# How to find planks constant from a given graph?

1. Apr 18, 2010

### phys02

Hi everyone, I can't seem to calculate Planks constant (h) from this graph. I thought it should simply be the gradient? So I used h= (f1-f2)/(Ua-Ua2) but found it to be nothing near the actual constant of 6.63*10^34!
Any help would be greatly appreciated!

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2. Apr 18, 2010

### jeppetrost

It depends on what the graph is showing.
When U is in volts, you can't use U = h*f, since U isn't the energy.

3. Apr 18, 2010

### RoyalCat

I'm assuming $$U_a$$ is the stopping potential.

Derive the relationship between the stopping potential and the frequency of the photons in a photoelectric effect experiment.

I'll help you get started, the stopping potential is that potential at which an electron with maximal kinetic energy cannot reach the other electrode. It doesn't have enough energy. Contemplate what energy it leaves the metal with, and how much energy it has left as it approaches the second electrode, and where that energy goes, and you should have your answer.

From there, the relationship between the stopping potential and frequency shown in the graph should be straight-forward to deduce. You're only off by a factor of the electron charge (e=1.602 * 10^-19 C)