Electricity & Megnetism

Picture on the TV tube

We can say that practically every home has a television. Today we know that they are increasingly sophisticated. A few years ago, televisions were huge, heavy, but what we see today are thin, flat-screen televisions with more advanced technology. Best of all, through TVs we can watch our favorite shows, movies, etc.

The image that is produced in the tube television depends on two forces: magnetic and electric, which act on moving electrons. Therefore, we say that the image is nothing more than the result of the transformation of electrical energy into kinetic energy and kinetic energy into light energy.

As shown in the figure above, we can see that the television tube is made of glass, and internally it is made in a vacuum. The inner front part of the tube is all covered with fluorescent material. Electrons are fired from the opposite end of the screen by an electron gun.

After being fired by the electronic gun, the electrons collide with the screen, thus creating luminous points on its surface. The electrons that are thrown towards the screen are produced by a superheated filament. The electrons that collide with the screen are the free electrons – they are able to move because they acquire extra kinetic energy by heating the filament. Therefore, with the gain of energy, they manage to escape the atoms, leaving the filament. The emission of these electrons by heating is called thermionic emission .

As we know, the escaping electrons could go in different directions, but this does not happen, because through an electric field the free electrons are directed in a single direction towards the screen. In physics we give the name electron beam to electrons that are accelerated. The speed of the electrons is controlled through the intensity of the electric field, and the more intense the field, the greater the speed of the electrons, because the greater the force that acts on them.

In order for the entire screen to be illuminated, the electronic beam must be very narrow, so it has to sweep across the entire surface of the screen. In order to direct the electronic beam over the entire surface of the screen, a magnetic field is used. The deviation caused by the magnetic field causes the electron beam to travel through all points on the screen depicting a horizontal line in a very short time interval. The different lighting of different points on the screen produces the effect of the entire image of a scene.

Even after the electron beam is deflected to another point on the screen, the glow of the fluorescent material on the initial point remains for a short time. In this way, we see that the two associated factors (the speed with which the screen is swept by the beam and the emission of light for a few moments after the incidence of the electronic beam on the fluorescent material) allow us to have the sensation of an integral and continuous image of a scene.

As we know, the first TVs that were manufactured displayed black and white images and their screen was coated internally with zinc sulfide or a mixture of cadmium and silver, which are fluorescent materials. Color televisions had an internal coating composed of three layers of fluorescent material. Thus, they created images in the colors blue, red and green. When the images were superimposed, the result was that of a single color image.

The TV picture tube is one of the applications of cathode ray tubes, which were built in 1897 by the German physicist Karl Ferdinand Braun. He observed the phenomenon of luminescence caused by the collision of electrons in fluorescent paints and the deviation of the luminous point generated by the action of the magnetic field. The beam of electrons emitted by a device called a cathode came to be called cathode rays. Cathode ray is also used in radar and oscilloscope displays, among other applications. It was used in TV tube imaging in 1928.

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