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Why the light increases when Prof. Lewin touch the end of the tube?

  1. Jun 15, 2013 #1
    Hi,
    Can you explain why the light increases when Prof. Lewin touch the
    end of the tube with his finger?
    He explained it but I am not quite understand it.
    The video is here at 45:40 : http://www.youtube.com/watch?v=ldJhMDuOGxY
     
  2. jcsd
  3. Jun 15, 2013 #2

    jedishrfu

    Staff: Mentor

    he's touching the tube and so the potential where he touches is at 0 and so more current flows and the light is much brighter.

    if you recall just before he did the experiment he said the potential at the close end was 100K v and at the middle 50K volts and at the far end 30k volts but when he touches the far end the 30k becomes 0k v so that the potential diff across the tube is 100K volts whereas when he didn't touch it the potential was 70K volts.
     
  4. Jun 15, 2013 #3
    Thanks jedishrfu!
    How can you know the potential of the Earth is zero volt?
    And when he doesn't touch the tube how electron move after go through the tube?
    I think it will go through the air but does it come back to the tube?
     
  5. Jun 15, 2013 #4

    jedishrfu

    Staff: Mentor

    Its zero by definition:

    http://en.wikipedia.org/wiki/Earth_ground

    Actually the Van degraff generator globe is positively charged so electrons are travelling from the ground thru the air to the tube endpoint.

    http://en.wikipedia.org/wiki/Van_de_Graaff_generator
     
    Last edited: Jun 15, 2013
  6. Jun 15, 2013 #5
    Hi,
    Is it mean that in this case there is a closed loop: Van degraff generator globe - air - tube - air - ground - air - Van degraff generator globe?
     
  7. Jun 16, 2013 #6

    jedishrfu

    Staff: Mentor

    yes although like lightning there could be multiple paths like ground - air - van degraff globe that aren't significant enough to be seen.
     
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