Why Do Voltage-Current Graphs in the Franck-Hertz Experiment Show Tops and Dips?

[Kristoffer Linder]

Homework Statement

I have two questions concerning the Franck-Hertz experiment on Mercury. The first one is to explain the tops and dips in the voltage-current graph (https://en.wikipedia.org/wiki/File:Franck-Hertz_en.svg). The second question consider the experimental setup, and why there is so few electrons in the collector.

Homework Equations

The Attempt at a Solution

My attempted answer to the first question, that was not recognized by my teacher, was that:

"as one increases the anode voltage, there is a sharp dip in the counter current at the energy at which such can occur. At higher voltages the counter current starts to rise again but eventually the electrons gain sufficient energy to once again inelastically collide with the Mercury atoms and thus excite them to a higher energy level. The observed current-voltage curve should be steadily rising with a superimposed series of dips; the separation of two dips will correspond to the difference in energy of the ground state and first excited state in the mercury atoms."

And the answer to the second question, which also was not recognized, was that:

"There is a potential difference between the grid anode and the collector (denoted as P in the figure) as well. This potential difference is called the reverse bias voltage. As the electrons makes it through the cloud of Mercury atoms they will be subject to a stopping force slowing down their speed and thus the energy. This reverse bias voltage may be seen as a filter that filters the electrons that does not have a sufficient energy to reach the collector. Finally the electrons that reach the collector will induce a current in the circuit following the collector"

Do anyone know another way of answering those two questions?

 

[darth boozer]

I would suggest reading other sources as the Wikipedia explanation is not very clear.
http://hyperphysics.phy-astr.gsu.edu/hbase/FrHz.html is a good place to start.

 

[Delta2]

About question 2, your answer explains what the reverse bias voltage does (it acts as a kind of filter, yes, to filter those electrons only that have high enough energy) but it doesn't seem to answer, at least explicitly, why there are few electrons at the collector, why the current measured is at the levels of milliampere.

 

[Kristoffer Linder]

I would suggest reading other sources as the Wikipedia explanation is not very clear.
http://hyperphysics.phy-astr.gsu.edu/hbase/FrHz.html is a good place to start.

I think this answer the first question, but the question with why there are so few electrons at the collector still remains.

 

[BvU]

why there are so few electrons at the collector

Thermal equilibrium