# Physical significance

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

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Hootenanny
Staff Emeritus
Gold Member
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?

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?

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,

h=E/f

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!

ideasrule
Homework Helper
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?

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?

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?

ideasrule
Homework Helper
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

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

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)

ideasrule
Homework Helper
Yup, it's the negative of the work function.

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...

i would looooove to know the answer to c) & d). i figured out a) & b)!!!