Questions About Franck-Hertz Experiment

[PatF]

I am not sure if this is the right place to post this so sorry if it is wrong. Be that as it may...

I have just completed a lab in which we do the Franck-Hertz experiment. I am trying to understand it so I have a couple of questions I hope you can help with.

1) There is an oven into which we insert a tube filled with mercury. It seems to me the only purpose of the oven is to vaporize the mercury. Is that correct?

2) The data we got seems to be fine. However, we were told that there were different spacings between the peaks because of "the thermocouple." I sort of kinda know what a thermocouple is, but why would this make a difference at lower voltages? Any ideas?

3) We got the usual data which looks like an increasing sinusoid. (Good.) We measured the distances between the peaks and we averaged about .343 volts. We performed a calibration between the measuring equipment on the computer and the electrometer and we got 14.07. (OK) If we multiply. .343 * 14.07 we get 4.82 which is close to 4.86eV which is what we are supposed to get. (Good). But 4.86 is in electron volts and the .343 is in volts. So what is the relation between volts and electron volts. Is there just a simple multiplicative factor? I am not sure this question makes sense even. It just seems like magic that we hit the correct number. I am not fond of magic and I would like to know what, if any relation there is between eV and the voltage in the experiment.

Thanks for any help of constructive criticism.

 

[naresh]

I've never actually done the experiment, so my answers may be way off.

1) That sounds right. You'd need vapour to do the experiment.

2) No idea about the thermocouple.

3) The definition of eV is that it is the energy acquired by an electron when accelerated through a potential of 1 V (hence electron-volt). In the Franck-Hertz experiment, you accelerate electrons through some voltage, say 4.9 V. Therefore the electrons would have energy ___ eV?

 

[PatF]

Naresh
"3) The definition of eV is that it is the energy acquired by an electron when accelerated through a potential of 1 V (hence electron-volt). In the Franck-Hertz experiment, you accelerate electrons through some voltage, say 4.9 V. Therefore the electrons would have energy ___ eV?"

Thanks much. That's what I needed reminding of.

 

[arbiter011]

you need the heat to warm the filament to liberate the electrons. If you try to keep a stable temperature, you can assume that you are releasing the same amount of electrons per unit time.

 

[paranormal]

I am happy to help clarify any questions you may have about the Franck-Hertz experiment. To answer your questions:

1) Yes, the purpose of the oven is to vaporize the mercury in the tube. This is necessary for the experiment because the mercury atoms need to be in a gaseous state to interact with the electrons passing through the tube.

2) The thermocouple is a device used to measure temperature. In this experiment, it is used to monitor the temperature of the oven and make sure it stays at a constant temperature throughout the experiment. This is important because the energy levels of the mercury atoms depend on the temperature, and any fluctuations could affect the spacing between the peaks in the data.

3) The relation between volts and electron volts is that one electron volt is equal to the energy gained by an electron when it is accelerated through a potential difference of one volt. So in this experiment, the voltage applied is directly related to the energy of the electrons passing through the tube. This is why when you multiply the voltage by the calibration factor, you get the correct energy value in electron volts.

I hope this helps clarify any confusion and please let me know if you have any further questions. The Franck-Hertz experiment is a fascinating one and I am glad you had the opportunity to conduct it. Keep asking questions and exploring the world of science!