# Lab report (photelectric effect to determine Planck's constant)

1. Apr 7, 2007

### silverthorne

1. The problem statement, all variables and given/known data
This isn't really a textbook or a homework problem...rather it is a question I have in trying to write up my lab report.

I am doing an experiment about the photoelectric effect in trying to determine Planck's constant h experimentally...and basically I am shining various values of light through different filters...and measuring the stopping voltage, then using that to find the value of h.

I have plotted the photocurrent as a function of the voltage...and it goes lower and lower as the voltage is increased until it just levels off horizontally starting at some threshold voltage. I took this threshold voltage as the stopping potential. Now my question is what is the effect of the "reverse current" here? How does it affect the determination of the stopping voltage? The current here actually drops below 0...what does it mean?

2. Relevant equations
maximum KE of electron = hf - W

h is Planck's constant, f is the frequency of the incident light, W is the work function of the particular metal.

total photocurrent I = Ir + If

Ir is the reverse current and If is the forward current

3. The attempt at a solution

I think the stopping voltage is still taken to be where the graph levels off horizontally...regardless of the Ir...that's my guess...