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I Franck-Hertz Experiment

  1. Sep 2, 2016 #1
    In the FH experiment, what is physically happening when the excited mercury atom hits the metal plate? why does it induce a current drop?
    Thank you for any help :)
  2. jcsd
  3. Sep 2, 2016 #2
    The excited mercury atom does not hit any metal plate. They bombarded a mercury atom vapor with electrons, and showed that the electrons lost energy only at discrete levels (corresponding to the energy needed to exite the mercury atoms). See this (pretty good) wikipedia article:

  4. Sep 2, 2016 #3
    In the FH experiment the electrons flow under a potential difference. In passing through the tube it encounters Mercury atoms and collide with them- initially as the PD is increased current grows but there is a sudden change at particular voltage .

    The sudden change in current suggests that the mercury atoms cannot accept energy until it reaches the threshold for transferring mercury to an excited state.

    And this happens only at certain voltage

    This happens at say 4.9 volt excited state of Hg atoms and corresponds to a strong emission line in the ultraviolet region of spectrum of mercury at 254 nm .

    The electron's energy is being absorbed in a packet corresponding to this value.

    Drops in the current occure since an accelerated electron which has 4.9 eV of energy gets removed and can not reach the collector plate.
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