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Homework Help: Radiation and electroscope

  1. Jan 23, 2010 #1
    If ultraviolet radiation is shone onto a freshly-cleaned zinc cap of an uncharged leaf electroscope, explain why the emission of electrons will soon stop

    I understand it is about the photoelectric effect, but I am not sure why the emission will ever stop...

    Thank You in advance!:biggrin:
  2. jcsd
  3. Jan 23, 2010 #2
    If the cap loses electrons (due to photoelectric emission) it will not stay uncharged.
    Think about the charge on the cap, and the effect this will have on the negative electrons trying to escape from it.

    (Welcome to Physics Forums, by the way)
  4. Jan 23, 2010 #3
    What do you mean?

    The cap is uncharged, so surely the UV light will just cause electrons to be emitted continuously...?
  5. Jan 24, 2010 #4
    It's uncharged initially; but doesn't stay so if it's losing electrons due to photoelectric emission.
  6. Jan 24, 2010 #5
    Yes, but I am not sure why this photoelectric emission will soon stop occuring...?

    maybe the UV radiation will ionize the surrounding air, so charge is more easily removed??
  7. Jan 24, 2010 #6
    If the cap becomes charged through losing those photoelectrons, what will the sign of the charge on it be? + or - ?
    What effect will this charged cap have on negative electrons trying to leave it?
    (Coulomb's Law?) Like charges repel, opposite charges attract.
    As more and more electrons leave the cap, what happens to this force?
    The photoelectrons only have a limited amount of "spare" energy when they leave the surface. What will happen eventually?
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