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Homework Help: Photoelectric effect find velocity??

  1. Nov 11, 2016 #1
    1. The problem statement, all variables and given/known data
    Light illuminates a metallic surface with work function of 1.20 eV. It takes 1.0 V of retarding potential to stop all of the photoelectrons from reaching the opposite plate. Find the maximum speed of the photoelectrons.

    2. Relevant equations
    hf = w

    hf = w + Ek

    3. The attempt at a solution

    First work function = 1.20eV(1.60x10-19 J/eV) = 1.92x10-19 J

    Not really sure how to do this problem... but giving it a go here:


    hf = w

    f = w/h

    f = 1.92x10-19 J / 6.63x10-34 Js

    f = 2.9x1014 Hz

    Now i got the frequency, i assume i use:
    hf = w+ Ek
    Ek = hf - w
    1/2mv2 = hf - w

    Is this correct so far? If yes do i just plug in the numbers and solve for "v" also not really sure what to do with the retarding potential?
  2. jcsd
  3. Nov 11, 2016 #2
    Or if the above isnt correct i tried it a different way...

    Ek = eV
    Ek = (1.60x10-19 C) (1.6V)
    Ek = 2.56x10-19 J
    ^^ Should i subtract the work function from this?

    Ek = 1/2mv2
    2.56x10-19 J = 1/2 (9.11x10-31 kg (v)2
    5.62x1011 Jkg = v2
    7.5x105 m/s = v

    Is this one correct?? Im just confused
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