# Homework Help: Franck Hertz Questions

1. Dec 2, 2008

### 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 sorta 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.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 2, 2008

### 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?

3. Dec 3, 2008

### 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.

4. Nov 10, 2010

### 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.