MadSci Network: Engineering |
Dear Adam, The picture tube you refer to is made of glass, and is sealed perfectly tight. All of the air is pumped out of the tube to that the pressure is less than 1 millionth of the pressure of the air at the earth's surface. Under this high vacuum electrons can move around and now bump into air molecules,so they can be made to move in straight lines to form an electron beam. The electrons are generated by heating a thin wire, called a filament, to over 1,000 degrees C. At this temperature the electrons have enough energy to come boiling out of the metal filament into the vacuum. In a metal there is a "sea" of electrons which can move about in the metal very easily. That is why metals conduct electricity so well-because of there free electrons. However they are trapped below the surface of the metal under room temperature conditions. But if you heat the metal very hot the electrons begin to boil out of the metal, just like the water boils out of a pot when you heat it to boiling temperature. As they boil out of the metal electric forces act on the electrons to focus them into a beam in much the same way a lens focuses the light that comes out of the bulb in a movie projector. This fine beam of electrons is accelerated to very high speed. The beam direction can be changed by applying voltages to metal electrodes surrounding the beam. The beam is directed at the large flat screen in front of the picture tube, and is controlled to scan across the tube face from inside the tube very quickly. The inner surface of the tube screen is coated with material which glows and gives off light when it is struck by the beam of electrons. The composition of the material which glows can be adjusted to give off different colors, particularly red, blue, and green. This material is called a phosphor, and we say the phosphor glows when the electrons strike it. The location of the beam is precisely controlled as to where it strikes the phosphor, so that the beam can be made to produce different colors as it moves across depending upon which phosphor it is striking. The Red/Green/Blue phosphors are deposited in a pattern on the screen and the beam is directed to strike the right color as it moves across the screen. There is no reaction with any gases-the colors come from the physical bombardment of the phosphors by the electrons. From the front of the tube you see the glowing phosphors! I hope you understand this OK. If you have more question for details let me know! R. Bersin.....
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