The cathode and anode are two components of the same process: the flow of electric current. All materials can be divided into two types - these are conductors, in the structure of which there is a large excess of free electrons, and dielectrics (there are practically no free electrons in them).
Concept of electric current
An electric current is an ordered movement of charged elementary particles in the structure of a substance under the influence of electromagnetic voltage. If a constant voltage is applied to the conductor, then the free electrons having a negative charge will begin to move orderly towards the anode (positively charged electrode) from the cathode (negatively charged electrode). Current, accordingly, will flow in the opposite direction. And the cathode and anode are two electrodes, between which a difference (difference) in electromagnetic voltage is formed.
Conductors and Dielectrics
Conductors and dielectrics can be solid, liquid and gaseous substances. This is not essential for the flow of electric current. With prolonged application of electromagnetic voltage to the material, an excess of electrons will form on the cathode, and its shortage will be formed on the anode. If the voltage is applied long enough, then bound electrons together with atoms will be pulled out from the structure of the material from which the anode is made, and the material itself will begin to enter into a chemical reaction with chemically active substances from the environment. This process is called electrolysis.
Electrolysis
The cathode and anode in electrochemistry are two poles of a constant electromagnetic voltage applied to salt solutions or melts. When a current arises from an excess of electrons, the anode begins to collapse, i.e. the positively charged atoms of the substance themselves will fall into the brine solution (the environment) and transferred to the cathode, where they settle in a purified form. This process is called galvanic. Using electroplating, various products are coated with a thin layer of zinc, copper, gold, silver and other metals.
What is a cathode and what are the tasks that it performs in electrolysis? This can be understood by performing the following steps: if you make an anode of bronze or tin, then on the cathode you will get a printed circuit board coated with a thin layer of copper or tin (used in the electronics industry). In the same way get gilded jewelry, copper-plated and even gilded aluminum tips for electrical engineering in order to increase electrical conductivity.
The answers to questions about what anode and cathode are during electrolysis are obvious: as a result of direct current flowing through the brine, the anode is destroyed, and the cathode takes over the anode material. Even such a term has arisen in the environment of electroplating - "anodization of the cathode". It does not have a physical meaning, but it reflects the actual essence of the issue perfectly.
Semiconductors
Semiconductors are materials that in the structure do not have free electrons, and atomic ones hold poorly in their places. If such a material in a liquid or gaseous state is placed in a magnetic field and then allowed to solidify, an electrically structured semiconductor will be obtained, which will transmit current only in one direction.
From this material, using the above property, diodes are made. They are of two types:
a) with "pnp" conductivity;
b) with "npn" conductivity.
In practice, this subtlety of the structure of the diodes does not matter. It is important to correctly connect the diode to the electrical circuit. Where is the anode, where is the cathode - a question that many are puzzled with. There are special notations on the diode: either A and K, or + and -. You can connect the diode in only two ways to the DC circuit. In one case, a working diode will conduct current, and in the other, it will not. Therefore, it is necessary to take a device on which it is obviously known where the cathode is and where the anode is, and connect it to the diode. If the device shows the presence of current, then the diode is connected correctly. This means that the cathode of the device and the cathode of the diode, as well as the anode of the device and the anode of the diode coincide. Otherwise, you need to swap the connections.
1. If the diode does not pass current in both directions, then it is blown, can not be repaired.
2. If on the contrary, misses, then it is broken. It must be thrown away.
Diodes are checked by testers and probes. In diodes, the cathode and anode are rigidly tied to their material performance, in contrast to galvanic power sources (batteries, batteries , etc.).
The cathode in the semiconductor elements (diodes) of the electrical circuit is an electrode (leg), from which a positive (+) potential emerges. Through the circuit, it is connected with the negative potential of the power source. This means that the current directly in the semiconductor of the diode flows in the direction from the anode to the cathode. On electrical circuits, this process is symbolically indicated.
If we connect a diode with one leg (electrode) to an alternating voltage, then on the second electrode we will get a positive or negative half-sine wave. If we connect two diodes to a bridge, we will observe a rectified electric almost constant current.
Galvanic sources of a direct current - accumulators (batteries)
The cathode and anode in these products change places depending on the direction of flow of electric current, because in one case the voltage does not come to them, and they themselves, due to the chemical reaction, serve as direct current sources. Here the anode will already be a negative electrode, and the cathode will be a positive one. In another case, the usual electrolysis process takes place in the battery.
When the battery is discharged and the chemical reaction that served as the source of electric current has stopped, it must be charged using an external power source. Thus, we start the electrolysis process, i.e. restore the original properties of a galvanic battery. A negative charge must already be applied to the cathode of the battery, and a positive charge to the anode, then the battery will be charged.
Thus, the answer to the question of how to determine the cathode and anode in a galvanic cell depends on whether it is charged or serves as a power source by electric current.
Conclusion
As a summation of all of the above, a cathode is an electrode on which an excess of electrons appears, and an anode is an electrode on which a shortage of electrons appears. But the plus or minus on the specific electrode of the element of the electric circuit is determined by the direction of flow of the electric current.