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Homework Help: Photoelectric effect :Millikan experiment

  1. Jul 15, 2009 #1
    Hi, I'm a newbie starting to study physics on my own (at a later age (45)).
    I've tried to find an answer to my question in this forum, the internet , books, etc... already.

    I'm reading Modern Physics, Tipler & LLewelyn. On p. 139 he presents a diagram illustrating Millikans experiment on work function (photoelectric effect). I've seen this same diagram on numerous other physics websites.
    (eg. http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c3

    The treshold frequency of the incident light for an electron of the cathode to be ejected is -in this particular experiment- 43.9 *10^13 Hz.
    Now, this corresponds to an energy of E=hf = 43.9 * 10^13 s-1 * 4.136*10^-15 eV s = 1.82 eV

    Now my question: which element (metal) was this? Which material has a work function of 1.82 eV? I can't find any.

    Am I messing up on this? I'm really stuck. Can someone bring some clarity?
  2. jcsd
  3. Jul 15, 2009 #2


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    Welcome to Physics Forums.

    That's a rather low number for the work function of a pure elemental metal. Alkalis and alkalines tend to be the lowest (2 to 3 eV), but are not suitable cathode materials.

    Can you check again on the frequency of 43.9 × 1013 Hz? That's not written in standard scientific notation (the numerical part is not between 1 and 10), or in engineering notation (the exponent is not a multiple of 3), so I'm wondering if it could be a typo as written.

    Another possibility is that the cathode is an alloy. Or, cathodes can have a non-metallic coating to lower the work function, but that would be an odd situation for an introductory physics problem.

    EDIT: are they asking you to find the metal, or are you just wondering out of curiosity?
  4. Jul 15, 2009 #3


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    Hi pepe1964,

    If I can just add to what Redbelly has said: The hyperphysics website that you got the 1.82eV value from says that the material was sodium, and also says the data is from Millikan.

    The plot is from Millikan's 1916 paper 'A Direct Photoelectric Determination of Planck's "h".' You can find it in the Physical Review. (This is one of the papers that helped him get the Nobel Prize.)

    So this measurement is from a very early experiment. Also, if I recall correctly, there is a lot of variations in reported work value functions, since the measured value depends largely on how the surface was prepared (how clean it was, etc).
  5. Jul 15, 2009 #4
    Thanks for the reply.
    I'm just wondering, I am not at school (I am an economist who has made a wrong study choice 25 years ago :-)

    The number they state is correct. It's also in the book MODERN PHYSICS, Tipler & Llewellyn, literally f treshold = 43.9*10^13 Hz.
    The number also corresponds with the diagram.
    I was also wondering if it was a typo, but since I found the same diagram in at least 5 serious websites (.edu), I became very puzzled...
    Does anyone have a copy the original paper? I can't find it.
    Many thanks again.
    Last edited: Jul 15, 2009
  6. Jul 15, 2009 #5
    Thanks for the reply.
    Are you saying :"Don't worry about the number, it's fuzzy anyway." ?
    But how could Millikan then have confirmed Einsteins PE-theory, on the basis of 'fuzzy' numbers?
  7. Jul 15, 2009 #6


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    No, I would never say to not worry about the number. I would say that when it comes to the work function, there is more involved than just looking up a number in a table. If people want to measure the mass of the electron, they (should) get the same number, but there is a natural variation in the measurements of work functions, because the work function of a material depends on more than just the material--it also depends on the conditions of the surface.

    For example, one handbook says that the work function values it lists are for a "reasonably clean surface". And you can imagine how much more control experimentalists have today than back in 1916.

    In the photoelectric equation, the work function is related to the intercept of the line, and planck's constant is the slope of the line. So Millikan was interested in the slope of the line, which did not depend on the work function.

    (And again, it's not that the numbers are "fuzzy", it's that a sample prepared one way will probably have a different work function than the same material prepared another way.)

    By the way, you might be interested in this (open access) paper:

    http://www.europhysicsnews.org/index.php?option=article&access=doi&doi=10.1051/epn:2000303 [Broken]

    which talks about that paper by Millikan, and includes a reproduction of the original plot. It also speaks a bit about how he made samples that were very similar to each other, and about what he thought about Einstein's theory.
    Last edited by a moderator: May 4, 2017
  8. Jul 16, 2009 #7
    OK, alphysicist,

    This is great help for me getting the bigger picture.
    The link is great too.

    Many thanks.
  9. Jul 16, 2009 #8


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    Sure, glad to help!
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