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Thread: discharge tube

  1. #1 discharge tube 
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    1)what makes the electron beam glow and emit visible radiation in discharge tube?

    2)as i read on the sit below
    http://www.aip.org/history/electron/jj1897.htm
    "attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example). Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all."
    i do not understand how gas behaves as electrical conductor on interacting with cathode rays.


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  3. #2  
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    An electron beam in which the electrons move at uniform velocity won't emit light. The light in a discharge tube arises when the electrons strike atoms or molecules in the tube and transfer energy to them. The light comes from the excited atoms or molecules. If the energy of the electron beam exceeds the ionization potential of the atoms or molecules, they will be ionized and able to conduct. The ionization potential of atoms and molecules is quite low. For hydrogen it is about 13 eV and for many molecules it is less than this.


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  4. #3  
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    thanks
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