MadSci Network: Physics

Re: Why the dust on the TV screen is jet black?

Date: Wed Feb 19 09:48:52 2003
Posted By: Dietmar Hildebrand, Post-doc/Fellow, Radiation Biophysics,
Area of science: Physics
ID: 1045417672.Ph

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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