The notion of cathode is used in the field of physics to refer to a negative electrode. The etymology of the term refers us to the Greek word káthodos, which translates as “downward path”.
An electrode is the end of an electrical conductor that collects or transfers a current when it is in contact with a medium. In the specific case of cathodes, they are electrodes that have a negative electrical charge.
The ends or terminals of a battery or a cell are called poles, which can be negative or positive. This quality is called polarity. The direction of circulation of the electric current was conventionally fixed as the flow of charges that goes from the positive pole to the negative pole.
In devices that provide power, such as batteries, the cathode has positive polarity. On the other hand, if the element consumes energy, the cathode has a negative polarity.
Redox reactions (reduction-oxidation) are generated in the cathodes that make a material, by obtaining electrons (elementary particles that have a negative charge), undergo a reduction in its oxidation state. In the anodes (positive electrodes), on the other hand, oxidation reactions take place, which lead a material to lose electrons and increase its oxidation state.
With regard to etymology, it is known that the term was coined by the physicist and chemist Michael Faraday, originally from Great Britain, who made great contributions to the fields of electrochemistry and electromagnetism. More specifically, Faraday first mentioned it in the context of his experimental investigations of electricity, in the seventh series.
The meaning he gave to the word cathode was “exit, downward path”, since its origin is found in a Greek word that can be translated as “path, downward”; in this case, it should be understood only in reference to the electrolyte of electrochemical cells.
Thermionic cathode is called that electrode that, from the thermionic effect generated by heat, emits electrons; This phenomenon is also known as the Edison effect. This type of cathode, for example, is the source of electrons used in thermionic valves.
One of the most important properties of the thermionic cathode is that it can increase its own temperature on its own; To do this, it makes a heating current circulate through it, or it uses a filament to which it is thermally coupled. Materials that manage to emit electrons at a not too high temperature are the most efficient to take advantage of the thermionic effect; some of the most common are alloys of tungsten (also called tungsten), thorium, and lanthanides; Another option is to coat the cathode with calcium oxide.
It is known as cathode rays, on the other hand, the currents of electrons that can be observed in vacuum tubes, those that are made of glass and that are equipped with a minimum of two electrodes, an anode and a cathode in a configuration that is called diode. When the cathode is heated, it emits radiation that travels in the direction of the anode; if the internal glass walls behind the latter are covered with some fluorescent material, then they produce a high gloss.
This concept is found in most TV screens and monitors of past decades as using tubes cathode ray, a technology that constantly emits rays into a glass screen coated with lead and phosphorous to reproduce images. Lead protects the person from lightning radiation while phosphorus enables the reproduction of images.