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Photo electric effect

  1. Jan 10, 2007 #1
    ok what i dont understand is that, after a certain amount of time, wont the piece of metal loose all its electrons becoming completly positive!, what will happen then, just beomes +vly charged, am in grade 12 and my teacher couldnt give me a satisfying answer, thanks
  2. jcsd
  3. Jan 10, 2007 #2
    Assuming no outside source of electrons, as you knock off more and more electrons, the metal becomes more and more positively charged. This means that the photons have to have more and more energy to knock off any more electrons. Pretty soon even cosmic rays (the photons with the most energy), won't even be able to knock off any more electrons and the whole thing comes to a halt. Keep in mind that the electricmagnetic field is VERY strong. It doesn't take much of an imbalance to bring the whole thing to a halt.

    Normally, if you want electrons to continue to fly off the metal, you have to supply an outside voltage
  4. Jan 10, 2007 #3
    cool, thanks bud
  5. Jan 28, 2007 #4
    Hmmm I posted a similar question just today. The electrons must eventually return since the metal is positively charged. I've been looking for more detailed discussion of how soon they return and how far the holes left behind can drift.
  6. Jan 28, 2007 #5


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    No they don't. The cathode is grounded, so none of the photoelectrons have to "return". If you look at a standard photoelectric setup, it is the anode that is biased to a positive potential. This is all you need to collect the photoelectrons.

  7. Jan 28, 2007 #6
    Dear Zz,

    Thanks for your response. I see what you mean. If the metal is grounded, electrons can "return" through ground rather than directly. Or another way to think of it is the holes also leave the metal through ground?

    What if the metal is floating (e.g.,placed on insulator)?
  8. Jan 28, 2007 #7


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    Then you get the charging effect and after a while, you'll stop getting any photoelectrons coming out, because the effective work function would have increased beyond the energy of the incoming photons.

    People who do photoemission spectroscopy on insulating material have to deal with this all the time. I certainly did.

  9. Jan 28, 2007 #8
    Great you're the right person to ask then! :)
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