Why is the first peak in the Franck-Hertz experiment longer than the others?

AI Thread Summary
The first peak in the Franck-Hertz experiment appears longer than subsequent peaks due to the influence of contact potential, which arises from the difference in work functions between the cathode and anode. This contact potential causes the voltage required to reach the first peak to exceed the average peak-to-peak voltage, leading to a larger spacing observed in the plot. While the expected excitation energy is around 4.9 eV, variations in experimental conditions can result in the first peak being recorded at a higher voltage. The discussion highlights confusion regarding the interpretation of the voltage values and the relationship between the peaks. Understanding the role of contact potential is crucial for clarifying this phenomenon in the experiment.
Terrycho
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Homework Statement
In Franck-Hertz Experiment, why is the spacing to the first peak different than the spacing between successive peaks?
Relevant Equations
λ=hc/E
In the experiment, I know that the spacing between successive valleys gives the excitation energy to be somewhere around 4.9eV. However, when you look at the plot, you can see that the spacing from zero to the first peak is much longer than any other spacings between two successive peaks. I was just wondering why that one is so much longer.
 
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It does look like the first peak is at 4.9V. However, when I did the experiment, the first peak was not 4.9V. It was larger than that. It seems like this plot also has the first peak is larger than 4.9V.
https://foothill.edu/psme/marasco/4dlabs/4dlab8.html
 
DrClaude said:
I don't understand what you mean. The first peak is at 4.9 V and the peaks are separated by 4.9 V.
https://en.wikipedia.org/wiki/Franck–Hertz_experiment#/media/File:Franck-Hertz_en.svg
I found this explanation,

The contact potential is the difference between the work functions of the cathode and anote, since they are oppositely directed in the electric field, that is, the electric field has to work against the cathode potential but is helped in the case of the anode. Thus we should expect that the voltage to the first peak will be greater than the average peak to peak voltage, due to the con- tact potential. The contact potential can be calculated as the average peak to peak voltage sub- tracted from the first peak voltage.

But does not quite make sense to me.

http://instructor.physics.lsa.umich.edu/adv-labs/Franck_Hertz/franck-hertz.pdf
 
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