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

[silverthorne]

Homework Statement

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?

Homework 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

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...

 

[Steve4Physics]

I’m answering this 15+ year old post in case the answer is of use to someone.

Without knowing the experimental details and seeing the results/graph, it is difficult to comment. However...

If you are using a typical vacuum photocell, the current should be one-way only. There should be no ‘reverse current’.

But if you are using a solid-state device (e.g. an LDR) the reverse conductance might be significant and can give you a ‘reverse current’.

In this case, you could measure the device's intrinsic I-V curve with the device covered (no light at all) and then use the data to deteemine the true photocurrent values: for a given voltage:
photocurrent = measured total current - intrinsic current

Note, it is incorrect to say that the stopping voltage is the voltage ‘where the graph levels off horizontally’, because the graph also levels off where saturation (maximum current) occurs.

The stopping voltage is (ideally) the smallest negative voltage which makes current exactly zero. It is V ₀ on the graph here:
https://upload.wikimedia.org/wikipe...asurement_apparatus_-_microscopic_picture.svg