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
Messages
20
Reaction score
2
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.
 
Physics news on Phys.org
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
 
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
Thread 'Trying to understand the logic behind adding vectors with an angle between them'
My initial calculation was to subtract V1 from V2 to show that from the perspective of the second aircraft the first one is -300km/h. So i checked with ChatGPT and it said I cant just subtract them because I have an angle between them. So I dont understand the reasoning of it. Like why should a velocity be dependent on an angle? I was thinking about how it would look like if the planes where parallel to each other, and then how it look like if one is turning away and I dont see it. Since...
Thread 'Correct statement about a reservoir with an outlet pipe'
The answer to this question is statements (ii) and (iv) are correct. (i) This is FALSE because the speed of water in the tap is greater than speed at the water surface (ii) I don't even understand this statement. What does the "seal" part have to do with water flowing out? Won't the water still flow out through the tap until the tank is empty whether the reservoir is sealed or not? (iii) In my opinion, this statement would be correct. Increasing the gravitational potential energy of the...
Back
Top