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Homework Help: Negative maximum kinetic energy in Millikan's data?

  1. Jun 19, 2011 #1
    What am I finding to be confusing..
    The graphs of Millikan's data were straight lines with equal slopes. The graph was plotted maimum kinetic energy (eV) versus the frequency of the wave. This was the graph that produced the linear relationship from the photoelectric effect.

    What does it mean on the graph when it is below the x-axis? The y-axis is the maximum kinetic energy, so if the y-values become negative, what is happening to the maximum kinetic energy?

    I understand that the y-intercept is for the work function of the metal, and that the x-intercept accounts for the threshold frequency, but I am confused as to what is the meaning behind "negative maximum kinetic energy" in the linear relationship.

    Ek(max) = hf - W
  2. jcsd
  3. Jun 19, 2011 #2


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    It's not physically meaningful. The work function is just the amount of energy required to liberate the electron from its parent atom. At f = W/h, the energy of the photon will be JUST enough to liberate the electron from its atom, and it will be a free electron, but with ZERO surplus kinetic energy.

    At f below W/h, the photon will NOT have enough energy to liberate the electron, and so it's not meaningful to talk about how much "extra kinetic energy' it gets freed with (since it doesn't get freed at all).

    The plot should not be extrapolated below f = W/h.
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