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Homework Help: Photoelectric effect

  1. Apr 8, 2010 #1
    Ok guys got a question on quantum physics - photoelectric effect i wanna run past you before i attempt a solution.

    The question is as follows;

    In a photoelectric-effect experiment, light of wavelength 420nm is incident on a sample of sodium. The work function of sodium is 3.67*10-19 J.
    Calculate the stopping potential required to stop all electrons from reaching the anode.

    I dont really know where to start as im not completly sure on what the question is asking for.
    I know how to work out the maximum kinetic energy of the ejected electrons by way of hf-work funtion, but i KNOW this isn't the way to approach it.
    Any ideas or suggestions?
  2. jcsd
  3. Apr 8, 2010 #2
    No you are correct. Calculate the max ke of the electrons as you suggest.
    Then it's a case of what pd would stop electrons with this ke.
    A hint: it's exactly the opposite case of what ke electrons would gain if accelerated from rest through that pd. Same formula. Same principle. Just deceleration rather than acceleration.
  4. Apr 8, 2010 #3
    hi thanks for the reply,
    when you say pd are do you mean the potential difference?
  5. Apr 8, 2010 #4
    Yes, sure. Pardon my abbreviations when in a hurry!
    The potential difference you calculate in this case (deceleration rather than acceleration) is the "stopping potential" you need.
  6. Apr 12, 2010 #5
    This problem seems like a trick to get you to calculate the electron energy in eV!
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