|MadSci Network: Physics|
Well, indeed carbon powder is black, but so are magnetic dust particles containing various metal oxides. You may remember from school that a TV is nothing but a cathode ray tube. The electron beam hitting the phosphors on the inside of the front screen is deflected horizontally and vertically by magnetic field oscillations, which create the nice images we see. Hold a magnetic compass near a TV and watch it deviate. Remember the product warnings on good VCR tape packages not to store them near the TV? It is the magnetic field from the rear end of the CRT, which causes the black dust to stick to the TV, whereas organic dust would be blown away by air drafts and convection as on any other surface. So over time a black sticky dust coat covers the TV. The front screen has an additional dust collection effect. The Brems- radiation (X-rays), which are generated, when the electrons hit the screen ionizes the air in front of the screen and in spite of the embedded conductors in the screen it is always on a negative potential (electrons are negative). So the positive parts of the ionized dust in front of the screen gets accelerated towards the screen. So, if you ionize a dust particle containing metal atoms, what do you get? oxidized metals on your screen. Now depending on the chemical nature of your dust this deposit can vary between pitch black or light gray.
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