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Homework Help: Physical significance

  1. May 20, 2008 #1
    Can some help me to solve this question
    I have no idea where to start
    a) What is the physical significance of the intercept of the graph with the frequency axis (x-axis)?
    b) What is the physical significance of the intercept of the graph with the kinetic energy axis (y-axis)?
    c) Use the graph to determine the value of Planck’s constant
    d) In a similar experiment, the cathode of the photocell is replaced with a cathode that has a stronger force of attraction for its electrons. Describe how a graph of kinetic energy against frequency would be
    i. Similar to the given graph
    ii. Different from the given graph

    Attached Files:

  2. jcsd
  3. May 21, 2008 #2


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    If you have no idea where to start, them I'm afraid we cannot help you. You must have some idea of how to start. Have you read your class notes/text on the subject matter?
  4. Jan 7, 2010 #3
    I am working on the same question right now. First off, I am a little confused as to what they are asking in parts a) and b)... As far as part c) goes I tried the following method to determine h (Planck's constant) :

    E=hf therefore,


    So referring to the graph, at 3 eV (4.8 x 10^-19 J) the f is 10 x 10^14 Hz

    h = 4.8 x 10^-19 J / 10 x 10^14 Hz
    h = 4.8 x 10^-34 Js

    This does not make sense as Planck's constant, as we know, is 6.63 x 10^-34 Js

    Can anyone shed some light on these issues? Thanks!
  5. Jan 7, 2010 #4


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    E does not equal hf; it equals hf-W, where W is the work function of the metal.

    I think parts a and b are pretty clear. The equation of the line in the graph is E=hf-W, so what do the intercepts represent?
  6. Jan 7, 2010 #5
    So the significance of the intercept of the x-axis is that is represents the threshold frequency, but there is no y intercept, so does that signify that there can be no Ek until the threshold freq. has been reached?
  7. Jan 7, 2010 #6


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    There IS a y-intercept. It's negative and doesn't represent an actual electron, but it does have physical significance.

    Again, look at the equation of the line: E=hf-W
  8. Jan 7, 2010 #7
    I'm hitting a brick wall here... I'll start with what I know for sure:

    h is constant, and f cannot be a negative
    With f=0, the y-axis will be negative the value of the work function

    So does the y-intercept represent the work function? (in the negative plane of course)
  9. Jan 7, 2010 #8


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    Yup, it's the negative of the work function.
  10. Jan 11, 2010 #9
    Perfect. So how would I prove Planck's constant without knowing the exact work function? Should I just use the value for W from the graph? Because it would be close but not exact...
  11. Mar 1, 2010 #10
    i would looooove to know the answer to c) & d). i figured out a) & b)!!!
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