# Determining Planck's Constant [Resolved]

1. Oct 10, 2013

### Hapablap

*Edit: Nevermind, I realize where I went wrong!

1. The problem statement, all variables and given/known data
The following graph shows the kinetic energy of the most energetic photoelectrons as a function of the frequency of light falling on the cathode in a photoelectric cell.

http://img208.imageshack.us/img208/3421/fwby.jpg [Broken]

Use the graph to determine the value of Planck's constant.

2. Relevant equations
E = hf
h = E/f

3. The attempt at a solution
I'll choose the point where E = 3.0 eV and f = 10.0 Hz for my calculation.

First I'll convert eV to Joules:
3.0 eV x (1.60 x 10^-19 J / 1 eV) = 4.8 x 10^-19 J

Now the calculation:
h = E/f
h = (4.8 x 10^-19 J) / (10.0 x 10^14 Hz)
h = 4.8 x 10^-34 J(s)

We already know that the value of Planck's constant is 6.63 x 10^-34 J(s), so my answer is off.

Any suggestions?

Last edited by a moderator: May 6, 2017
2. Oct 10, 2013

### Staff: Mentor

looks like you need the slope of the line but what you calculated was the slope of the line going thru 0,0