used to increase the corrosion resistance of the product. Galvanizing can be cold, hot, galvanic, gas-thermal and thermal diffusion. Gold is used mainly for protective and decorative purposes. Silver increases the resistance of electrical appliance contacts to oxidation. Chrome - for increased wear resistance and protection against corrosion. Chrome plating gives the products a beautiful and expensive look. Used to apply on handles, faucets, wheel rims, etc. The chromium plating process is toxic, therefore it is strictly regulated by law different countries. The picture below shows the method of electroplating with nickel.

Application in vacuum electronic devices

Here the cathode acts as a source of free electrodes. They are formed in the course of their knocking out of the metal during high temperatures. The positively charged electrode attracts the electrons released by the negative conductor. In different devices, he varying degrees collects them. In electron tubes, it completely attracts negatively charged particles, and in cathode-ray devices - partially, forming an electron beam at the end of the process.

Those involved in practical electronics need to know about the anode and cathode of the power source. What and how is it called? Why exactly? There will be an in-depth consideration of the topic from the point of view of not only amateur radio, but also chemistry. The most popular explanation is that the anode is the positive electrode and the cathode is the negative. Alas, this is not always true and incomplete. To be able to determine the anode and cathode, you must have a theoretical basis and know what and how. Let's take a look at this in the article.

Anode

Let's turn to GOST 15596-82, which deals with chemicals. We are interested in the information posted on the third page. According to GOST, the anode is the negative electrode. That's it! Why exactly? The fact is that it is through it that the electric current enters from the external circuit into the source itself. As you can see, not everything is as easy as it seems at first glance. It can be advised to carefully consider the pictures presented in the article if the content seems too complicated - they will help you understand what the author wants to convey to you.

Cathode

We turn to the same GOST 15596-82. positive electrode chemical source current is the one from which it exits into an external circuit. As you can see, the data contained in GOST 15596-82 consider the situation from a different perspective. Therefore, when consulting with other people about certain structures, you must be very careful.

The emergence of terms

They were introduced by Faraday in January 1834 in order to avoid ambiguity and achieve greater accuracy. He also offered his own version of memorization using the example of the Sun. So, his anode is sunrise. The sun moves up (current enters). The cathode is the entry. The sun moves down (current goes out).

Example of a radio tube and a diode

We continue to understand what is used to denote what. Suppose we have one of these energy consumers in open state(in direct connection). So, from the external circuit of the diode, an electric current enters the element through the anode. But don't get confused by this explanation with the direction of the electrons. Through the cathode, an electric current flows out of the used element into the external circuit. The situation that has developed now is reminiscent of cases when people look at an inverted picture. If these designations are complex, remember that it is only chemists who must understand them in this way. Now let's do the reverse. It can be seen that semiconductor diodes will practically not conduct current. The only possible exception here is the reverse breakdown of elements. And electrovacuum diodes (kenotrons, radio tubes) will not conduct reverse current at all. Therefore, it is considered (conditionally) that he does not go through them. Therefore, formally, the conclusions of the anode and cathode of the diode do not fulfill their functions.

Why is there confusion?

Specifically to facilitate learning and practical use, it was decided that the diode elements of the pin names will not change depending on their switching scheme, and they will be “attached” to the physical pins. But this does not apply to batteries. So, for semiconductor diodes, everything depends on the type of conductivity of the crystal. In vacuum tubes, this question is tied to the electrode that emits electrons at the location of the filament. Of course, there are certain nuances here: for example, through such as a suppressor and a zener diode, a reverse current may flow a little, but there is a specificity here that is clearly beyond the scope of the article.

Dealing with an electric battery

It's for real classic example a chemical source of electric current that is renewable. The battery is in one of two modes: charge / discharge. In both of these cases, there will be a different direction of electric current. But note that the polarity of the electrodes will not change. And they can play different roles:

1. During charging, the positive electrode receives an electric current and is the anode, and the negative one releases it and is called the cathode.
2. In the absence of movement, there is no point in talking about them.
3. During discharge, the positive electrode releases an electric current and is the cathode, while the negative electrode receives and is called the anode.

Let's say a word about electrochemistry

Slightly different definitions are used here. Thus, the anode is considered as an electrode where oxidative processes take place. And remembering school course chemistry, can you answer what happens in the other part? The electrode on which the leak recovery processes is called the cathode. But there is no reference to electronic devices. Let's consider the value of redox reactions for us:

1. Oxidation. There is a process of recoil of an electron by a particle. The neutral turns into a positive ion, and the negative is neutralized.
2. Recovery. There is a process of obtaining an electron by a particle. A positive turns into a neutral ion, and then into a negative with repetition.
3. Both processes are interconnected (for example, the number of electrons that are given away is equal to their number added).

Faraday also introduced names for the elements that take part in chemical reactions for designation:

1. Cations. This is the name of positively charged ions that move towards the negative pole (cathode).
2. Anions. This is the name of the negatively charged ions that move in the electrolyte solution towards the positive pole (anode).

How do chemical reactions take place?

Oxidative and reducing half-reactions are separated in space. The transition of electrons between the cathode and the anode is not carried out directly, but due to the conductor of the external circuit, on which an electric current is created. Here you can observe the mutual transformation of electrical and chemical form energy. Therefore, to form an external circuit of the system of conductors different kind(which are the electrodes in the electrolyte) and it is necessary to use metal. You see, the voltage between the anode and the cathode exists, as well as one nuance. And if there were no element that prevents them from directly carrying out the necessary process, then the value of the sources of chemical current would be very low. And so, due to the fact that the charge needs to go through that scheme, the equipment was assembled and works.

What is what: step 1

Now let's define what is what. Let's take galvanic cell Jacobi-Daniel. On the one hand, it consists of a zinc electrode, which is immersed in a solution of zinc sulfate. Then comes the porous partition. And on the other side there is a copper electrode, which is located in a solution. They are in contact with each other, but chemical features and the partition is not allowed to mix.

Step 2: Process

Oxidation of zinc occurs, and electrons move along the external circuit to copper. So it turns out that the galvanic cell has a negatively charged anode and a positive cathode. Moreover, this process can proceed only in cases where the electrons have somewhere to "go". The fact is that the presence of "isolation" prevents getting directly from the electrode to another.

Step 3: Electrolysis

Let's look at the process of electrolysis. The installation for its passage is a vessel in which there is a solution or an electrolyte melt. Two electrodes are lowered into it. They are connected to the source. direct current. The anode in this case is the electrode that is connected to the positive pole. This is where oxidation takes place. The negatively charged electrode is the cathode. This is where the reduction reaction takes place.

Step 4: Finally

Therefore, when operating with these concepts, it must always be taken into account that the anode is not used in 100% of cases to denote the negative electrode. Also, the cathode can periodically lose its positive charge. It all depends on what process takes place on the electrode: reductive or oxidative.

Conclusion

This is how everything is - not very difficult, but you can’t say that it’s simple. We examined the galvanic cell, anode and cathode from the point of view of the circuit, and now you should not have problems connecting power supplies with operating time. And finally, you need to leave some more valuable information for you. You always have to take into account the difference that the anode has. The thing is, the first one will always be a little big. This is because the coefficient useful action does not work with an indicator of 100% and some of the charges are dissipated. It is because of this that you can see that batteries have a limit on the number of times they can be charged and discharged.