# Planck's constant

1. May 22, 2010

### Cudi1

1. The problem statement, all variables and given/known data

2. Relevant equations
Ek=hf-W

3. The attempt at a solution
Using the equation Ek= hf-W ( W is work function) , in the given graph the x axis is the threshold frequency so f= 10, and Ek is given by 3ev, i tried to isolate variables but i have no clue what W is equal to

for part c) a stronger force of attraction means greater threshold and kinetic energy so i would imagine a diagonal line going through (0,0) , it would be different in the case that it would start at (0,0) since the cathode ray has a stronger force of attraction.

Im unsure how to go on about doing it , although I believe that the physical signifcance of the y intercept is that it is the negative of the work function .
1. The problem statement, all variables and given/known data

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2. May 22, 2010

### jegues

I'm gonna approach this with solely a mathematics prespective.

The graph you have is a linear function, the equation you provided is also in the format of a linear function.

With that alone you've got enough information to solve whatever you're looking for.

3. May 22, 2010

### Cudi1

I understand that,as you mentioned its a linear function so assuming the cathode has a stronger force of attraction with its electrons, doesn't it mean that the slope is greater since there is greater attractive force b/w the x axis ( threshold frequency) and the y -axis( kinetic energy) but how would i determine the value of planck's constant using
Ek=hf-w

4. May 22, 2010

### jegues

You have enough information to find everything in that equation from that graph... You should be able to solve for h.

5. May 22, 2010

### Cudi1

so h=(Ek+W)/f , the thing is i know that from the given graph f= 3, Ek=3ev and what is W?

6. May 22, 2010

### jegues

Can't you simply use the slope to "extrapolate" back and find the intercept?

7. May 22, 2010

### Cudi1

yes but only thing im confused is with how to find W still, in the mathematical way not so , but using physics i am

8. May 22, 2010

### jegues

I'm not sure what you're confused about... You can solve it using an "mathematical" approach but you want to solve it using a "physics" approach? What's wrong with using the "mathematical" approach?

9. May 22, 2010

### Cudi1

ok im confused solving mathematically, so basically take the points of 2 points and find the slope after that what would i do, and by doing this would it give me the work function?

10. May 22, 2010

### jegues

The slope of that graph is plancks constant.

11. May 22, 2010

### Cudi1

okay got that but using the equation i mention Ek=hf-W where "w" represent the work function rearranging for planck's constant we can solve it using that formula but the only unknown is the work function :S

12. May 22, 2010

### jegues

You solve planck's constant by solving the slope. You solve the work function by using planck's constant.

13. May 22, 2010

### Cudi1

we already know planck's constant though which is 6.63*10^-34Js so why solve planck's constant when we are finding it?

14. May 23, 2010

### Jokerhelper

One good reason is to verify the accuracy of your experiment based on the collected data.

15. May 23, 2010

### Cudi1

yes that is true, to verify the validity the only thing that is confusing is finding what the work function is...

16. May 23, 2010

### Cudi1

still don't understand would appreciate any help whatsoever

17. May 23, 2010

### Cudi1

Okay, if anyone can have a look as mentioned i just find the slope which should give me planck's constant does it matter that i have a margin error of around 10%? im getting a value of h=6.7*10^-34, is that ok?

18. May 23, 2010

### Cudi1

sorry a slight margin error of approximately 1% is that fine, and is that how i solve by finding the slope which should give me planck's constant?