“The combined magnetic fields of all these aligned electrons add to the original magnetic field and enhance it drastically” In a Television the magnets that direct electrons are made from solenoids within the CRT with the power to direct the electrons to any point on the screen. The magnets effect the electrons perpendicular to the direction of their current Interesting Facts www. sciencejoywagon. com claims that 900,000 electrons are needed for one image on a colour Television. Using this fact I can predict the current that is likely to be travelling through the vacuum in the cathode ray tube.
Each electron has a current of -1. 610-19C. Every second there are 24 images on a television screen:- 24900,000 = 21. 6million electrons per second Charge transferred/ second (Current2 Amps Phosphors Looking at a television set in a dark room I noticed something else. If the television set is on and then turned off, the screen glows faintly and a vague outline of the last image remains. (I noticed this occurred more noticeably on a black and white television). The continuing glow is very much like that given out by ‘glow in the dark’ paint.
When the TV set has been turned on and the phosphors have been ‘charged’ with electrons the phosphor will continue to glow, strongly at first before eventually becoming dull. Glow in the dark paint shares the same characteristics, when the paint is exposed to light it seems to take in energy from the light and then glow in the dark. Howstuffworks. com had an article stating that there are three characteristics that make phosphors differ. 1. The type of energy they absorb to become ‘energised’. (In television this must be an electric charge given by the electrons.
) 2. The frequency of light they emit. (The manipulation of the substances by doping gives a range of light. This being the important factor that makes the phosphors suitable for television. ) 3. The length of time they glow after they have been energised. I expect that the phosphors on a television set have a short ‘glowing time’. Without this I expect the picture would be very unclear as light would be emitted when it is not wanted. Phosphors are man made materials. I found several examples of these including Zinc sulphide doped with small amounts of silver.
The silver in this case is the ‘activator’ which causes the substance to glow in particular colour. Television signal The Television signal is sent as a voltage so that in Black and white Television all that is needed, for the picture, is one varying voltage. On a black and white television the screen is black when it is off so when the desired pixel shade is black no electrons need be sent to the screen. When the screen is white the maximum voltage is used to send the maximum number of electrons to the pixel, resulting in the brightest possible phosphor glow.
The scanning pattern of a Television screen is shown by the image below. The electron beams are all aimed along these lines, starting from the upper left corner and finishing at the bottom right. The blue lines represent the lines of pixels where the beam must strike. The red dotted line represents the movement of the gun (not emitting electrons) as it travels from line to line, called flyback. The blue line represents the electron guns movement after it has scanned a full image and prepares to start again. Analogue televisions receive a continuous signal that represents the whole screen.
In Europe the image on the screen changes 25 times a second, therefore a continuous signal must represent a whole image every 0. 04seconds. This meaning that receiving Televisions do not need to share the same number of pixels per line, as long as the electron beam scans the screen at the same rate, a similar picture will be shown. Figure GSIT demonstrates this. In the period in which the beam changes line the electron gun has to be inactive or there would be interference on the screen. At this time the signal has to be ‘blanked’.