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HELP photoelectric effect

  1. Apr 24, 2010 #1
    HELP!!! photoelectric effect

    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.

    Ok so im planig to use einsteins photoelectric equation of 1/2 mev2 = hf-psi(as in work function)
    Im guessing i need to convert the wavelength to frequency so have come up with the solution 7.137915667e11

    so this in the equation gives me 6.63*10-34 * 7.14285713*1014 - 3.67*10-19 J = n = -3.665267562

    So this is the result of the maximum kinetic energy E of the electrons.
    I am a bit worried as my result is a minus!!!!!!!! which i imagine is impossible

    after that i believe i am to work out the potential differnence to stop the electrons.

    That i am a bit unclear on how to do so any help would be much appreiciated :)
  2. jcsd
  3. Apr 24, 2010 #2


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    Re: HELP!!! photoelectric effect

    You have miscalculated the KE of the electrons. It is

    6.63*10-34 * 7.14*1014 - 3.67*10-19 =1.07*10-19

  4. Apr 24, 2010 #3
    Re: HELP!!! photoelectric effect

    ahhh ok thanks :)

    for the potential differnce is it just KE=eV and therefore i need a force of the opposite charge to stop the electrons............this right? is this question that simple?
  5. Apr 24, 2010 #4


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    Homework Helper

    Re: HELP!!! photoelectric effect

    Yes. it is that simple :)

  6. Apr 24, 2010 #5
    Re: HELP!!! photoelectric effect

    sweeeeet thanks :)

    any good at schrodingers wave mechanics ha ha :)
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