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B Does the photoelectric effect eventually stop?

  1. Jul 3, 2017 #1
    Assume that a light shining on a metal initially resulted in electrons being emitted from the surface of the metal due to the photoelectric effect. If the light is left on indefinitely, will the emission of electrons eventually cease? (Assume the metal is insulated on all surfaces apart from the surface where the light is shining).
     
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  3. Jul 3, 2017 #2

    Merlin3189

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    Unless the electrons are attracted away by an external field, many would fall back anyway. If some escape then the metal will become more positively charged and attract electrons back to it more strongly. When it reaches the stopping potential (determined by the frequency and independent of the intensity) electrons no longer have enough energy to leave.

    Electrons are not liberated by light with lower frequency than the threshold. As the frequency gets higher than the threshold, the emitted electrons have greater energy and so more chance of escaping. You need a higher stopping potential. But as electrons leave, the metal will become more positively charged and will eventually exceed the stopping potential.

    In a photocell the light provides the energy to produce the free electrons (the work function energy), but then a strong external field pulls the electrons away. If this field is strong enough,all the electrons emitted are attracted to the anode and you get a saturation current proportional to the intensity of the light.
     
  4. Jul 3, 2017 #3
    Thanks for the reply. Ignoring external fields, are you saying
    (a) that all emissions will eventually stop because all the electrons that could be emitted (using this wavelength of light) will have been emitted
    or
    (b) that due to electrons escaping and not returning, the metal surface becomes positively charged. This attracts many of the escaping electrons back to the surface. Therefore after a while, the same electrons are emitted and attracted back over and over again, in a kind of dynamic equilibrium?
     
  5. Jul 3, 2017 #4

    Merlin3189

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    I'm saying (b)
    I don't think you could come anywhere near driving off electrons from all the atoms in the metal. I think the positive charge on the metal would be enormous and that, I think, would effectively increase the work function energy, requiring shorter wavelength light even to get an electron off the surface.
     
  6. Jul 7, 2017 #5

    sophiecentaur

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    If every atom in the metal became a +ion, there would be no net binding force to hold the metal together. It would just disperse and become an expanding cloud of ions.
    The energies and forces that are implied by these questions about what would happen in such extreme situations tend to make a nonsense.
     
  7. Jul 7, 2017 #6
    @sophiecentaur
    The question simply asks whether the photoelectric effect eventually ceases. All the atoms in the metal being ionised has not been mentioned.

    Reply #3 makes two suggestions as to what happens in the long run and the current consensus seems to be (b) that emissions will never cease because eventually the same electrons will be emitted and attracted back in an endless cycle.

    It would be interesting to hear your suggestion for what would happen in the long term.

    .
     
  8. Jul 7, 2017 #7

    sophiecentaur

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    I thought that the 'equilibrium' situation had been discussed. The only other long term situation would be to maintain the loss of electrons. This would require higher and higher photon energies of course.
    But there are practicalities involved.
     
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