Anode and cathode: what are they, how to identify and remember them

Among the terms in electrical engineering, there are concepts such 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 terminals or contacts of the devices and what electrical sign they have. In this article, we will tell you what it is an anode and a cathode, as well as how to determine where they are in an electrolyzer, a diode and a battery, which of them is a plus and what is a minus.

Electrochemistry and electroplating

There are two main sections in electrochemistry:

1. Galvanic cells - the production of electricity through a chemical reaction. These items include batteries and accumulators. They are often referred to as chemical current sources.
2. Electrolysis - the impact on a chemical reaction with electricity, in simple words - with the help of a power source, some kind of reaction is triggered.

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

• Anode - the electrode on which oxidative reaction, that is, he donates electrons. The electrode on which the oxidative reaction occurs is called reducing agent.
• Cathode - the electrode on which flows restorative reaction, that is, he accepts electrons. The electrode on which the reduction reaction occurs is called oxidizing agent.

This raises the question - where is the plus and where is the minus in the battery? Based on the definition, for a galvanic cell the 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 a plus at the cathode, and a minus at the anode.

In this case, the flow of electric current is considered on the external circuit conductor from oxidizer (cathode) To reducer(anode). Since the electrons in the circuit flow from minus to plus, and the electric current is vice versa, 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:

Electrolysis or battery charging process

These processes are similar and inverse to the galvanic cell, since no energy is supplied here due to chemical reaction, but on the contrary - a chemical reaction occurs due to an external source electricity.

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

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

Since the current from the positive terminal of the power supply goes to the positive terminal of the battery, the latter can no longer be a cathode. Referring to the above, we can conclude that in this case, the electrodes of the battery are conditionally swapped during charging.

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

Electroplating

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 plated with other metals ornaments and details. This process is used for both decorative and applied purposes - to improve the corrosion resistance of various components and assemblies of mechanisms.

The principle of operation of installations for applying galvanic coating lies in the use of solutions of salts of the elements, which will cover the part, as an electrolyte.

In electroplating, the anode is also an electrode to which the positive terminal of the power supply 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

The electrodes or legs of semiconductor and vacuum electronic devices are also often referred to as 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 side of the battery. It is so called for the same reason - current flows into this output from the diode in any case. On a real element on the cathode there is a marking in the form of a strip or a dot.

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

The situation is the same with the thyristor, the assignment of the terminals and the "unipolar" application of these three-legged components make it a controlled diode:

In 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 the opposite direction, albeit insignificant.

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

Conclusion

So, let's summarize by answering the question: how to remember where is the plus, where is the minus at the cathode with the anode? There is a handy 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.

Do not 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 electronic elements, as well as electrolyzers and in electroplating - in general, for all energy consumers, the anode is called the output connected to the plus. This is where the differences end, now it's easier for you to figure out what is plus and what is minus between the outputs of elements and devices.

Finally, we recommend watching a useful video on the topic of the article:

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

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